from math import factorial as _factorial, log, prod from itertools import chain, islice, product from sympy.combinatorics import Permutation from sympy.combinatorics.permutations import (_af_commutes_with, _af_invert, _af_rmul, _af_rmuln, _af_pow, Cycle) from sympy.combinatorics.util import (_check_cycles_alt_sym, _distribute_gens_by_base, _orbits_transversals_from_bsgs, _handle_precomputed_bsgs, _base_ordering, _strong_gens_from_distr, _strip, _strip_af) from sympy.core import Basic from sympy.core.random import _randrange, randrange, choice from sympy.core.symbol import Symbol from sympy.core.sympify import _sympify from sympy.functions.combinatorial.factorials import factorial from sympy.ntheory import primefactors, sieve from sympy.ntheory.factor_ import (factorint, multiplicity) from sympy.ntheory.primetest import isprime from sympy.utilities.iterables import has_variety, is_sequence, uniq rmul = Permutation.rmul_with_af _af_new = Permutation._af_new class PermutationGroup(Basic): r"""The class defining a Permutation group. Explanation =========== ``PermutationGroup([p1, p2, ..., pn])`` returns the permutation group generated by the list of permutations. This group can be supplied to Polyhedron if one desires to decorate the elements to which the indices of the permutation refer. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> from sympy.combinatorics import Polyhedron The permutations corresponding to motion of the front, right and bottom face of a $2 \times 2$ Rubik's cube are defined: >>> F = Permutation(2, 19, 21, 8)(3, 17, 20, 10)(4, 6, 7, 5) >>> R = Permutation(1, 5, 21, 14)(3, 7, 23, 12)(8, 10, 11, 9) >>> D = Permutation(6, 18, 14, 10)(7, 19, 15, 11)(20, 22, 23, 21) These are passed as permutations to PermutationGroup: >>> G = PermutationGroup(F, R, D) >>> G.order() 3674160 The group can be supplied to a Polyhedron in order to track the objects being moved. An example involving the $2 \times 2$ Rubik's cube is given there, but here is a simple demonstration: >>> a = Permutation(2, 1) >>> b = Permutation(1, 0) >>> G = PermutationGroup(a, b) >>> P = Polyhedron(list('ABC'), pgroup=G) >>> P.corners (A, B, C) >>> P.rotate(0) # apply permutation 0 >>> P.corners (A, C, B) >>> P.reset() >>> P.corners (A, B, C) Or one can make a permutation as a product of selected permutations and apply them to an iterable directly: >>> P10 = G.make_perm([0, 1]) >>> P10('ABC') ['C', 'A', 'B'] See Also ======== sympy.combinatorics.polyhedron.Polyhedron, sympy.combinatorics.permutations.Permutation References ========== .. [1] Holt, D., Eick, B., O'Brien, E. "Handbook of Computational Group Theory" .. [2] Seress, A. "Permutation Group Algorithms" .. [3] https://en.wikipedia.org/wiki/Schreier_vector .. [4] https://en.wikipedia.org/wiki/Nielsen_transformation#Product_replacement_algorithm .. [5] Frank Celler, Charles R.Leedham-Green, Scott H.Murray, Alice C.Niemeyer, and E.A.O'Brien. "Generating Random Elements of a Finite Group" .. [6] https://en.wikipedia.org/wiki/Block_%28permutation_group_theory%29 .. [7] https://algorithmist.com/wiki/Union_find .. [8] https://en.wikipedia.org/wiki/Multiply_transitive_group#Multiply_transitive_groups .. [9] https://en.wikipedia.org/wiki/Center_%28group_theory%29 .. [10] https://en.wikipedia.org/wiki/Centralizer_and_normalizer .. [11] https://groupprops.subwiki.org/wiki/Derived_subgroup .. [12] https://en.wikipedia.org/wiki/Nilpotent_group .. [13] https://www.math.colostate.edu/~hulpke/CGT/cgtnotes.pdf .. [14] https://docs.gap-system.org/doc/ref/manual.pdf """ is_group = True def __new__(cls, *args, dups=True, **kwargs): """The default constructor. Accepts Cycle and Permutation forms. Removes duplicates unless ``dups`` keyword is ``False``. """ if not args: args = [Permutation()] else: args = list(args[0] if is_sequence(args[0]) else args) if not args: args = [Permutation()] if any(isinstance(a, Cycle) for a in args): args = [Permutation(a) for a in args] if has_variety(a.size for a in args): degree = kwargs.pop('degree', None) if degree is None: degree = max(a.size for a in args) for i in range(len(args)): if args[i].size != degree: args[i] = Permutation(args[i], size=degree) if dups: args = list(uniq([_af_new(list(a)) for a in args])) if len(args) > 1: args = [g for g in args if not g.is_identity] return Basic.__new__(cls, *args, **kwargs) def __init__(self, *args, **kwargs): self._generators = list(self.args) self._order = None self._center = [] self._is_abelian = None self._is_transitive = None self._is_sym = None self._is_alt = None self._is_primitive = None self._is_nilpotent = None self._is_solvable = None self._is_trivial = None self._transitivity_degree = None self._max_div = None self._is_perfect = None self._is_cyclic = None self._is_dihedral = None self._r = len(self._generators) self._degree = self._generators[0].size # these attributes are assigned after running schreier_sims self._base = [] self._strong_gens = [] self._strong_gens_slp = [] self._basic_orbits = [] self._transversals = [] self._transversal_slp = [] # these attributes are assigned after running _random_pr_init self._random_gens = [] # finite presentation of the group as an instance of `FpGroup` self._fp_presentation = None def __getitem__(self, i): return self._generators[i] def __contains__(self, i): """Return ``True`` if *i* is contained in PermutationGroup. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> p = Permutation(1, 2, 3) >>> Permutation(3) in PermutationGroup(p) True """ if not isinstance(i, Permutation): raise TypeError("A PermutationGroup contains only Permutations as " "elements, not elements of type %s" % type(i)) return self.contains(i) def __len__(self): return len(self._generators) def equals(self, other): """Return ``True`` if PermutationGroup generated by elements in the group are same i.e they represent the same PermutationGroup. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> p = Permutation(0, 1, 2, 3, 4, 5) >>> G = PermutationGroup([p, p**2]) >>> H = PermutationGroup([p**2, p]) >>> G.generators == H.generators False >>> G.equals(H) True """ if not isinstance(other, PermutationGroup): return False set_self_gens = set(self.generators) set_other_gens = set(other.generators) # before reaching the general case there are also certain # optimisation and obvious cases requiring less or no actual # computation. if set_self_gens == set_other_gens: return True # in the most general case it will check that each generator of # one group belongs to the other PermutationGroup and vice-versa for gen1 in set_self_gens: if not other.contains(gen1): return False for gen2 in set_other_gens: if not self.contains(gen2): return False return True def __mul__(self, other): """ Return the direct product of two permutation groups as a permutation group. Explanation =========== This implementation realizes the direct product by shifting the index set for the generators of the second group: so if we have ``G`` acting on ``n1`` points and ``H`` acting on ``n2`` points, ``G*H`` acts on ``n1 + n2`` points. Examples ======== >>> from sympy.combinatorics.named_groups import CyclicGroup >>> G = CyclicGroup(5) >>> H = G*G >>> H PermutationGroup([ (9)(0 1 2 3 4), (5 6 7 8 9)]) >>> H.order() 25 """ if isinstance(other, Permutation): return Coset(other, self, dir='+') gens1 = [perm._array_form for perm in self.generators] gens2 = [perm._array_form for perm in other.generators] n1 = self._degree n2 = other._degree start = list(range(n1)) end = list(range(n1, n1 + n2)) for i in range(len(gens2)): gens2[i] = [x + n1 for x in gens2[i]] gens2 = [start + gen for gen in gens2] gens1 = [gen + end for gen in gens1] together = gens1 + gens2 gens = [_af_new(x) for x in together] return PermutationGroup(gens) def _random_pr_init(self, r, n, _random_prec_n=None): r"""Initialize random generators for the product replacement algorithm. Explanation =========== The implementation uses a modification of the original product replacement algorithm due to Leedham-Green, as described in [1], pp. 69-71; also, see [2], pp. 27-29 for a detailed theoretical analysis of the original product replacement algorithm, and [4]. The product replacement algorithm is used for producing random, uniformly distributed elements of a group `G` with a set of generators `S`. For the initialization ``_random_pr_init``, a list ``R`` of `\max\{r, |S|\}` group generators is created as the attribute ``G._random_gens``, repeating elements of `S` if necessary, and the identity element of `G` is appended to ``R`` - we shall refer to this last element as the accumulator. Then the function ``random_pr()`` is called ``n`` times, randomizing the list ``R`` while preserving the generation of `G` by ``R``. The function ``random_pr()`` itself takes two random elements ``g, h`` among all elements of ``R`` but the accumulator and replaces ``g`` with a randomly chosen element from `\{gh, g(~h), hg, (~h)g\}`. Then the accumulator is multiplied by whatever ``g`` was replaced by. The new value of the accumulator is then returned by ``random_pr()``. The elements returned will eventually (for ``n`` large enough) become uniformly distributed across `G` ([5]). For practical purposes however, the values ``n = 50, r = 11`` are suggested in [1]. Notes ===== THIS FUNCTION HAS SIDE EFFECTS: it changes the attribute self._random_gens See Also ======== random_pr """ deg = self.degree random_gens = [x._array_form for x in self.generators] k = len(random_gens) if k < r: for i in range(k, r): random_gens.append(random_gens[i - k]) acc = list(range(deg)) random_gens.append(acc) self._random_gens = random_gens # handle randomized input for testing purposes if _random_prec_n is None: for i in range(n): self.random_pr() else: for i in range(n): self.random_pr(_random_prec=_random_prec_n[i]) def _union_find_merge(self, first, second, ranks, parents, not_rep): """Merges two classes in a union-find data structure. Explanation =========== Used in the implementation of Atkinson's algorithm as suggested in [1], pp. 83-87. The class merging process uses union by rank as an optimization. ([7]) Notes ===== THIS FUNCTION HAS SIDE EFFECTS: the list of class representatives, ``parents``, the list of class sizes, ``ranks``, and the list of elements that are not representatives, ``not_rep``, are changed due to class merging. See Also ======== minimal_block, _union_find_rep References ========== .. [1] Holt, D., Eick, B., O'Brien, E. "Handbook of computational group theory" .. [7] https://algorithmist.com/wiki/Union_find """ rep_first = self._union_find_rep(first, parents) rep_second = self._union_find_rep(second, parents) if rep_first != rep_second: # union by rank if ranks[rep_first] >= ranks[rep_second]: new_1, new_2 = rep_first, rep_second else: new_1, new_2 = rep_second, rep_first total_rank = ranks[new_1] + ranks[new_2] if total_rank > self.max_div: return -1 parents[new_2] = new_1 ranks[new_1] = total_rank not_rep.append(new_2) return 1 return 0 def _union_find_rep(self, num, parents): """Find representative of a class in a union-find data structure. Explanation =========== Used in the implementation of Atkinson's algorithm as suggested in [1], pp. 83-87. After the representative of the class to which ``num`` belongs is found, path compression is performed as an optimization ([7]). Notes ===== THIS FUNCTION HAS SIDE EFFECTS: the list of class representatives, ``parents``, is altered due to path compression. See Also ======== minimal_block, _union_find_merge References ========== .. [1] Holt, D., Eick, B., O'Brien, E. "Handbook of computational group theory" .. [7] https://algorithmist.com/wiki/Union_find """ rep, parent = num, parents[num] while parent != rep: rep = parent parent = parents[rep] # path compression temp, parent = num, parents[num] while parent != rep: parents[temp] = rep temp = parent parent = parents[temp] return rep @property def base(self): r"""Return a base from the Schreier-Sims algorithm. Explanation =========== For a permutation group `G`, a base is a sequence of points `B = (b_1, b_2, \dots, b_k)` such that no element of `G` apart from the identity fixes all the points in `B`. The concepts of a base and strong generating set and their applications are discussed in depth in [1], pp. 87-89 and [2], pp. 55-57. An alternative way to think of `B` is that it gives the indices of the stabilizer cosets that contain more than the identity permutation. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> G = PermutationGroup([Permutation(0, 1, 3)(2, 4)]) >>> G.base [0, 2] See Also ======== strong_gens, basic_transversals, basic_orbits, basic_stabilizers """ if self._base == []: self.schreier_sims() return self._base def baseswap(self, base, strong_gens, pos, randomized=False, transversals=None, basic_orbits=None, strong_gens_distr=None): r"""Swap two consecutive base points in base and strong generating set. Explanation =========== If a base for a group `G` is given by `(b_1, b_2, \dots, b_k)`, this function returns a base `(b_1, b_2, \dots, b_{i+1}, b_i, \dots, b_k)`, where `i` is given by ``pos``, and a strong generating set relative to that base. The original base and strong generating set are not modified. The randomized version (default) is of Las Vegas type. Parameters ========== base, strong_gens The base and strong generating set. pos The position at which swapping is performed. randomized A switch between randomized and deterministic version. transversals The transversals for the basic orbits, if known. basic_orbits The basic orbits, if known. strong_gens_distr The strong generators distributed by basic stabilizers, if known. Returns ======= (base, strong_gens) ``base`` is the new base, and ``strong_gens`` is a generating set relative to it. Examples ======== >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> from sympy.combinatorics.testutil import _verify_bsgs >>> from sympy.combinatorics.perm_groups import PermutationGroup >>> S = SymmetricGroup(4) >>> S.schreier_sims() >>> S.base [0, 1, 2] >>> base, gens = S.baseswap(S.base, S.strong_gens, 1, randomized=False) >>> base, gens ([0, 2, 1], [(0 1 2 3), (3)(0 1), (1 3 2), (2 3), (1 3)]) check that base, gens is a BSGS >>> S1 = PermutationGroup(gens) >>> _verify_bsgs(S1, base, gens) True See Also ======== schreier_sims Notes ===== The deterministic version of the algorithm is discussed in [1], pp. 102-103; the randomized version is discussed in [1], p.103, and [2], p.98. It is of Las Vegas type. Notice that [1] contains a mistake in the pseudocode and discussion of BASESWAP: on line 3 of the pseudocode, `|\beta_{i+1}^{\left\langle T\right\rangle}|` should be replaced by `|\beta_{i}^{\left\langle T\right\rangle}|`, and the same for the discussion of the algorithm. """ # construct the basic orbits, generators for the stabilizer chain # and transversal elements from whatever was provided transversals, basic_orbits, strong_gens_distr = \ _handle_precomputed_bsgs(base, strong_gens, transversals, basic_orbits, strong_gens_distr) base_len = len(base) degree = self.degree # size of orbit of base[pos] under the stabilizer we seek to insert # in the stabilizer chain at position pos + 1 size = len(basic_orbits[pos])*len(basic_orbits[pos + 1]) \ //len(_orbit(degree, strong_gens_distr[pos], base[pos + 1])) # initialize the wanted stabilizer by a subgroup if pos + 2 > base_len - 1: T = [] else: T = strong_gens_distr[pos + 2][:] # randomized version if randomized is True: stab_pos = PermutationGroup(strong_gens_distr[pos]) schreier_vector = stab_pos.schreier_vector(base[pos + 1]) # add random elements of the stabilizer until they generate it while len(_orbit(degree, T, base[pos])) != size: new = stab_pos.random_stab(base[pos + 1], schreier_vector=schreier_vector) T.append(new) # deterministic version else: Gamma = set(basic_orbits[pos]) Gamma.remove(base[pos]) if base[pos + 1] in Gamma: Gamma.remove(base[pos + 1]) # add elements of the stabilizer until they generate it by # ruling out member of the basic orbit of base[pos] along the way while len(_orbit(degree, T, base[pos])) != size: gamma = next(iter(Gamma)) x = transversals[pos][gamma] temp = x._array_form.index(base[pos + 1]) # (~x)(base[pos + 1]) if temp not in basic_orbits[pos + 1]: Gamma = Gamma - _orbit(degree, T, gamma) else: y = transversals[pos + 1][temp] el = rmul(x, y) if el(base[pos]) not in _orbit(degree, T, base[pos]): T.append(el) Gamma = Gamma - _orbit(degree, T, base[pos]) # build the new base and strong generating set strong_gens_new_distr = strong_gens_distr[:] strong_gens_new_distr[pos + 1] = T base_new = base[:] base_new[pos], base_new[pos + 1] = base_new[pos + 1], base_new[pos] strong_gens_new = _strong_gens_from_distr(strong_gens_new_distr) for gen in T: if gen not in strong_gens_new: strong_gens_new.append(gen) return base_new, strong_gens_new @property def basic_orbits(self): r""" Return the basic orbits relative to a base and strong generating set. Explanation =========== If `(b_1, b_2, \dots, b_k)` is a base for a group `G`, and `G^{(i)} = G_{b_1, b_2, \dots, b_{i-1}}` is the ``i``-th basic stabilizer (so that `G^{(1)} = G`), the ``i``-th basic orbit relative to this base is the orbit of `b_i` under `G^{(i)}`. See [1], pp. 87-89 for more information. Examples ======== >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> S = SymmetricGroup(4) >>> S.basic_orbits [[0, 1, 2, 3], [1, 2, 3], [2, 3]] See Also ======== base, strong_gens, basic_transversals, basic_stabilizers """ if self._basic_orbits == []: self.schreier_sims() return self._basic_orbits @property def basic_stabilizers(self): r""" Return a chain of stabilizers relative to a base and strong generating set. Explanation =========== The ``i``-th basic stabilizer `G^{(i)}` relative to a base `(b_1, b_2, \dots, b_k)` is `G_{b_1, b_2, \dots, b_{i-1}}`. For more information, see [1], pp. 87-89. Examples ======== >>> from sympy.combinatorics.named_groups import AlternatingGroup >>> A = AlternatingGroup(4) >>> A.schreier_sims() >>> A.base [0, 1] >>> for g in A.basic_stabilizers: ... print(g) ... PermutationGroup([ (3)(0 1 2), (1 2 3)]) PermutationGroup([ (1 2 3)]) See Also ======== base, strong_gens, basic_orbits, basic_transversals """ if self._transversals == []: self.schreier_sims() strong_gens = self._strong_gens base = self._base if not base: # e.g. if self is trivial return [] strong_gens_distr = _distribute_gens_by_base(base, strong_gens) basic_stabilizers = [] for gens in strong_gens_distr: basic_stabilizers.append(PermutationGroup(gens)) return basic_stabilizers @property def basic_transversals(self): """ Return basic transversals relative to a base and strong generating set. Explanation =========== The basic transversals are transversals of the basic orbits. They are provided as a list of dictionaries, each dictionary having keys - the elements of one of the basic orbits, and values - the corresponding transversal elements. See [1], pp. 87-89 for more information. Examples ======== >>> from sympy.combinatorics.named_groups import AlternatingGroup >>> A = AlternatingGroup(4) >>> A.basic_transversals [{0: (3), 1: (3)(0 1 2), 2: (3)(0 2 1), 3: (0 3 1)}, {1: (3), 2: (1 2 3), 3: (1 3 2)}] See Also ======== strong_gens, base, basic_orbits, basic_stabilizers """ if self._transversals == []: self.schreier_sims() return self._transversals def composition_series(self): r""" Return the composition series for a group as a list of permutation groups. Explanation =========== The composition series for a group `G` is defined as a subnormal series `G = H_0 > H_1 > H_2 \ldots` A composition series is a subnormal series such that each factor group `H(i+1) / H(i)` is simple. A subnormal series is a composition series only if it is of maximum length. The algorithm works as follows: Starting with the derived series the idea is to fill the gap between `G = der[i]` and `H = der[i+1]` for each `i` independently. Since, all subgroups of the abelian group `G/H` are normal so, first step is to take the generators `g` of `G` and add them to generators of `H` one by one. The factor groups formed are not simple in general. Each group is obtained from the previous one by adding one generator `g`, if the previous group is denoted by `H` then the next group `K` is generated by `g` and `H`. The factor group `K/H` is cyclic and it's order is `K.order()//G.order()`. The series is then extended between `K` and `H` by groups generated by powers of `g` and `H`. The series formed is then prepended to the already existing series. Examples ======== >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> from sympy.combinatorics.named_groups import CyclicGroup >>> S = SymmetricGroup(12) >>> G = S.sylow_subgroup(2) >>> C = G.composition_series() >>> [H.order() for H in C] [1024, 512, 256, 128, 64, 32, 16, 8, 4, 2, 1] >>> G = S.sylow_subgroup(3) >>> C = G.composition_series() >>> [H.order() for H in C] [243, 81, 27, 9, 3, 1] >>> G = CyclicGroup(12) >>> C = G.composition_series() >>> [H.order() for H in C] [12, 6, 3, 1] """ der = self.derived_series() if not all(g.is_identity for g in der[-1].generators): raise NotImplementedError('Group should be solvable') series = [] for i in range(len(der)-1): H = der[i+1] up_seg = [] for g in der[i].generators: K = PermutationGroup([g] + H.generators) order = K.order() // H.order() down_seg = [] for p, e in factorint(order).items(): for _ in range(e): down_seg.append(PermutationGroup([g] + H.generators)) g = g**p up_seg = down_seg + up_seg H = K up_seg[0] = der[i] series.extend(up_seg) series.append(der[-1]) return series def coset_transversal(self, H): """Return a transversal of the right cosets of self by its subgroup H using the second method described in [1], Subsection 4.6.7 """ if not H.is_subgroup(self): raise ValueError("The argument must be a subgroup") if H.order() == 1: return self._elements self._schreier_sims(base=H.base) # make G.base an extension of H.base base = self.base base_ordering = _base_ordering(base, self.degree) identity = Permutation(self.degree - 1) transversals = self.basic_transversals[:] # transversals is a list of dictionaries. Get rid of the keys # so that it is a list of lists and sort each list in # the increasing order of base[l]^x for l, t in enumerate(transversals): transversals[l] = sorted(t.values(), key = lambda x: base_ordering[base[l]^x]) orbits = H.basic_orbits h_stabs = H.basic_stabilizers g_stabs = self.basic_stabilizers indices = [x.order()//y.order() for x, y in zip(g_stabs, h_stabs)] # T^(l) should be a right transversal of H^(l) in G^(l) for # 1<=l<=len(base). While H^(l) is the trivial group, T^(l) # contains all the elements of G^(l) so we might just as well # start with l = len(h_stabs)-1 if len(g_stabs) > len(h_stabs): T = g_stabs[len(h_stabs)]._elements else: T = [identity] l = len(h_stabs)-1 t_len = len(T) while l > -1: T_next = [] for u in transversals[l]: if u == identity: continue b = base_ordering[base[l]^u] for t in T: p = t*u if all(base_ordering[h^p] >= b for h in orbits[l]): T_next.append(p) if t_len + len(T_next) == indices[l]: break if t_len + len(T_next) == indices[l]: break T += T_next t_len += len(T_next) l -= 1 T.remove(identity) T = [identity] + T return T def _coset_representative(self, g, H): """Return the representative of Hg from the transversal that would be computed by ``self.coset_transversal(H)``. """ if H.order() == 1: return g # The base of self must be an extension of H.base. if not(self.base[:len(H.base)] == H.base): self._schreier_sims(base=H.base) orbits = H.basic_orbits[:] h_transversals = [list(_.values()) for _ in H.basic_transversals] transversals = [list(_.values()) for _ in self.basic_transversals] base = self.base base_ordering = _base_ordering(base, self.degree) def step(l, x): gamma = sorted(orbits[l], key = lambda y: base_ordering[y^x])[0] i = [base[l]^h for h in h_transversals[l]].index(gamma) x = h_transversals[l][i]*x if l < len(orbits)-1: for u in transversals[l]: if base[l]^u == base[l]^x: break x = step(l+1, x*u**-1)*u return x return step(0, g) def coset_table(self, H): """Return the standardised (right) coset table of self in H as a list of lists. """ # Maybe this should be made to return an instance of CosetTable # from fp_groups.py but the class would need to be changed first # to be compatible with PermutationGroups if not H.is_subgroup(self): raise ValueError("The argument must be a subgroup") T = self.coset_transversal(H) n = len(T) A = list(chain.from_iterable((gen, gen**-1) for gen in self.generators)) table = [] for i in range(n): row = [self._coset_representative(T[i]*x, H) for x in A] row = [T.index(r) for r in row] table.append(row) # standardize (this is the same as the algorithm used in coset_table) # If CosetTable is made compatible with PermutationGroups, this # should be replaced by table.standardize() A = range(len(A)) gamma = 1 for alpha, a in product(range(n), A): beta = table[alpha][a] if beta >= gamma: if beta > gamma: for x in A: z = table[gamma][x] table[gamma][x] = table[beta][x] table[beta][x] = z for i in range(n): if table[i][x] == beta: table[i][x] = gamma elif table[i][x] == gamma: table[i][x] = beta gamma += 1 if gamma >= n-1: return table def center(self): r""" Return the center of a permutation group. Explanation =========== The center for a group `G` is defined as `Z(G) = \{z\in G | \forall g\in G, zg = gz \}`, the set of elements of `G` that commute with all elements of `G`. It is equal to the centralizer of `G` inside `G`, and is naturally a subgroup of `G` ([9]). Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> D = DihedralGroup(4) >>> G = D.center() >>> G.order() 2 See Also ======== centralizer Notes ===== This is a naive implementation that is a straightforward application of ``.centralizer()`` """ return self.centralizer(self) def centralizer(self, other): r""" Return the centralizer of a group/set/element. Explanation =========== The centralizer of a set of permutations ``S`` inside a group ``G`` is the set of elements of ``G`` that commute with all elements of ``S``:: `C_G(S) = \{ g \in G | gs = sg \forall s \in S\}` ([10]) Usually, ``S`` is a subset of ``G``, but if ``G`` is a proper subgroup of the full symmetric group, we allow for ``S`` to have elements outside ``G``. It is naturally a subgroup of ``G``; the centralizer of a permutation group is equal to the centralizer of any set of generators for that group, since any element commuting with the generators commutes with any product of the generators. Parameters ========== other a permutation group/list of permutations/single permutation Examples ======== >>> from sympy.combinatorics.named_groups import (SymmetricGroup, ... CyclicGroup) >>> S = SymmetricGroup(6) >>> C = CyclicGroup(6) >>> H = S.centralizer(C) >>> H.is_subgroup(C) True See Also ======== subgroup_search Notes ===== The implementation is an application of ``.subgroup_search()`` with tests using a specific base for the group ``G``. """ if hasattr(other, 'generators'): if other.is_trivial or self.is_trivial: return self degree = self.degree identity = _af_new(list(range(degree))) orbits = other.orbits() num_orbits = len(orbits) orbits.sort(key=lambda x: -len(x)) long_base = [] orbit_reps = [None]*num_orbits orbit_reps_indices = [None]*num_orbits orbit_descr = [None]*degree for i in range(num_orbits): orbit = list(orbits[i]) orbit_reps[i] = orbit[0] orbit_reps_indices[i] = len(long_base) for point in orbit: orbit_descr[point] = i long_base = long_base + orbit base, strong_gens = self.schreier_sims_incremental(base=long_base) strong_gens_distr = _distribute_gens_by_base(base, strong_gens) i = 0 for i in range(len(base)): if strong_gens_distr[i] == [identity]: break base = base[:i] base_len = i for j in range(num_orbits): if base[base_len - 1] in orbits[j]: break rel_orbits = orbits[: j + 1] num_rel_orbits = len(rel_orbits) transversals = [None]*num_rel_orbits for j in range(num_rel_orbits): rep = orbit_reps[j] transversals[j] = dict( other.orbit_transversal(rep, pairs=True)) trivial_test = lambda x: True tests = [None]*base_len for l in range(base_len): if base[l] in orbit_reps: tests[l] = trivial_test else: def test(computed_words, l=l): g = computed_words[l] rep_orb_index = orbit_descr[base[l]] rep = orbit_reps[rep_orb_index] im = g._array_form[base[l]] im_rep = g._array_form[rep] tr_el = transversals[rep_orb_index][base[l]] # using the definition of transversal, # base[l]^g = rep^(tr_el*g); # if g belongs to the centralizer, then # base[l]^g = (rep^g)^tr_el return im == tr_el._array_form[im_rep] tests[l] = test def prop(g): return [rmul(g, gen) for gen in other.generators] == \ [rmul(gen, g) for gen in other.generators] return self.subgroup_search(prop, base=base, strong_gens=strong_gens, tests=tests) elif hasattr(other, '__getitem__'): gens = list(other) return self.centralizer(PermutationGroup(gens)) elif hasattr(other, 'array_form'): return self.centralizer(PermutationGroup([other])) def commutator(self, G, H): """ Return the commutator of two subgroups. Explanation =========== For a permutation group ``K`` and subgroups ``G``, ``H``, the commutator of ``G`` and ``H`` is defined as the group generated by all the commutators `[g, h] = hgh^{-1}g^{-1}` for ``g`` in ``G`` and ``h`` in ``H``. It is naturally a subgroup of ``K`` ([1], p.27). Examples ======== >>> from sympy.combinatorics.named_groups import (SymmetricGroup, ... AlternatingGroup) >>> S = SymmetricGroup(5) >>> A = AlternatingGroup(5) >>> G = S.commutator(S, A) >>> G.is_subgroup(A) True See Also ======== derived_subgroup Notes ===== The commutator of two subgroups `H, G` is equal to the normal closure of the commutators of all the generators, i.e. `hgh^{-1}g^{-1}` for `h` a generator of `H` and `g` a generator of `G` ([1], p.28) """ ggens = G.generators hgens = H.generators commutators = [] for ggen in ggens: for hgen in hgens: commutator = rmul(hgen, ggen, ~hgen, ~ggen) if commutator not in commutators: commutators.append(commutator) res = self.normal_closure(commutators) return res def coset_factor(self, g, factor_index=False): """Return ``G``'s (self's) coset factorization of ``g`` Explanation =========== If ``g`` is an element of ``G`` then it can be written as the product of permutations drawn from the Schreier-Sims coset decomposition, The permutations returned in ``f`` are those for which the product gives ``g``: ``g = f[n]*...f[1]*f[0]`` where ``n = len(B)`` and ``B = G.base``. f[i] is one of the permutations in ``self._basic_orbits[i]``. If factor_index==True, returns a tuple ``[b[0],..,b[n]]``, where ``b[i]`` belongs to ``self._basic_orbits[i]`` Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation(0, 1, 3, 7, 6, 4)(2, 5) >>> b = Permutation(0, 1, 3, 2)(4, 5, 7, 6) >>> G = PermutationGroup([a, b]) Define g: >>> g = Permutation(7)(1, 2, 4)(3, 6, 5) Confirm that it is an element of G: >>> G.contains(g) True Thus, it can be written as a product of factors (up to 3) drawn from u. See below that a factor from u1 and u2 and the Identity permutation have been used: >>> f = G.coset_factor(g) >>> f[2]*f[1]*f[0] == g True >>> f1 = G.coset_factor(g, True); f1 [0, 4, 4] >>> tr = G.basic_transversals >>> f[0] == tr[0][f1[0]] True If g is not an element of G then [] is returned: >>> c = Permutation(5, 6, 7) >>> G.coset_factor(c) [] See Also ======== sympy.combinatorics.util._strip """ if isinstance(g, (Cycle, Permutation)): g = g.list() if len(g) != self._degree: # this could either adjust the size or return [] immediately # but we don't choose between the two and just signal a possible # error raise ValueError('g should be the same size as permutations of G') I = list(range(self._degree)) basic_orbits = self.basic_orbits transversals = self._transversals factors = [] base = self.base h = g for i in range(len(base)): beta = h[base[i]] if beta == base[i]: factors.append(beta) continue if beta not in basic_orbits[i]: return [] u = transversals[i][beta]._array_form h = _af_rmul(_af_invert(u), h) factors.append(beta) if h != I: return [] if factor_index: return factors tr = self.basic_transversals factors = [tr[i][factors[i]] for i in range(len(base))] return factors def generator_product(self, g, original=False): r''' Return a list of strong generators `[s1, \dots, sn]` s.t `g = sn \times \dots \times s1`. If ``original=True``, make the list contain only the original group generators ''' product = [] if g.is_identity: return [] if g in self.strong_gens: if not original or g in self.generators: return [g] else: slp = self._strong_gens_slp[g] for s in slp: product.extend(self.generator_product(s, original=True)) return product elif g**-1 in self.strong_gens: g = g**-1 if not original or g in self.generators: return [g**-1] else: slp = self._strong_gens_slp[g] for s in slp: product.extend(self.generator_product(s, original=True)) l = len(product) product = [product[l-i-1]**-1 for i in range(l)] return product f = self.coset_factor(g, True) for i, j in enumerate(f): slp = self._transversal_slp[i][j] for s in slp: if not original: product.append(self.strong_gens[s]) else: s = self.strong_gens[s] product.extend(self.generator_product(s, original=True)) return product def coset_rank(self, g): """rank using Schreier-Sims representation. Explanation =========== The coset rank of ``g`` is the ordering number in which it appears in the lexicographic listing according to the coset decomposition The ordering is the same as in G.generate(method='coset'). If ``g`` does not belong to the group it returns None. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation(0, 1, 3, 7, 6, 4)(2, 5) >>> b = Permutation(0, 1, 3, 2)(4, 5, 7, 6) >>> G = PermutationGroup([a, b]) >>> c = Permutation(7)(2, 4)(3, 5) >>> G.coset_rank(c) 16 >>> G.coset_unrank(16) (7)(2 4)(3 5) See Also ======== coset_factor """ factors = self.coset_factor(g, True) if not factors: return None rank = 0 b = 1 transversals = self._transversals base = self._base basic_orbits = self._basic_orbits for i in range(len(base)): k = factors[i] j = basic_orbits[i].index(k) rank += b*j b = b*len(transversals[i]) return rank def coset_unrank(self, rank, af=False): """unrank using Schreier-Sims representation coset_unrank is the inverse operation of coset_rank if 0 <= rank < order; otherwise it returns None. """ if rank < 0 or rank >= self.order(): return None base = self.base transversals = self.basic_transversals basic_orbits = self.basic_orbits m = len(base) v = [0]*m for i in range(m): rank, c = divmod(rank, len(transversals[i])) v[i] = basic_orbits[i][c] a = [transversals[i][v[i]]._array_form for i in range(m)] h = _af_rmuln(*a) if af: return h else: return _af_new(h) @property def degree(self): """Returns the size of the permutations in the group. Explanation =========== The number of permutations comprising the group is given by ``len(group)``; the number of permutations that can be generated by the group is given by ``group.order()``. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([1, 0, 2]) >>> G = PermutationGroup([a]) >>> G.degree 3 >>> len(G) 1 >>> G.order() 2 >>> list(G.generate()) [(2), (2)(0 1)] See Also ======== order """ return self._degree @property def identity(self): ''' Return the identity element of the permutation group. ''' return _af_new(list(range(self.degree))) @property def elements(self): """Returns all the elements of the permutation group as a set Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> p = PermutationGroup(Permutation(1, 3), Permutation(1, 2)) >>> p.elements {(1 2 3), (1 3 2), (1 3), (2 3), (3), (3)(1 2)} """ return set(self._elements) @property def _elements(self): """Returns all the elements of the permutation group as a list Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> p = PermutationGroup(Permutation(1, 3), Permutation(1, 2)) >>> p._elements [(3), (3)(1 2), (1 3), (2 3), (1 2 3), (1 3 2)] """ return list(islice(self.generate(), None)) def derived_series(self): r"""Return the derived series for the group. Explanation =========== The derived series for a group `G` is defined as `G = G_0 > G_1 > G_2 > \ldots` where `G_i = [G_{i-1}, G_{i-1}]`, i.e. `G_i` is the derived subgroup of `G_{i-1}`, for `i\in\mathbb{N}`. When we have `G_k = G_{k-1}` for some `k\in\mathbb{N}`, the series terminates. Returns ======= A list of permutation groups containing the members of the derived series in the order `G = G_0, G_1, G_2, \ldots`. Examples ======== >>> from sympy.combinatorics.named_groups import (SymmetricGroup, ... AlternatingGroup, DihedralGroup) >>> A = AlternatingGroup(5) >>> len(A.derived_series()) 1 >>> S = SymmetricGroup(4) >>> len(S.derived_series()) 4 >>> S.derived_series()[1].is_subgroup(AlternatingGroup(4)) True >>> S.derived_series()[2].is_subgroup(DihedralGroup(2)) True See Also ======== derived_subgroup """ res = [self] current = self nxt = self.derived_subgroup() while not current.is_subgroup(nxt): res.append(nxt) current = nxt nxt = nxt.derived_subgroup() return res def derived_subgroup(self): r"""Compute the derived subgroup. Explanation =========== The derived subgroup, or commutator subgroup is the subgroup generated by all commutators `[g, h] = hgh^{-1}g^{-1}` for `g, h\in G` ; it is equal to the normal closure of the set of commutators of the generators ([1], p.28, [11]). Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([1, 0, 2, 4, 3]) >>> b = Permutation([0, 1, 3, 2, 4]) >>> G = PermutationGroup([a, b]) >>> C = G.derived_subgroup() >>> list(C.generate(af=True)) [[0, 1, 2, 3, 4], [0, 1, 3, 4, 2], [0, 1, 4, 2, 3]] See Also ======== derived_series """ r = self._r gens = [p._array_form for p in self.generators] set_commutators = set() degree = self._degree rng = list(range(degree)) for i in range(r): for j in range(r): p1 = gens[i] p2 = gens[j] c = list(range(degree)) for k in rng: c[p2[p1[k]]] = p1[p2[k]] ct = tuple(c) if ct not in set_commutators: set_commutators.add(ct) cms = [_af_new(p) for p in set_commutators] G2 = self.normal_closure(cms) return G2 def generate(self, method="coset", af=False): """Return iterator to generate the elements of the group. Explanation =========== Iteration is done with one of these methods:: method='coset' using the Schreier-Sims coset representation method='dimino' using the Dimino method If ``af = True`` it yields the array form of the permutations Examples ======== >>> from sympy.combinatorics import PermutationGroup >>> from sympy.combinatorics.polyhedron import tetrahedron The permutation group given in the tetrahedron object is also true groups: >>> G = tetrahedron.pgroup >>> G.is_group True Also the group generated by the permutations in the tetrahedron pgroup -- even the first two -- is a proper group: >>> H = PermutationGroup(G[0], G[1]) >>> J = PermutationGroup(list(H.generate())); J PermutationGroup([ (0 1)(2 3), (1 2 3), (1 3 2), (0 3 1), (0 2 3), (0 3)(1 2), (0 1 3), (3)(0 2 1), (0 3 2), (3)(0 1 2), (0 2)(1 3)]) >>> _.is_group True """ if method == "coset": return self.generate_schreier_sims(af) elif method == "dimino": return self.generate_dimino(af) else: raise NotImplementedError('No generation defined for %s' % method) def generate_dimino(self, af=False): """Yield group elements using Dimino's algorithm. If ``af == True`` it yields the array form of the permutations. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1, 3]) >>> b = Permutation([0, 2, 3, 1]) >>> g = PermutationGroup([a, b]) >>> list(g.generate_dimino(af=True)) [[0, 1, 2, 3], [0, 2, 1, 3], [0, 2, 3, 1], [0, 1, 3, 2], [0, 3, 2, 1], [0, 3, 1, 2]] References ========== .. [1] The Implementation of Various Algorithms for Permutation Groups in the Computer Algebra System: AXIOM, N.J. Doye, M.Sc. Thesis """ idn = list(range(self.degree)) order = 0 element_list = [idn] set_element_list = {tuple(idn)} if af: yield idn else: yield _af_new(idn) gens = [p._array_form for p in self.generators] for i in range(len(gens)): # D elements of the subgroup G_i generated by gens[:i] D = element_list[:] N = [idn] while N: A = N N = [] for a in A: for g in gens[:i + 1]: ag = _af_rmul(a, g) if tuple(ag) not in set_element_list: # produce G_i*g for d in D: order += 1 ap = _af_rmul(d, ag) if af: yield ap else: p = _af_new(ap) yield p element_list.append(ap) set_element_list.add(tuple(ap)) N.append(ap) self._order = len(element_list) def generate_schreier_sims(self, af=False): """Yield group elements using the Schreier-Sims representation in coset_rank order If ``af = True`` it yields the array form of the permutations Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1, 3]) >>> b = Permutation([0, 2, 3, 1]) >>> g = PermutationGroup([a, b]) >>> list(g.generate_schreier_sims(af=True)) [[0, 1, 2, 3], [0, 2, 1, 3], [0, 3, 2, 1], [0, 1, 3, 2], [0, 2, 3, 1], [0, 3, 1, 2]] """ n = self._degree u = self.basic_transversals basic_orbits = self._basic_orbits if len(u) == 0: for x in self.generators: if af: yield x._array_form else: yield x return if len(u) == 1: for i in basic_orbits[0]: if af: yield u[0][i]._array_form else: yield u[0][i] return u = list(reversed(u)) basic_orbits = basic_orbits[::-1] # stg stack of group elements stg = [list(range(n))] posmax = [len(x) for x in u] n1 = len(posmax) - 1 pos = [0]*n1 h = 0 while 1: # backtrack when finished iterating over coset if pos[h] >= posmax[h]: if h == 0: return pos[h] = 0 h -= 1 stg.pop() continue p = _af_rmul(u[h][basic_orbits[h][pos[h]]]._array_form, stg[-1]) pos[h] += 1 stg.append(p) h += 1 if h == n1: if af: for i in basic_orbits[-1]: p = _af_rmul(u[-1][i]._array_form, stg[-1]) yield p else: for i in basic_orbits[-1]: p = _af_rmul(u[-1][i]._array_form, stg[-1]) p1 = _af_new(p) yield p1 stg.pop() h -= 1 @property def generators(self): """Returns the generators of the group. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1]) >>> b = Permutation([1, 0, 2]) >>> G = PermutationGroup([a, b]) >>> G.generators [(1 2), (2)(0 1)] """ return self._generators def contains(self, g, strict=True): """Test if permutation ``g`` belong to self, ``G``. Explanation =========== If ``g`` is an element of ``G`` it can be written as a product of factors drawn from the cosets of ``G``'s stabilizers. To see if ``g`` is one of the actual generators defining the group use ``G.has(g)``. If ``strict`` is not ``True``, ``g`` will be resized, if necessary, to match the size of permutations in ``self``. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation(1, 2) >>> b = Permutation(2, 3, 1) >>> G = PermutationGroup(a, b, degree=5) >>> G.contains(G[0]) # trivial check True >>> elem = Permutation([[2, 3]], size=5) >>> G.contains(elem) True >>> G.contains(Permutation(4)(0, 1, 2, 3)) False If strict is False, a permutation will be resized, if necessary: >>> H = PermutationGroup(Permutation(5)) >>> H.contains(Permutation(3)) False >>> H.contains(Permutation(3), strict=False) True To test if a given permutation is present in the group: >>> elem in G.generators False >>> G.has(elem) False See Also ======== coset_factor, sympy.core.basic.Basic.has, __contains__ """ if not isinstance(g, Permutation): return False if g.size != self.degree: if strict: return False g = Permutation(g, size=self.degree) if g in self.generators: return True return bool(self.coset_factor(g.array_form, True)) @property def is_perfect(self): """Return ``True`` if the group is perfect. A group is perfect if it equals to its derived subgroup. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation(1,2,3)(4,5) >>> b = Permutation(1,2,3,4,5) >>> G = PermutationGroup([a, b]) >>> G.is_perfect False """ if self._is_perfect is None: self._is_perfect = self.equals(self.derived_subgroup()) return self._is_perfect @property def is_abelian(self): """Test if the group is Abelian. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1]) >>> b = Permutation([1, 0, 2]) >>> G = PermutationGroup([a, b]) >>> G.is_abelian False >>> a = Permutation([0, 2, 1]) >>> G = PermutationGroup([a]) >>> G.is_abelian True """ if self._is_abelian is not None: return self._is_abelian self._is_abelian = True gens = [p._array_form for p in self.generators] for x in gens: for y in gens: if y <= x: continue if not _af_commutes_with(x, y): self._is_abelian = False return False return True def abelian_invariants(self): """ Returns the abelian invariants for the given group. Let ``G`` be a nontrivial finite abelian group. Then G is isomorphic to the direct product of finitely many nontrivial cyclic groups of prime-power order. Explanation =========== The prime-powers that occur as the orders of the factors are uniquely determined by G. More precisely, the primes that occur in the orders of the factors in any such decomposition of ``G`` are exactly the primes that divide ``|G|`` and for any such prime ``p``, if the orders of the factors that are p-groups in one such decomposition of ``G`` are ``p^{t_1} >= p^{t_2} >= ... p^{t_r}``, then the orders of the factors that are p-groups in any such decomposition of ``G`` are ``p^{t_1} >= p^{t_2} >= ... p^{t_r}``. The uniquely determined integers ``p^{t_1} >= p^{t_2} >= ... p^{t_r}``, taken for all primes that divide ``|G|`` are called the invariants of the nontrivial group ``G`` as suggested in ([14], p. 542). Notes ===== We adopt the convention that the invariants of a trivial group are []. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1]) >>> b = Permutation([1, 0, 2]) >>> G = PermutationGroup([a, b]) >>> G.abelian_invariants() [2] >>> from sympy.combinatorics import CyclicGroup >>> G = CyclicGroup(7) >>> G.abelian_invariants() [7] """ if self.is_trivial: return [] gns = self.generators inv = [] G = self H = G.derived_subgroup() Hgens = H.generators for p in primefactors(G.order()): ranks = [] while True: pows = [] for g in gns: elm = g**p if not H.contains(elm): pows.append(elm) K = PermutationGroup(Hgens + pows) if pows else H r = G.order()//K.order() G = K gns = pows if r == 1: break ranks.append(multiplicity(p, r)) if ranks: pows = [1]*ranks[0] for i in ranks: for j in range(i): pows[j] = pows[j]*p inv.extend(pows) inv.sort() return inv def is_elementary(self, p): """Return ``True`` if the group is elementary abelian. An elementary abelian group is a finite abelian group, where every nontrivial element has order `p`, where `p` is a prime. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1]) >>> G = PermutationGroup([a]) >>> G.is_elementary(2) True >>> a = Permutation([0, 2, 1, 3]) >>> b = Permutation([3, 1, 2, 0]) >>> G = PermutationGroup([a, b]) >>> G.is_elementary(2) True >>> G.is_elementary(3) False """ return self.is_abelian and all(g.order() == p for g in self.generators) def _eval_is_alt_sym_naive(self, only_sym=False, only_alt=False): """A naive test using the group order.""" if only_sym and only_alt: raise ValueError( "Both {} and {} cannot be set to True" .format(only_sym, only_alt)) n = self.degree sym_order = _factorial(n) order = self.order() if order == sym_order: self._is_sym = True self._is_alt = False if only_alt: return False return True elif 2*order == sym_order: self._is_sym = False self._is_alt = True if only_sym: return False return True return False def _eval_is_alt_sym_monte_carlo(self, eps=0.05, perms=None): """A test using monte-carlo algorithm. Parameters ========== eps : float, optional The criterion for the incorrect ``False`` return. perms : list[Permutation], optional If explicitly given, it tests over the given candidates for testing. If ``None``, it randomly computes ``N_eps`` and chooses ``N_eps`` sample of the permutation from the group. See Also ======== _check_cycles_alt_sym """ if perms is None: n = self.degree if n < 17: c_n = 0.34 else: c_n = 0.57 d_n = (c_n*log(2))/log(n) N_eps = int(-log(eps)/d_n) perms = (self.random_pr() for i in range(N_eps)) return self._eval_is_alt_sym_monte_carlo(perms=perms) for perm in perms: if _check_cycles_alt_sym(perm): return True return False def is_alt_sym(self, eps=0.05, _random_prec=None): r"""Monte Carlo test for the symmetric/alternating group for degrees >= 8. Explanation =========== More specifically, it is one-sided Monte Carlo with the answer True (i.e., G is symmetric/alternating) guaranteed to be correct, and the answer False being incorrect with probability eps. For degree < 8, the order of the group is checked so the test is deterministic. Notes ===== The algorithm itself uses some nontrivial results from group theory and number theory: 1) If a transitive group ``G`` of degree ``n`` contains an element with a cycle of length ``n/2 < p < n-2`` for ``p`` a prime, ``G`` is the symmetric or alternating group ([1], pp. 81-82) 2) The proportion of elements in the symmetric/alternating group having the property described in 1) is approximately `\log(2)/\log(n)` ([1], p.82; [2], pp. 226-227). The helper function ``_check_cycles_alt_sym`` is used to go over the cycles in a permutation and look for ones satisfying 1). Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> D = DihedralGroup(10) >>> D.is_alt_sym() False See Also ======== _check_cycles_alt_sym """ if _random_prec is not None: N_eps = _random_prec['N_eps'] perms= (_random_prec[i] for i in range(N_eps)) return self._eval_is_alt_sym_monte_carlo(perms=perms) if self._is_sym or self._is_alt: return True if self._is_sym is False and self._is_alt is False: return False n = self.degree if n < 8: return self._eval_is_alt_sym_naive() elif self.is_transitive(): return self._eval_is_alt_sym_monte_carlo(eps=eps) self._is_sym, self._is_alt = False, False return False @property def is_nilpotent(self): """Test if the group is nilpotent. Explanation =========== A group `G` is nilpotent if it has a central series of finite length. Alternatively, `G` is nilpotent if its lower central series terminates with the trivial group. Every nilpotent group is also solvable ([1], p.29, [12]). Examples ======== >>> from sympy.combinatorics.named_groups import (SymmetricGroup, ... CyclicGroup) >>> C = CyclicGroup(6) >>> C.is_nilpotent True >>> S = SymmetricGroup(5) >>> S.is_nilpotent False See Also ======== lower_central_series, is_solvable """ if self._is_nilpotent is None: lcs = self.lower_central_series() terminator = lcs[len(lcs) - 1] gens = terminator.generators degree = self.degree identity = _af_new(list(range(degree))) if all(g == identity for g in gens): self._is_solvable = True self._is_nilpotent = True return True else: self._is_nilpotent = False return False else: return self._is_nilpotent def is_normal(self, gr, strict=True): """Test if ``G=self`` is a normal subgroup of ``gr``. Explanation =========== G is normal in gr if for each g2 in G, g1 in gr, ``g = g1*g2*g1**-1`` belongs to G It is sufficient to check this for each g1 in gr.generators and g2 in G.generators. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([1, 2, 0]) >>> b = Permutation([1, 0, 2]) >>> G = PermutationGroup([a, b]) >>> G1 = PermutationGroup([a, Permutation([2, 0, 1])]) >>> G1.is_normal(G) True """ if not self.is_subgroup(gr, strict=strict): return False d_self = self.degree d_gr = gr.degree if self.is_trivial and (d_self == d_gr or not strict): return True if self._is_abelian: return True new_self = self.copy() if not strict and d_self != d_gr: if d_self < d_gr: new_self = PermGroup(new_self.generators + [Permutation(d_gr - 1)]) else: gr = PermGroup(gr.generators + [Permutation(d_self - 1)]) gens2 = [p._array_form for p in new_self.generators] gens1 = [p._array_form for p in gr.generators] for g1 in gens1: for g2 in gens2: p = _af_rmuln(g1, g2, _af_invert(g1)) if not new_self.coset_factor(p, True): return False return True def is_primitive(self, randomized=True): r"""Test if a group is primitive. Explanation =========== A permutation group ``G`` acting on a set ``S`` is called primitive if ``S`` contains no nontrivial block under the action of ``G`` (a block is nontrivial if its cardinality is more than ``1``). Notes ===== The algorithm is described in [1], p.83, and uses the function minimal_block to search for blocks of the form `\{0, k\}` for ``k`` ranging over representatives for the orbits of `G_0`, the stabilizer of ``0``. This algorithm has complexity `O(n^2)` where ``n`` is the degree of the group, and will perform badly if `G_0` is small. There are two implementations offered: one finds `G_0` deterministically using the function ``stabilizer``, and the other (default) produces random elements of `G_0` using ``random_stab``, hoping that they generate a subgroup of `G_0` with not too many more orbits than `G_0` (this is suggested in [1], p.83). Behavior is changed by the ``randomized`` flag. Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> D = DihedralGroup(10) >>> D.is_primitive() False See Also ======== minimal_block, random_stab """ if self._is_primitive is not None: return self._is_primitive if self.is_transitive() is False: return False if randomized: random_stab_gens = [] v = self.schreier_vector(0) for _ in range(len(self)): random_stab_gens.append(self.random_stab(0, v)) stab = PermutationGroup(random_stab_gens) else: stab = self.stabilizer(0) orbits = stab.orbits() for orb in orbits: x = orb.pop() if x != 0 and any(e != 0 for e in self.minimal_block([0, x])): self._is_primitive = False return False self._is_primitive = True return True def minimal_blocks(self, randomized=True): ''' For a transitive group, return the list of all minimal block systems. If a group is intransitive, return `False`. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> from sympy.combinatorics.named_groups import DihedralGroup >>> DihedralGroup(6).minimal_blocks() [[0, 1, 0, 1, 0, 1], [0, 1, 2, 0, 1, 2]] >>> G = PermutationGroup(Permutation(1,2,5)) >>> G.minimal_blocks() False See Also ======== minimal_block, is_transitive, is_primitive ''' def _number_blocks(blocks): # number the blocks of a block system # in order and return the number of # blocks and the tuple with the # reordering n = len(blocks) appeared = {} m = 0 b = [None]*n for i in range(n): if blocks[i] not in appeared: appeared[blocks[i]] = m b[i] = m m += 1 else: b[i] = appeared[blocks[i]] return tuple(b), m if not self.is_transitive(): return False blocks = [] num_blocks = [] rep_blocks = [] if randomized: random_stab_gens = [] v = self.schreier_vector(0) for i in range(len(self)): random_stab_gens.append(self.random_stab(0, v)) stab = PermutationGroup(random_stab_gens) else: stab = self.stabilizer(0) orbits = stab.orbits() for orb in orbits: x = orb.pop() if x != 0: block = self.minimal_block([0, x]) num_block, _ = _number_blocks(block) # a representative block (containing 0) rep = {j for j in range(self.degree) if num_block[j] == 0} # check if the system is minimal with # respect to the already discovere ones minimal = True blocks_remove_mask = [False] * len(blocks) for i, r in enumerate(rep_blocks): if len(r) > len(rep) and rep.issubset(r): # i-th block system is not minimal blocks_remove_mask[i] = True elif len(r) < len(rep) and r.issubset(rep): # the system being checked is not minimal minimal = False break # remove non-minimal representative blocks blocks = [b for i, b in enumerate(blocks) if not blocks_remove_mask[i]] num_blocks = [n for i, n in enumerate(num_blocks) if not blocks_remove_mask[i]] rep_blocks = [r for i, r in enumerate(rep_blocks) if not blocks_remove_mask[i]] if minimal and num_block not in num_blocks: blocks.append(block) num_blocks.append(num_block) rep_blocks.append(rep) return blocks @property def is_solvable(self): """Test if the group is solvable. ``G`` is solvable if its derived series terminates with the trivial group ([1], p.29). Examples ======== >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> S = SymmetricGroup(3) >>> S.is_solvable True See Also ======== is_nilpotent, derived_series """ if self._is_solvable is None: if self.order() % 2 != 0: return True ds = self.derived_series() terminator = ds[len(ds) - 1] gens = terminator.generators degree = self.degree identity = _af_new(list(range(degree))) if all(g == identity for g in gens): self._is_solvable = True return True else: self._is_solvable = False return False else: return self._is_solvable def is_subgroup(self, G, strict=True): """Return ``True`` if all elements of ``self`` belong to ``G``. If ``strict`` is ``False`` then if ``self``'s degree is smaller than ``G``'s, the elements will be resized to have the same degree. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> from sympy.combinatorics import SymmetricGroup, CyclicGroup Testing is strict by default: the degree of each group must be the same: >>> p = Permutation(0, 1, 2, 3, 4, 5) >>> G1 = PermutationGroup([Permutation(0, 1, 2), Permutation(0, 1)]) >>> G2 = PermutationGroup([Permutation(0, 2), Permutation(0, 1, 2)]) >>> G3 = PermutationGroup([p, p**2]) >>> assert G1.order() == G2.order() == G3.order() == 6 >>> G1.is_subgroup(G2) True >>> G1.is_subgroup(G3) False >>> G3.is_subgroup(PermutationGroup(G3[1])) False >>> G3.is_subgroup(PermutationGroup(G3[0])) True To ignore the size, set ``strict`` to ``False``: >>> S3 = SymmetricGroup(3) >>> S5 = SymmetricGroup(5) >>> S3.is_subgroup(S5, strict=False) True >>> C7 = CyclicGroup(7) >>> G = S5*C7 >>> S5.is_subgroup(G, False) True >>> C7.is_subgroup(G, 0) False """ if isinstance(G, SymmetricPermutationGroup): if self.degree != G.degree: return False return True if not isinstance(G, PermutationGroup): return False if self == G or self.generators[0]==Permutation(): return True if G.order() % self.order() != 0: return False if self.degree == G.degree or \ (self.degree < G.degree and not strict): gens = self.generators else: return False return all(G.contains(g, strict=strict) for g in gens) @property def is_polycyclic(self): """Return ``True`` if a group is polycyclic. A group is polycyclic if it has a subnormal series with cyclic factors. For finite groups, this is the same as if the group is solvable. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1, 3]) >>> b = Permutation([2, 0, 1, 3]) >>> G = PermutationGroup([a, b]) >>> G.is_polycyclic True """ return self.is_solvable def is_transitive(self, strict=True): """Test if the group is transitive. Explanation =========== A group is transitive if it has a single orbit. If ``strict`` is ``False`` the group is transitive if it has a single orbit of length different from 1. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1, 3]) >>> b = Permutation([2, 0, 1, 3]) >>> G1 = PermutationGroup([a, b]) >>> G1.is_transitive() False >>> G1.is_transitive(strict=False) True >>> c = Permutation([2, 3, 0, 1]) >>> G2 = PermutationGroup([a, c]) >>> G2.is_transitive() True >>> d = Permutation([1, 0, 2, 3]) >>> e = Permutation([0, 1, 3, 2]) >>> G3 = PermutationGroup([d, e]) >>> G3.is_transitive() or G3.is_transitive(strict=False) False """ if self._is_transitive: # strict or not, if True then True return self._is_transitive if strict: if self._is_transitive is not None: # we only store strict=True return self._is_transitive ans = len(self.orbit(0)) == self.degree self._is_transitive = ans return ans got_orb = False for x in self.orbits(): if len(x) > 1: if got_orb: return False got_orb = True return got_orb @property def is_trivial(self): """Test if the group is the trivial group. This is true if the group contains only the identity permutation. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> G = PermutationGroup([Permutation([0, 1, 2])]) >>> G.is_trivial True """ if self._is_trivial is None: self._is_trivial = len(self) == 1 and self[0].is_Identity return self._is_trivial def lower_central_series(self): r"""Return the lower central series for the group. The lower central series for a group `G` is the series `G = G_0 > G_1 > G_2 > \ldots` where `G_k = [G, G_{k-1}]`, i.e. every term after the first is equal to the commutator of `G` and the previous term in `G1` ([1], p.29). Returns ======= A list of permutation groups in the order `G = G_0, G_1, G_2, \ldots` Examples ======== >>> from sympy.combinatorics.named_groups import (AlternatingGroup, ... DihedralGroup) >>> A = AlternatingGroup(4) >>> len(A.lower_central_series()) 2 >>> A.lower_central_series()[1].is_subgroup(DihedralGroup(2)) True See Also ======== commutator, derived_series """ res = [self] current = self nxt = self.commutator(self, current) while not current.is_subgroup(nxt): res.append(nxt) current = nxt nxt = self.commutator(self, current) return res @property def max_div(self): """Maximum proper divisor of the degree of a permutation group. Explanation =========== Obviously, this is the degree divided by its minimal proper divisor (larger than ``1``, if one exists). As it is guaranteed to be prime, the ``sieve`` from ``sympy.ntheory`` is used. This function is also used as an optimization tool for the functions ``minimal_block`` and ``_union_find_merge``. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> G = PermutationGroup([Permutation([0, 2, 1, 3])]) >>> G.max_div 2 See Also ======== minimal_block, _union_find_merge """ if self._max_div is not None: return self._max_div n = self.degree if n == 1: return 1 for x in sieve: if n % x == 0: d = n//x self._max_div = d return d def minimal_block(self, points): r"""For a transitive group, finds the block system generated by ``points``. Explanation =========== If a group ``G`` acts on a set ``S``, a nonempty subset ``B`` of ``S`` is called a block under the action of ``G`` if for all ``g`` in ``G`` we have ``gB = B`` (``g`` fixes ``B``) or ``gB`` and ``B`` have no common points (``g`` moves ``B`` entirely). ([1], p.23; [6]). The distinct translates ``gB`` of a block ``B`` for ``g`` in ``G`` partition the set ``S`` and this set of translates is known as a block system. Moreover, we obviously have that all blocks in the partition have the same size, hence the block size divides ``|S|`` ([1], p.23). A ``G``-congruence is an equivalence relation ``~`` on the set ``S`` such that ``a ~ b`` implies ``g(a) ~ g(b)`` for all ``g`` in ``G``. For a transitive group, the equivalence classes of a ``G``-congruence and the blocks of a block system are the same thing ([1], p.23). The algorithm below checks the group for transitivity, and then finds the ``G``-congruence generated by the pairs ``(p_0, p_1), (p_0, p_2), ..., (p_0,p_{k-1})`` which is the same as finding the maximal block system (i.e., the one with minimum block size) such that ``p_0, ..., p_{k-1}`` are in the same block ([1], p.83). It is an implementation of Atkinson's algorithm, as suggested in [1], and manipulates an equivalence relation on the set ``S`` using a union-find data structure. The running time is just above `O(|points||S|)`. ([1], pp. 83-87; [7]). Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> D = DihedralGroup(10) >>> D.minimal_block([0, 5]) [0, 1, 2, 3, 4, 0, 1, 2, 3, 4] >>> D.minimal_block([0, 1]) [0, 0, 0, 0, 0, 0, 0, 0, 0, 0] See Also ======== _union_find_rep, _union_find_merge, is_transitive, is_primitive """ if not self.is_transitive(): return False n = self.degree gens = self.generators # initialize the list of equivalence class representatives parents = list(range(n)) ranks = [1]*n not_rep = [] k = len(points) # the block size must divide the degree of the group if k > self.max_div: return [0]*n for i in range(k - 1): parents[points[i + 1]] = points[0] not_rep.append(points[i + 1]) ranks[points[0]] = k i = 0 len_not_rep = k - 1 while i < len_not_rep: gamma = not_rep[i] i += 1 for gen in gens: # find has side effects: performs path compression on the list # of representatives delta = self._union_find_rep(gamma, parents) # union has side effects: performs union by rank on the list # of representatives temp = self._union_find_merge(gen(gamma), gen(delta), ranks, parents, not_rep) if temp == -1: return [0]*n len_not_rep += temp for i in range(n): # force path compression to get the final state of the equivalence # relation self._union_find_rep(i, parents) # rewrite result so that block representatives are minimal new_reps = {} return [new_reps.setdefault(r, i) for i, r in enumerate(parents)] def conjugacy_class(self, x): r"""Return the conjugacy class of an element in the group. Explanation =========== The conjugacy class of an element ``g`` in a group ``G`` is the set of elements ``x`` in ``G`` that are conjugate with ``g``, i.e. for which ``g = xax^{-1}`` for some ``a`` in ``G``. Note that conjugacy is an equivalence relation, and therefore that conjugacy classes are partitions of ``G``. For a list of all the conjugacy classes of the group, use the conjugacy_classes() method. In a permutation group, each conjugacy class corresponds to a particular `cycle structure': for example, in ``S_3``, the conjugacy classes are: * the identity class, ``{()}`` * all transpositions, ``{(1 2), (1 3), (2 3)}`` * all 3-cycles, ``{(1 2 3), (1 3 2)}`` Examples ======== >>> from sympy.combinatorics import Permutation, SymmetricGroup >>> S3 = SymmetricGroup(3) >>> S3.conjugacy_class(Permutation(0, 1, 2)) {(0 1 2), (0 2 1)} Notes ===== This procedure computes the conjugacy class directly by finding the orbit of the element under conjugation in G. This algorithm is only feasible for permutation groups of relatively small order, but is like the orbit() function itself in that respect. """ # Ref: "Computing the conjugacy classes of finite groups"; Butler, G. # Groups '93 Galway/St Andrews; edited by Campbell, C. M. new_class = {x} last_iteration = new_class while len(last_iteration) > 0: this_iteration = set() for y in last_iteration: for s in self.generators: conjugated = s * y * (~s) if conjugated not in new_class: this_iteration.add(conjugated) new_class.update(last_iteration) last_iteration = this_iteration return new_class def conjugacy_classes(self): r"""Return the conjugacy classes of the group. Explanation =========== As described in the documentation for the .conjugacy_class() function, conjugacy is an equivalence relation on a group G which partitions the set of elements. This method returns a list of all these conjugacy classes of G. Examples ======== >>> from sympy.combinatorics import SymmetricGroup >>> SymmetricGroup(3).conjugacy_classes() [{(2)}, {(0 1 2), (0 2 1)}, {(0 2), (1 2), (2)(0 1)}] """ identity = _af_new(list(range(self.degree))) known_elements = {identity} classes = [known_elements.copy()] for x in self.generate(): if x not in known_elements: new_class = self.conjugacy_class(x) classes.append(new_class) known_elements.update(new_class) return classes def normal_closure(self, other, k=10): r"""Return the normal closure of a subgroup/set of permutations. Explanation =========== If ``S`` is a subset of a group ``G``, the normal closure of ``A`` in ``G`` is defined as the intersection of all normal subgroups of ``G`` that contain ``A`` ([1], p.14). Alternatively, it is the group generated by the conjugates ``x^{-1}yx`` for ``x`` a generator of ``G`` and ``y`` a generator of the subgroup ``\left\langle S\right\rangle`` generated by ``S`` (for some chosen generating set for ``\left\langle S\right\rangle``) ([1], p.73). Parameters ========== other a subgroup/list of permutations/single permutation k an implementation-specific parameter that determines the number of conjugates that are adjoined to ``other`` at once Examples ======== >>> from sympy.combinatorics.named_groups import (SymmetricGroup, ... CyclicGroup, AlternatingGroup) >>> S = SymmetricGroup(5) >>> C = CyclicGroup(5) >>> G = S.normal_closure(C) >>> G.order() 60 >>> G.is_subgroup(AlternatingGroup(5)) True See Also ======== commutator, derived_subgroup, random_pr Notes ===== The algorithm is described in [1], pp. 73-74; it makes use of the generation of random elements for permutation groups by the product replacement algorithm. """ if hasattr(other, 'generators'): degree = self.degree identity = _af_new(list(range(degree))) if all(g == identity for g in other.generators): return other Z = PermutationGroup(other.generators[:]) base, strong_gens = Z.schreier_sims_incremental() strong_gens_distr = _distribute_gens_by_base(base, strong_gens) basic_orbits, basic_transversals = \ _orbits_transversals_from_bsgs(base, strong_gens_distr) self._random_pr_init(r=10, n=20) _loop = True while _loop: Z._random_pr_init(r=10, n=10) for _ in range(k): g = self.random_pr() h = Z.random_pr() conj = h^g res = _strip(conj, base, basic_orbits, basic_transversals) if res[0] != identity or res[1] != len(base) + 1: gens = Z.generators gens.append(conj) Z = PermutationGroup(gens) strong_gens.append(conj) temp_base, temp_strong_gens = \ Z.schreier_sims_incremental(base, strong_gens) base, strong_gens = temp_base, temp_strong_gens strong_gens_distr = \ _distribute_gens_by_base(base, strong_gens) basic_orbits, basic_transversals = \ _orbits_transversals_from_bsgs(base, strong_gens_distr) _loop = False for g in self.generators: for h in Z.generators: conj = h^g res = _strip(conj, base, basic_orbits, basic_transversals) if res[0] != identity or res[1] != len(base) + 1: _loop = True break if _loop: break return Z elif hasattr(other, '__getitem__'): return self.normal_closure(PermutationGroup(other)) elif hasattr(other, 'array_form'): return self.normal_closure(PermutationGroup([other])) def orbit(self, alpha, action='tuples'): r"""Compute the orbit of alpha `\{g(\alpha) | g \in G\}` as a set. Explanation =========== The time complexity of the algorithm used here is `O(|Orb|*r)` where `|Orb|` is the size of the orbit and ``r`` is the number of generators of the group. For a more detailed analysis, see [1], p.78, [2], pp. 19-21. Here alpha can be a single point, or a list of points. If alpha is a single point, the ordinary orbit is computed. if alpha is a list of points, there are three available options: 'union' - computes the union of the orbits of the points in the list 'tuples' - computes the orbit of the list interpreted as an ordered tuple under the group action ( i.e., g((1,2,3)) = (g(1), g(2), g(3)) ) 'sets' - computes the orbit of the list interpreted as a sets Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([1, 2, 0, 4, 5, 6, 3]) >>> G = PermutationGroup([a]) >>> G.orbit(0) {0, 1, 2} >>> G.orbit([0, 4], 'union') {0, 1, 2, 3, 4, 5, 6} See Also ======== orbit_transversal """ return _orbit(self.degree, self.generators, alpha, action) def orbit_rep(self, alpha, beta, schreier_vector=None): """Return a group element which sends ``alpha`` to ``beta``. Explanation =========== If ``beta`` is not in the orbit of ``alpha``, the function returns ``False``. This implementation makes use of the schreier vector. For a proof of correctness, see [1], p.80 Examples ======== >>> from sympy.combinatorics.named_groups import AlternatingGroup >>> G = AlternatingGroup(5) >>> G.orbit_rep(0, 4) (0 4 1 2 3) See Also ======== schreier_vector """ if schreier_vector is None: schreier_vector = self.schreier_vector(alpha) if schreier_vector[beta] is None: return False k = schreier_vector[beta] gens = [x._array_form for x in self.generators] a = [] while k != -1: a.append(gens[k]) beta = gens[k].index(beta) # beta = (~gens[k])(beta) k = schreier_vector[beta] if a: return _af_new(_af_rmuln(*a)) else: return _af_new(list(range(self._degree))) def orbit_transversal(self, alpha, pairs=False): r"""Computes a transversal for the orbit of ``alpha`` as a set. Explanation =========== For a permutation group `G`, a transversal for the orbit `Orb = \{g(\alpha) | g \in G\}` is a set `\{g_\beta | g_\beta(\alpha) = \beta\}` for `\beta \in Orb`. Note that there may be more than one possible transversal. If ``pairs`` is set to ``True``, it returns the list of pairs `(\beta, g_\beta)`. For a proof of correctness, see [1], p.79 Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> G = DihedralGroup(6) >>> G.orbit_transversal(0) [(5), (0 1 2 3 4 5), (0 5)(1 4)(2 3), (0 2 4)(1 3 5), (5)(0 4)(1 3), (0 3)(1 4)(2 5)] See Also ======== orbit """ return _orbit_transversal(self._degree, self.generators, alpha, pairs) def orbits(self, rep=False): """Return the orbits of ``self``, ordered according to lowest element in each orbit. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation(1, 5)(2, 3)(4, 0, 6) >>> b = Permutation(1, 5)(3, 4)(2, 6, 0) >>> G = PermutationGroup([a, b]) >>> G.orbits() [{0, 2, 3, 4, 6}, {1, 5}] """ return _orbits(self._degree, self._generators) def order(self): """Return the order of the group: the number of permutations that can be generated from elements of the group. The number of permutations comprising the group is given by ``len(group)``; the length of each permutation in the group is given by ``group.size``. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([1, 0, 2]) >>> G = PermutationGroup([a]) >>> G.degree 3 >>> len(G) 1 >>> G.order() 2 >>> list(G.generate()) [(2), (2)(0 1)] >>> a = Permutation([0, 2, 1]) >>> b = Permutation([1, 0, 2]) >>> G = PermutationGroup([a, b]) >>> G.order() 6 See Also ======== degree """ if self._order is not None: return self._order if self._is_sym: n = self._degree self._order = factorial(n) return self._order if self._is_alt: n = self._degree self._order = factorial(n)/2 return self._order m = prod([len(x) for x in self.basic_transversals]) self._order = m return m def index(self, H): """ Returns the index of a permutation group. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation(1,2,3) >>> b =Permutation(3) >>> G = PermutationGroup([a]) >>> H = PermutationGroup([b]) >>> G.index(H) 3 """ if H.is_subgroup(self): return self.order()//H.order() @property def is_symmetric(self): """Return ``True`` if the group is symmetric. Examples ======== >>> from sympy.combinatorics import SymmetricGroup >>> g = SymmetricGroup(5) >>> g.is_symmetric True >>> from sympy.combinatorics import Permutation, PermutationGroup >>> g = PermutationGroup( ... Permutation(0, 1, 2, 3, 4), ... Permutation(2, 3)) >>> g.is_symmetric True Notes ===== This uses a naive test involving the computation of the full group order. If you need more quicker taxonomy for large groups, you can use :meth:`PermutationGroup.is_alt_sym`. However, :meth:`PermutationGroup.is_alt_sym` may not be accurate and is not able to distinguish between an alternating group and a symmetric group. See Also ======== is_alt_sym """ _is_sym = self._is_sym if _is_sym is not None: return _is_sym n = self.degree if n >= 8: if self.is_transitive(): _is_alt_sym = self._eval_is_alt_sym_monte_carlo() if _is_alt_sym: if any(g.is_odd for g in self.generators): self._is_sym, self._is_alt = True, False return True self._is_sym, self._is_alt = False, True return False return self._eval_is_alt_sym_naive(only_sym=True) self._is_sym, self._is_alt = False, False return False return self._eval_is_alt_sym_naive(only_sym=True) @property def is_alternating(self): """Return ``True`` if the group is alternating. Examples ======== >>> from sympy.combinatorics import AlternatingGroup >>> g = AlternatingGroup(5) >>> g.is_alternating True >>> from sympy.combinatorics import Permutation, PermutationGroup >>> g = PermutationGroup( ... Permutation(0, 1, 2, 3, 4), ... Permutation(2, 3, 4)) >>> g.is_alternating True Notes ===== This uses a naive test involving the computation of the full group order. If you need more quicker taxonomy for large groups, you can use :meth:`PermutationGroup.is_alt_sym`. However, :meth:`PermutationGroup.is_alt_sym` may not be accurate and is not able to distinguish between an alternating group and a symmetric group. See Also ======== is_alt_sym """ _is_alt = self._is_alt if _is_alt is not None: return _is_alt n = self.degree if n >= 8: if self.is_transitive(): _is_alt_sym = self._eval_is_alt_sym_monte_carlo() if _is_alt_sym: if all(g.is_even for g in self.generators): self._is_sym, self._is_alt = False, True return True self._is_sym, self._is_alt = True, False return False return self._eval_is_alt_sym_naive(only_alt=True) self._is_sym, self._is_alt = False, False return False return self._eval_is_alt_sym_naive(only_alt=True) @classmethod def _distinct_primes_lemma(cls, primes): """Subroutine to test if there is only one cyclic group for the order.""" primes = sorted(primes) l = len(primes) for i in range(l): for j in range(i+1, l): if primes[j] % primes[i] == 1: return None return True @property def is_cyclic(self): r""" Return ``True`` if the group is Cyclic. Examples ======== >>> from sympy.combinatorics.named_groups import AbelianGroup >>> G = AbelianGroup(3, 4) >>> G.is_cyclic True >>> G = AbelianGroup(4, 4) >>> G.is_cyclic False Notes ===== If the order of a group $n$ can be factored into the distinct primes $p_1, p_2, \dots , p_s$ and if .. math:: \forall i, j \in \{1, 2, \dots, s \}: p_i \not \equiv 1 \pmod {p_j} holds true, there is only one group of the order $n$ which is a cyclic group [1]_. This is a generalization of the lemma that the group of order $15, 35, \dots$ are cyclic. And also, these additional lemmas can be used to test if a group is cyclic if the order of the group is already found. - If the group is abelian and the order of the group is square-free, the group is cyclic. - If the order of the group is less than $6$ and is not $4$, the group is cyclic. - If the order of the group is prime, the group is cyclic. References ========== .. [1] 1978: John S. Rose: A Course on Group Theory, Introduction to Finite Group Theory: 1.4 """ if self._is_cyclic is not None: return self._is_cyclic if len(self.generators) == 1: self._is_cyclic = True self._is_abelian = True return True if self._is_abelian is False: self._is_cyclic = False return False order = self.order() if order < 6: self._is_abelian = True if order != 4: self._is_cyclic = True return True factors = factorint(order) if all(v == 1 for v in factors.values()): if self._is_abelian: self._is_cyclic = True return True primes = list(factors.keys()) if PermutationGroup._distinct_primes_lemma(primes) is True: self._is_cyclic = True self._is_abelian = True return True if not self.is_abelian: self._is_cyclic = False return False self._is_cyclic = all( any(g**(order//p) != self.identity for g in self.generators) for p, e in factors.items() if e > 1 ) return self._is_cyclic @property def is_dihedral(self): r""" Return ``True`` if the group is dihedral. Examples ======== >>> from sympy.combinatorics.perm_groups import PermutationGroup >>> from sympy.combinatorics.permutations import Permutation >>> from sympy.combinatorics.named_groups import SymmetricGroup, CyclicGroup >>> G = PermutationGroup(Permutation(1, 6)(2, 5)(3, 4), Permutation(0, 1, 2, 3, 4, 5, 6)) >>> G.is_dihedral True >>> G = SymmetricGroup(3) >>> G.is_dihedral True >>> G = CyclicGroup(6) >>> G.is_dihedral False References ========== .. [Di1] https://math.stackexchange.com/a/827273 .. [Di2] https://kconrad.math.uconn.edu/blurbs/grouptheory/dihedral.pdf .. [Di3] https://kconrad.math.uconn.edu/blurbs/grouptheory/dihedral2.pdf .. [Di4] https://en.wikipedia.org/wiki/Dihedral_group """ if self._is_dihedral is not None: return self._is_dihedral order = self.order() if order % 2 == 1: self._is_dihedral = False return False if order == 2: self._is_dihedral = True return True if order == 4: # The dihedral group of order 4 is the Klein 4-group. self._is_dihedral = not self.is_cyclic return self._is_dihedral if self.is_abelian: # The only abelian dihedral groups are the ones of orders 2 and 4. self._is_dihedral = False return False # Now we know the group is of even order >= 6, and nonabelian. n = order // 2 # Handle special cases where there are exactly two generators. gens = self.generators if len(gens) == 2: x, y = gens a, b = x.order(), y.order() # Make a >= b if a < b: x, y, a, b = y, x, b, a # Using Theorem 2.1 of [Di3]: if a == 2 == b: self._is_dihedral = True return True # Using Theorem 1.1 of [Di3]: if a == n and b == 2 and y*x*y == ~x: self._is_dihedral = True return True # Proceed with algorithm of [Di1] # Find elements of orders 2 and n order_2, order_n = [], [] for p in self.elements: k = p.order() if k == 2: order_2.append(p) elif k == n: order_n.append(p) if len(order_2) != n + 1 - (n % 2): self._is_dihedral = False return False if not order_n: self._is_dihedral = False return False x = order_n[0] # Want an element y of order 2 that is not a power of x # (i.e. that is not the 180-deg rotation, when n is even). y = order_2[0] if n % 2 == 0 and y == x**(n//2): y = order_2[1] self._is_dihedral = (y*x*y == ~x) return self._is_dihedral def pointwise_stabilizer(self, points, incremental=True): r"""Return the pointwise stabilizer for a set of points. Explanation =========== For a permutation group `G` and a set of points `\{p_1, p_2,\ldots, p_k\}`, the pointwise stabilizer of `p_1, p_2, \ldots, p_k` is defined as `G_{p_1,\ldots, p_k} = \{g\in G | g(p_i) = p_i \forall i\in\{1, 2,\ldots,k\}\}` ([1],p20). It is a subgroup of `G`. Examples ======== >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> S = SymmetricGroup(7) >>> Stab = S.pointwise_stabilizer([2, 3, 5]) >>> Stab.is_subgroup(S.stabilizer(2).stabilizer(3).stabilizer(5)) True See Also ======== stabilizer, schreier_sims_incremental Notes ===== When incremental == True, rather than the obvious implementation using successive calls to ``.stabilizer()``, this uses the incremental Schreier-Sims algorithm to obtain a base with starting segment - the given points. """ if incremental: base, strong_gens = self.schreier_sims_incremental(base=points) stab_gens = [] degree = self.degree for gen in strong_gens: if [gen(point) for point in points] == points: stab_gens.append(gen) if not stab_gens: stab_gens = _af_new(list(range(degree))) return PermutationGroup(stab_gens) else: gens = self._generators degree = self.degree for x in points: gens = _stabilizer(degree, gens, x) return PermutationGroup(gens) def make_perm(self, n, seed=None): """ Multiply ``n`` randomly selected permutations from pgroup together, starting with the identity permutation. If ``n`` is a list of integers, those integers will be used to select the permutations and they will be applied in L to R order: make_perm((A, B, C)) will give CBA(I) where I is the identity permutation. ``seed`` is used to set the seed for the random selection of permutations from pgroup. If this is a list of integers, the corresponding permutations from pgroup will be selected in the order give. This is mainly used for testing purposes. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a, b = [Permutation([1, 0, 3, 2]), Permutation([1, 3, 0, 2])] >>> G = PermutationGroup([a, b]) >>> G.make_perm(1, [0]) (0 1)(2 3) >>> G.make_perm(3, [0, 1, 0]) (0 2 3 1) >>> G.make_perm([0, 1, 0]) (0 2 3 1) See Also ======== random """ if is_sequence(n): if seed is not None: raise ValueError('If n is a sequence, seed should be None') n, seed = len(n), n else: try: n = int(n) except TypeError: raise ValueError('n must be an integer or a sequence.') randomrange = _randrange(seed) # start with the identity permutation result = Permutation(list(range(self.degree))) m = len(self) for _ in range(n): p = self[randomrange(m)] result = rmul(result, p) return result def random(self, af=False): """Return a random group element """ rank = randrange(self.order()) return self.coset_unrank(rank, af) def random_pr(self, gen_count=11, iterations=50, _random_prec=None): """Return a random group element using product replacement. Explanation =========== For the details of the product replacement algorithm, see ``_random_pr_init`` In ``random_pr`` the actual 'product replacement' is performed. Notice that if the attribute ``_random_gens`` is empty, it needs to be initialized by ``_random_pr_init``. See Also ======== _random_pr_init """ if self._random_gens == []: self._random_pr_init(gen_count, iterations) random_gens = self._random_gens r = len(random_gens) - 1 # handle randomized input for testing purposes if _random_prec is None: s = randrange(r) t = randrange(r - 1) if t == s: t = r - 1 x = choice([1, 2]) e = choice([-1, 1]) else: s = _random_prec['s'] t = _random_prec['t'] if t == s: t = r - 1 x = _random_prec['x'] e = _random_prec['e'] if x == 1: random_gens[s] = _af_rmul(random_gens[s], _af_pow(random_gens[t], e)) random_gens[r] = _af_rmul(random_gens[r], random_gens[s]) else: random_gens[s] = _af_rmul(_af_pow(random_gens[t], e), random_gens[s]) random_gens[r] = _af_rmul(random_gens[s], random_gens[r]) return _af_new(random_gens[r]) def random_stab(self, alpha, schreier_vector=None, _random_prec=None): """Random element from the stabilizer of ``alpha``. The schreier vector for ``alpha`` is an optional argument used for speeding up repeated calls. The algorithm is described in [1], p.81 See Also ======== random_pr, orbit_rep """ if schreier_vector is None: schreier_vector = self.schreier_vector(alpha) if _random_prec is None: rand = self.random_pr() else: rand = _random_prec['rand'] beta = rand(alpha) h = self.orbit_rep(alpha, beta, schreier_vector) return rmul(~h, rand) def schreier_sims(self): """Schreier-Sims algorithm. Explanation =========== It computes the generators of the chain of stabilizers `G > G_{b_1} > .. > G_{b1,..,b_r} > 1` in which `G_{b_1,..,b_i}` stabilizes `b_1,..,b_i`, and the corresponding ``s`` cosets. An element of the group can be written as the product `h_1*..*h_s`. We use the incremental Schreier-Sims algorithm. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([0, 2, 1]) >>> b = Permutation([1, 0, 2]) >>> G = PermutationGroup([a, b]) >>> G.schreier_sims() >>> G.basic_transversals [{0: (2)(0 1), 1: (2), 2: (1 2)}, {0: (2), 2: (0 2)}] """ if self._transversals: return self._schreier_sims() return def _schreier_sims(self, base=None): schreier = self.schreier_sims_incremental(base=base, slp_dict=True) base, strong_gens = schreier[:2] self._base = base self._strong_gens = strong_gens self._strong_gens_slp = schreier[2] if not base: self._transversals = [] self._basic_orbits = [] return strong_gens_distr = _distribute_gens_by_base(base, strong_gens) basic_orbits, transversals, slps = _orbits_transversals_from_bsgs(base,\ strong_gens_distr, slp=True) # rewrite the indices stored in slps in terms of strong_gens for i, slp in enumerate(slps): gens = strong_gens_distr[i] for k in slp: slp[k] = [strong_gens.index(gens[s]) for s in slp[k]] self._transversals = transversals self._basic_orbits = [sorted(x) for x in basic_orbits] self._transversal_slp = slps def schreier_sims_incremental(self, base=None, gens=None, slp_dict=False): """Extend a sequence of points and generating set to a base and strong generating set. Parameters ========== base The sequence of points to be extended to a base. Optional parameter with default value ``[]``. gens The generating set to be extended to a strong generating set relative to the base obtained. Optional parameter with default value ``self.generators``. slp_dict If `True`, return a dictionary `{g: gens}` for each strong generator `g` where `gens` is a list of strong generators coming before `g` in `strong_gens`, such that the product of the elements of `gens` is equal to `g`. Returns ======= (base, strong_gens) ``base`` is the base obtained, and ``strong_gens`` is the strong generating set relative to it. The original parameters ``base``, ``gens`` remain unchanged. Examples ======== >>> from sympy.combinatorics.named_groups import AlternatingGroup >>> from sympy.combinatorics.testutil import _verify_bsgs >>> A = AlternatingGroup(7) >>> base = [2, 3] >>> seq = [2, 3] >>> base, strong_gens = A.schreier_sims_incremental(base=seq) >>> _verify_bsgs(A, base, strong_gens) True >>> base[:2] [2, 3] Notes ===== This version of the Schreier-Sims algorithm runs in polynomial time. There are certain assumptions in the implementation - if the trivial group is provided, ``base`` and ``gens`` are returned immediately, as any sequence of points is a base for the trivial group. If the identity is present in the generators ``gens``, it is removed as it is a redundant generator. The implementation is described in [1], pp. 90-93. See Also ======== schreier_sims, schreier_sims_random """ if base is None: base = [] if gens is None: gens = self.generators[:] degree = self.degree id_af = list(range(degree)) # handle the trivial group if len(gens) == 1 and gens[0].is_Identity: if slp_dict: return base, gens, {gens[0]: [gens[0]]} return base, gens # prevent side effects _base, _gens = base[:], gens[:] # remove the identity as a generator _gens = [x for x in _gens if not x.is_Identity] # make sure no generator fixes all base points for gen in _gens: if all(x == gen._array_form[x] for x in _base): for new in id_af: if gen._array_form[new] != new: break else: assert None # can this ever happen? _base.append(new) # distribute generators according to basic stabilizers strong_gens_distr = _distribute_gens_by_base(_base, _gens) strong_gens_slp = [] # initialize the basic stabilizers, basic orbits and basic transversals orbs = {} transversals = {} slps = {} base_len = len(_base) for i in range(base_len): transversals[i], slps[i] = _orbit_transversal(degree, strong_gens_distr[i], _base[i], pairs=True, af=True, slp=True) transversals[i] = dict(transversals[i]) orbs[i] = list(transversals[i].keys()) # main loop: amend the stabilizer chain until we have generators # for all stabilizers i = base_len - 1 while i >= 0: # this flag is used to continue with the main loop from inside # a nested loop continue_i = False # test the generators for being a strong generating set db = {} for beta, u_beta in list(transversals[i].items()): for j, gen in enumerate(strong_gens_distr[i]): gb = gen._array_form[beta] u1 = transversals[i][gb] g1 = _af_rmul(gen._array_form, u_beta) slp = [(i, g) for g in slps[i][beta]] slp = [(i, j)] + slp if g1 != u1: # test if the schreier generator is in the i+1-th # would-be basic stabilizer y = True try: u1_inv = db[gb] except KeyError: u1_inv = db[gb] = _af_invert(u1) schreier_gen = _af_rmul(u1_inv, g1) u1_inv_slp = slps[i][gb][:] u1_inv_slp.reverse() u1_inv_slp = [(i, (g,)) for g in u1_inv_slp] slp = u1_inv_slp + slp h, j, slp = _strip_af(schreier_gen, _base, orbs, transversals, i, slp=slp, slps=slps) if j <= base_len: # new strong generator h at level j y = False elif h: # h fixes all base points y = False moved = 0 while h[moved] == moved: moved += 1 _base.append(moved) base_len += 1 strong_gens_distr.append([]) if y is False: # if a new strong generator is found, update the # data structures and start over h = _af_new(h) strong_gens_slp.append((h, slp)) for l in range(i + 1, j): strong_gens_distr[l].append(h) transversals[l], slps[l] =\ _orbit_transversal(degree, strong_gens_distr[l], _base[l], pairs=True, af=True, slp=True) transversals[l] = dict(transversals[l]) orbs[l] = list(transversals[l].keys()) i = j - 1 # continue main loop using the flag continue_i = True if continue_i is True: break if continue_i is True: break if continue_i is True: continue i -= 1 strong_gens = _gens[:] if slp_dict: # create the list of the strong generators strong_gens and # rewrite the indices of strong_gens_slp in terms of the # elements of strong_gens for k, slp in strong_gens_slp: strong_gens.append(k) for i in range(len(slp)): s = slp[i] if isinstance(s[1], tuple): slp[i] = strong_gens_distr[s[0]][s[1][0]]**-1 else: slp[i] = strong_gens_distr[s[0]][s[1]] strong_gens_slp = dict(strong_gens_slp) # add the original generators for g in _gens: strong_gens_slp[g] = [g] return (_base, strong_gens, strong_gens_slp) strong_gens.extend([k for k, _ in strong_gens_slp]) return _base, strong_gens def schreier_sims_random(self, base=None, gens=None, consec_succ=10, _random_prec=None): r"""Randomized Schreier-Sims algorithm. Explanation =========== The randomized Schreier-Sims algorithm takes the sequence ``base`` and the generating set ``gens``, and extends ``base`` to a base, and ``gens`` to a strong generating set relative to that base with probability of a wrong answer at most `2^{-consec\_succ}`, provided the random generators are sufficiently random. Parameters ========== base The sequence to be extended to a base. gens The generating set to be extended to a strong generating set. consec_succ The parameter defining the probability of a wrong answer. _random_prec An internal parameter used for testing purposes. Returns ======= (base, strong_gens) ``base`` is the base and ``strong_gens`` is the strong generating set relative to it. Examples ======== >>> from sympy.combinatorics.testutil import _verify_bsgs >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> S = SymmetricGroup(5) >>> base, strong_gens = S.schreier_sims_random(consec_succ=5) >>> _verify_bsgs(S, base, strong_gens) #doctest: +SKIP True Notes ===== The algorithm is described in detail in [1], pp. 97-98. It extends the orbits ``orbs`` and the permutation groups ``stabs`` to basic orbits and basic stabilizers for the base and strong generating set produced in the end. The idea of the extension process is to "sift" random group elements through the stabilizer chain and amend the stabilizers/orbits along the way when a sift is not successful. The helper function ``_strip`` is used to attempt to decompose a random group element according to the current state of the stabilizer chain and report whether the element was fully decomposed (successful sift) or not (unsuccessful sift). In the latter case, the level at which the sift failed is reported and used to amend ``stabs``, ``base``, ``gens`` and ``orbs`` accordingly. The halting condition is for ``consec_succ`` consecutive successful sifts to pass. This makes sure that the current ``base`` and ``gens`` form a BSGS with probability at least `1 - 1/\text{consec\_succ}`. See Also ======== schreier_sims """ if base is None: base = [] if gens is None: gens = self.generators base_len = len(base) n = self.degree # make sure no generator fixes all base points for gen in gens: if all(gen(x) == x for x in base): new = 0 while gen._array_form[new] == new: new += 1 base.append(new) base_len += 1 # distribute generators according to basic stabilizers strong_gens_distr = _distribute_gens_by_base(base, gens) # initialize the basic stabilizers, basic transversals and basic orbits transversals = {} orbs = {} for i in range(base_len): transversals[i] = dict(_orbit_transversal(n, strong_gens_distr[i], base[i], pairs=True)) orbs[i] = list(transversals[i].keys()) # initialize the number of consecutive elements sifted c = 0 # start sifting random elements while the number of consecutive sifts # is less than consec_succ while c < consec_succ: if _random_prec is None: g = self.random_pr() else: g = _random_prec['g'].pop() h, j = _strip(g, base, orbs, transversals) y = True # determine whether a new base point is needed if j <= base_len: y = False elif not h.is_Identity: y = False moved = 0 while h(moved) == moved: moved += 1 base.append(moved) base_len += 1 strong_gens_distr.append([]) # if the element doesn't sift, amend the strong generators and # associated stabilizers and orbits if y is False: for l in range(1, j): strong_gens_distr[l].append(h) transversals[l] = dict(_orbit_transversal(n, strong_gens_distr[l], base[l], pairs=True)) orbs[l] = list(transversals[l].keys()) c = 0 else: c += 1 # build the strong generating set strong_gens = strong_gens_distr[0][:] for gen in strong_gens_distr[1]: if gen not in strong_gens: strong_gens.append(gen) return base, strong_gens def schreier_vector(self, alpha): """Computes the schreier vector for ``alpha``. Explanation =========== The Schreier vector efficiently stores information about the orbit of ``alpha``. It can later be used to quickly obtain elements of the group that send ``alpha`` to a particular element in the orbit. Notice that the Schreier vector depends on the order in which the group generators are listed. For a definition, see [3]. Since list indices start from zero, we adopt the convention to use "None" instead of 0 to signify that an element does not belong to the orbit. For the algorithm and its correctness, see [2], pp.78-80. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([2, 4, 6, 3, 1, 5, 0]) >>> b = Permutation([0, 1, 3, 5, 4, 6, 2]) >>> G = PermutationGroup([a, b]) >>> G.schreier_vector(0) [-1, None, 0, 1, None, 1, 0] See Also ======== orbit """ n = self.degree v = [None]*n v[alpha] = -1 orb = [alpha] used = [False]*n used[alpha] = True gens = self.generators r = len(gens) for b in orb: for i in range(r): temp = gens[i]._array_form[b] if used[temp] is False: orb.append(temp) used[temp] = True v[temp] = i return v def stabilizer(self, alpha): r"""Return the stabilizer subgroup of ``alpha``. Explanation =========== The stabilizer of `\alpha` is the group `G_\alpha = \{g \in G | g(\alpha) = \alpha\}`. For a proof of correctness, see [1], p.79. Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> G = DihedralGroup(6) >>> G.stabilizer(5) PermutationGroup([ (5)(0 4)(1 3)]) See Also ======== orbit """ return PermGroup(_stabilizer(self._degree, self._generators, alpha)) @property def strong_gens(self): r"""Return a strong generating set from the Schreier-Sims algorithm. Explanation =========== A generating set `S = \{g_1, g_2, \dots, g_t\}` for a permutation group `G` is a strong generating set relative to the sequence of points (referred to as a "base") `(b_1, b_2, \dots, b_k)` if, for `1 \leq i \leq k` we have that the intersection of the pointwise stabilizer `G^{(i+1)} := G_{b_1, b_2, \dots, b_i}` with `S` generates the pointwise stabilizer `G^{(i+1)}`. The concepts of a base and strong generating set and their applications are discussed in depth in [1], pp. 87-89 and [2], pp. 55-57. Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> D = DihedralGroup(4) >>> D.strong_gens [(0 1 2 3), (0 3)(1 2), (1 3)] >>> D.base [0, 1] See Also ======== base, basic_transversals, basic_orbits, basic_stabilizers """ if self._strong_gens == []: self.schreier_sims() return self._strong_gens def subgroup(self, gens): """ Return the subgroup generated by `gens` which is a list of elements of the group """ if not all(g in self for g in gens): raise ValueError("The group does not contain the supplied generators") G = PermutationGroup(gens) return G def subgroup_search(self, prop, base=None, strong_gens=None, tests=None, init_subgroup=None): """Find the subgroup of all elements satisfying the property ``prop``. Explanation =========== This is done by a depth-first search with respect to base images that uses several tests to prune the search tree. Parameters ========== prop The property to be used. Has to be callable on group elements and always return ``True`` or ``False``. It is assumed that all group elements satisfying ``prop`` indeed form a subgroup. base A base for the supergroup. strong_gens A strong generating set for the supergroup. tests A list of callables of length equal to the length of ``base``. These are used to rule out group elements by partial base images, so that ``tests[l](g)`` returns False if the element ``g`` is known not to satisfy prop base on where g sends the first ``l + 1`` base points. init_subgroup if a subgroup of the sought group is known in advance, it can be passed to the function as this parameter. Returns ======= res The subgroup of all elements satisfying ``prop``. The generating set for this group is guaranteed to be a strong generating set relative to the base ``base``. Examples ======== >>> from sympy.combinatorics.named_groups import (SymmetricGroup, ... AlternatingGroup) >>> from sympy.combinatorics.testutil import _verify_bsgs >>> S = SymmetricGroup(7) >>> prop_even = lambda x: x.is_even >>> base, strong_gens = S.schreier_sims_incremental() >>> G = S.subgroup_search(prop_even, base=base, strong_gens=strong_gens) >>> G.is_subgroup(AlternatingGroup(7)) True >>> _verify_bsgs(G, base, G.generators) True Notes ===== This function is extremely lengthy and complicated and will require some careful attention. The implementation is described in [1], pp. 114-117, and the comments for the code here follow the lines of the pseudocode in the book for clarity. The complexity is exponential in general, since the search process by itself visits all members of the supergroup. However, there are a lot of tests which are used to prune the search tree, and users can define their own tests via the ``tests`` parameter, so in practice, and for some computations, it's not terrible. A crucial part in the procedure is the frequent base change performed (this is line 11 in the pseudocode) in order to obtain a new basic stabilizer. The book mentiones that this can be done by using ``.baseswap(...)``, however the current implementation uses a more straightforward way to find the next basic stabilizer - calling the function ``.stabilizer(...)`` on the previous basic stabilizer. """ # initialize BSGS and basic group properties def get_reps(orbits): # get the minimal element in the base ordering return [min(orbit, key = lambda x: base_ordering[x]) \ for orbit in orbits] def update_nu(l): temp_index = len(basic_orbits[l]) + 1 -\ len(res_basic_orbits_init_base[l]) # this corresponds to the element larger than all points if temp_index >= len(sorted_orbits[l]): nu[l] = base_ordering[degree] else: nu[l] = sorted_orbits[l][temp_index] if base is None: base, strong_gens = self.schreier_sims_incremental() base_len = len(base) degree = self.degree identity = _af_new(list(range(degree))) base_ordering = _base_ordering(base, degree) # add an element larger than all points base_ordering.append(degree) # add an element smaller than all points base_ordering.append(-1) # compute BSGS-related structures strong_gens_distr = _distribute_gens_by_base(base, strong_gens) basic_orbits, transversals = _orbits_transversals_from_bsgs(base, strong_gens_distr) # handle subgroup initialization and tests if init_subgroup is None: init_subgroup = PermutationGroup([identity]) if tests is None: trivial_test = lambda x: True tests = [] for i in range(base_len): tests.append(trivial_test) # line 1: more initializations. res = init_subgroup f = base_len - 1 l = base_len - 1 # line 2: set the base for K to the base for G res_base = base[:] # line 3: compute BSGS and related structures for K res_base, res_strong_gens = res.schreier_sims_incremental( base=res_base) res_strong_gens_distr = _distribute_gens_by_base(res_base, res_strong_gens) res_generators = res.generators res_basic_orbits_init_base = \ [_orbit(degree, res_strong_gens_distr[i], res_base[i])\ for i in range(base_len)] # initialize orbit representatives orbit_reps = [None]*base_len # line 4: orbit representatives for f-th basic stabilizer of K orbits = _orbits(degree, res_strong_gens_distr[f]) orbit_reps[f] = get_reps(orbits) # line 5: remove the base point from the representatives to avoid # getting the identity element as a generator for K orbit_reps[f].remove(base[f]) # line 6: more initializations c = [0]*base_len u = [identity]*base_len sorted_orbits = [None]*base_len for i in range(base_len): sorted_orbits[i] = basic_orbits[i][:] sorted_orbits[i].sort(key=lambda point: base_ordering[point]) # line 7: initializations mu = [None]*base_len nu = [None]*base_len # this corresponds to the element smaller than all points mu[l] = degree + 1 update_nu(l) # initialize computed words computed_words = [identity]*base_len # line 8: main loop while True: # apply all the tests while l < base_len - 1 and \ computed_words[l](base[l]) in orbit_reps[l] and \ base_ordering[mu[l]] < \ base_ordering[computed_words[l](base[l])] < \ base_ordering[nu[l]] and \ tests[l](computed_words): # line 11: change the (partial) base of K new_point = computed_words[l](base[l]) res_base[l] = new_point new_stab_gens = _stabilizer(degree, res_strong_gens_distr[l], new_point) res_strong_gens_distr[l + 1] = new_stab_gens # line 12: calculate minimal orbit representatives for the # l+1-th basic stabilizer orbits = _orbits(degree, new_stab_gens) orbit_reps[l + 1] = get_reps(orbits) # line 13: amend sorted orbits l += 1 temp_orbit = [computed_words[l - 1](point) for point in basic_orbits[l]] temp_orbit.sort(key=lambda point: base_ordering[point]) sorted_orbits[l] = temp_orbit # lines 14 and 15: update variables used minimality tests new_mu = degree + 1 for i in range(l): if base[l] in res_basic_orbits_init_base[i]: candidate = computed_words[i](base[i]) if base_ordering[candidate] > base_ordering[new_mu]: new_mu = candidate mu[l] = new_mu update_nu(l) # line 16: determine the new transversal element c[l] = 0 temp_point = sorted_orbits[l][c[l]] gamma = computed_words[l - 1]._array_form.index(temp_point) u[l] = transversals[l][gamma] # update computed words computed_words[l] = rmul(computed_words[l - 1], u[l]) # lines 17 & 18: apply the tests to the group element found g = computed_words[l] temp_point = g(base[l]) if l == base_len - 1 and \ base_ordering[mu[l]] < \ base_ordering[temp_point] < base_ordering[nu[l]] and \ temp_point in orbit_reps[l] and \ tests[l](computed_words) and \ prop(g): # line 19: reset the base of K res_generators.append(g) res_base = base[:] # line 20: recalculate basic orbits (and transversals) res_strong_gens.append(g) res_strong_gens_distr = _distribute_gens_by_base(res_base, res_strong_gens) res_basic_orbits_init_base = \ [_orbit(degree, res_strong_gens_distr[i], res_base[i]) \ for i in range(base_len)] # line 21: recalculate orbit representatives # line 22: reset the search depth orbit_reps[f] = get_reps(orbits) l = f # line 23: go up the tree until in the first branch not fully # searched while l >= 0 and c[l] == len(basic_orbits[l]) - 1: l = l - 1 # line 24: if the entire tree is traversed, return K if l == -1: return PermutationGroup(res_generators) # lines 25-27: update orbit representatives if l < f: # line 26 f = l c[l] = 0 # line 27 temp_orbits = _orbits(degree, res_strong_gens_distr[f]) orbit_reps[f] = get_reps(temp_orbits) # line 28: update variables used for minimality testing mu[l] = degree + 1 temp_index = len(basic_orbits[l]) + 1 - \ len(res_basic_orbits_init_base[l]) if temp_index >= len(sorted_orbits[l]): nu[l] = base_ordering[degree] else: nu[l] = sorted_orbits[l][temp_index] # line 29: set the next element from the current branch and update # accordingly c[l] += 1 if l == 0: gamma = sorted_orbits[l][c[l]] else: gamma = computed_words[l - 1]._array_form.index(sorted_orbits[l][c[l]]) u[l] = transversals[l][gamma] if l == 0: computed_words[l] = u[l] else: computed_words[l] = rmul(computed_words[l - 1], u[l]) @property def transitivity_degree(self): r"""Compute the degree of transitivity of the group. Explanation =========== A permutation group `G` acting on `\Omega = \{0, 1, \dots, n-1\}` is ``k``-fold transitive, if, for any `k` points `(a_1, a_2, \dots, a_k) \in \Omega` and any `k` points `(b_1, b_2, \dots, b_k) \in \Omega` there exists `g \in G` such that `g(a_1) = b_1, g(a_2) = b_2, \dots, g(a_k) = b_k` The degree of transitivity of `G` is the maximum ``k`` such that `G` is ``k``-fold transitive. ([8]) Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> a = Permutation([1, 2, 0]) >>> b = Permutation([1, 0, 2]) >>> G = PermutationGroup([a, b]) >>> G.transitivity_degree 3 See Also ======== is_transitive, orbit """ if self._transitivity_degree is None: n = self.degree G = self # if G is k-transitive, a tuple (a_0,..,a_k) # can be brought to (b_0,...,b_(k-1), b_k) # where b_0,...,b_(k-1) are fixed points; # consider the group G_k which stabilizes b_0,...,b_(k-1) # if G_k is transitive on the subset excluding b_0,...,b_(k-1) # then G is (k+1)-transitive for i in range(n): orb = G.orbit(i) if len(orb) != n - i: self._transitivity_degree = i return i G = G.stabilizer(i) self._transitivity_degree = n return n else: return self._transitivity_degree def _p_elements_group(self, p): ''' For an abelian p-group, return the subgroup consisting of all elements of order p (and the identity) ''' gens = self.generators[:] gens = sorted(gens, key=lambda x: x.order(), reverse=True) gens_p = [g**(g.order()/p) for g in gens] gens_r = [] for i in range(len(gens)): x = gens[i] x_order = x.order() # x_p has order p x_p = x**(x_order/p) if i > 0: P = PermutationGroup(gens_p[:i]) else: P = PermutationGroup(self.identity) if x**(x_order/p) not in P: gens_r.append(x**(x_order/p)) else: # replace x by an element of order (x.order()/p) # so that gens still generates G g = P.generator_product(x_p, original=True) for s in g: x = x*s**-1 x_order = x_order/p # insert x to gens so that the sorting is preserved del gens[i] del gens_p[i] j = i - 1 while j < len(gens) and gens[j].order() >= x_order: j += 1 gens = gens[:j] + [x] + gens[j:] gens_p = gens_p[:j] + [x] + gens_p[j:] return PermutationGroup(gens_r) def _sylow_alt_sym(self, p): ''' Return a p-Sylow subgroup of a symmetric or an alternating group. Explanation =========== The algorithm for this is hinted at in [1], Chapter 4, Exercise 4. For Sym(n) with n = p^i, the idea is as follows. Partition the interval [0..n-1] into p equal parts, each of length p^(i-1): [0..p^(i-1)-1], [p^(i-1)..2*p^(i-1)-1]...[(p-1)*p^(i-1)..p^i-1]. Find a p-Sylow subgroup of Sym(p^(i-1)) (treated as a subgroup of ``self``) acting on each of the parts. Call the subgroups P_1, P_2...P_p. The generators for the subgroups P_2...P_p can be obtained from those of P_1 by applying a "shifting" permutation to them, that is, a permutation mapping [0..p^(i-1)-1] to the second part (the other parts are obtained by using the shift multiple times). The union of this permutation and the generators of P_1 is a p-Sylow subgroup of ``self``. For n not equal to a power of p, partition [0..n-1] in accordance with how n would be written in base p. E.g. for p=2 and n=11, 11 = 2^3 + 2^2 + 1 so the partition is [[0..7], [8..9], {10}]. To generate a p-Sylow subgroup, take the union of the generators for each of the parts. For the above example, {(0 1), (0 2)(1 3), (0 4), (1 5)(2 7)} from the first part, {(8 9)} from the second part and nothing from the third. This gives 4 generators in total, and the subgroup they generate is p-Sylow. Alternating groups are treated the same except when p=2. In this case, (0 1)(s s+1) should be added for an appropriate s (the start of a part) for each part in the partitions. See Also ======== sylow_subgroup, is_alt_sym ''' n = self.degree gens = [] identity = Permutation(n-1) # the case of 2-sylow subgroups of alternating groups # needs special treatment alt = p == 2 and all(g.is_even for g in self.generators) # find the presentation of n in base p coeffs = [] m = n while m > 0: coeffs.append(m % p) m = m // p power = len(coeffs)-1 # for a symmetric group, gens[:i] is the generating # set for a p-Sylow subgroup on [0..p**(i-1)-1]. For # alternating groups, the same is given by gens[:2*(i-1)] for i in range(1, power+1): if i == 1 and alt: # (0 1) shouldn't be added for alternating groups continue gen = Permutation([(j + p**(i-1)) % p**i for j in range(p**i)]) gens.append(identity*gen) if alt: gen = Permutation(0, 1)*gen*Permutation(0, 1)*gen gens.append(gen) # the first point in the current part (see the algorithm # description in the docstring) start = 0 while power > 0: a = coeffs[power] # make the permutation shifting the start of the first # part ([0..p^i-1] for some i) to the current one for _ in range(a): shift = Permutation() if start > 0: for i in range(p**power): shift = shift(i, start + i) if alt: gen = Permutation(0, 1)*shift*Permutation(0, 1)*shift gens.append(gen) j = 2*(power - 1) else: j = power for i, gen in enumerate(gens[:j]): if alt and i % 2 == 1: continue # shift the generator to the start of the # partition part gen = shift*gen*shift gens.append(gen) start += p**power power = power-1 return gens def sylow_subgroup(self, p): ''' Return a p-Sylow subgroup of the group. The algorithm is described in [1], Chapter 4, Section 7 Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> from sympy.combinatorics.named_groups import AlternatingGroup >>> D = DihedralGroup(6) >>> S = D.sylow_subgroup(2) >>> S.order() 4 >>> G = SymmetricGroup(6) >>> S = G.sylow_subgroup(5) >>> S.order() 5 >>> G1 = AlternatingGroup(3) >>> G2 = AlternatingGroup(5) >>> G3 = AlternatingGroup(9) >>> S1 = G1.sylow_subgroup(3) >>> S2 = G2.sylow_subgroup(3) >>> S3 = G3.sylow_subgroup(3) >>> len1 = len(S1.lower_central_series()) >>> len2 = len(S2.lower_central_series()) >>> len3 = len(S3.lower_central_series()) >>> len1 == len2 True >>> len1 < len3 True ''' from sympy.combinatorics.homomorphisms import ( orbit_homomorphism, block_homomorphism) if not isprime(p): raise ValueError("p must be a prime") def is_p_group(G): # check if the order of G is a power of p # and return the power m = G.order() n = 0 while m % p == 0: m = m/p n += 1 if m == 1: return True, n return False, n def _sylow_reduce(mu, nu): # reduction based on two homomorphisms # mu and nu with trivially intersecting # kernels Q = mu.image().sylow_subgroup(p) Q = mu.invert_subgroup(Q) nu = nu.restrict_to(Q) R = nu.image().sylow_subgroup(p) return nu.invert_subgroup(R) order = self.order() if order % p != 0: return PermutationGroup([self.identity]) p_group, n = is_p_group(self) if p_group: return self if self.is_alt_sym(): return PermutationGroup(self._sylow_alt_sym(p)) # if there is a non-trivial orbit with size not divisible # by p, the sylow subgroup is contained in its stabilizer # (by orbit-stabilizer theorem) orbits = self.orbits() non_p_orbits = [o for o in orbits if len(o) % p != 0 and len(o) != 1] if non_p_orbits: G = self.stabilizer(list(non_p_orbits[0]).pop()) return G.sylow_subgroup(p) if not self.is_transitive(): # apply _sylow_reduce to orbit actions orbits = sorted(orbits, key=len) omega1 = orbits.pop() omega2 = orbits[0].union(*orbits) mu = orbit_homomorphism(self, omega1) nu = orbit_homomorphism(self, omega2) return _sylow_reduce(mu, nu) blocks = self.minimal_blocks() if len(blocks) > 1: # apply _sylow_reduce to block system actions mu = block_homomorphism(self, blocks[0]) nu = block_homomorphism(self, blocks[1]) return _sylow_reduce(mu, nu) elif len(blocks) == 1: block = list(blocks)[0] if any(e != 0 for e in block): # self is imprimitive mu = block_homomorphism(self, block) if not is_p_group(mu.image())[0]: S = mu.image().sylow_subgroup(p) return mu.invert_subgroup(S).sylow_subgroup(p) # find an element of order p g = self.random() g_order = g.order() while g_order % p != 0 or g_order == 0: g = self.random() g_order = g.order() g = g**(g_order // p) if order % p**2 != 0: return PermutationGroup(g) C = self.centralizer(g) while C.order() % p**n != 0: S = C.sylow_subgroup(p) s_order = S.order() Z = S.center() P = Z._p_elements_group(p) h = P.random() C_h = self.centralizer(h) while C_h.order() % p*s_order != 0: h = P.random() C_h = self.centralizer(h) C = C_h return C.sylow_subgroup(p) def _block_verify(self, L, alpha): delta = sorted(self.orbit(alpha)) # p[i] will be the number of the block # delta[i] belongs to p = [-1]*len(delta) blocks = [-1]*len(delta) B = [[]] # future list of blocks u = [0]*len(delta) # u[i] in L s.t. alpha^u[i] = B[0][i] t = L.orbit_transversal(alpha, pairs=True) for a, beta in t: B[0].append(a) i_a = delta.index(a) p[i_a] = 0 blocks[i_a] = alpha u[i_a] = beta rho = 0 m = 0 # number of blocks - 1 while rho <= m: beta = B[rho][0] for g in self.generators: d = beta^g i_d = delta.index(d) sigma = p[i_d] if sigma < 0: # define a new block m += 1 sigma = m u[i_d] = u[delta.index(beta)]*g p[i_d] = sigma rep = d blocks[i_d] = rep newb = [rep] for gamma in B[rho][1:]: i_gamma = delta.index(gamma) d = gamma^g i_d = delta.index(d) if p[i_d] < 0: u[i_d] = u[i_gamma]*g p[i_d] = sigma blocks[i_d] = rep newb.append(d) else: # B[rho] is not a block s = u[i_gamma]*g*u[i_d]**(-1) return False, s B.append(newb) else: for h in B[rho][1:]: if h^g not in B[sigma]: # B[rho] is not a block s = u[delta.index(beta)]*g*u[i_d]**(-1) return False, s rho += 1 return True, blocks def _verify(H, K, phi, z, alpha): ''' Return a list of relators ``rels`` in generators ``gens`_h` that are mapped to ``H.generators`` by ``phi`` so that given a finite presentation of ``K`` on a subset of ``gens_h`` is a finite presentation of ``H``. Explanation =========== ``H`` should be generated by the union of ``K.generators`` and ``z`` (a single generator), and ``H.stabilizer(alpha) == K``; ``phi`` is a canonical injection from a free group into a permutation group containing ``H``. The algorithm is described in [1], Chapter 6. Examples ======== >>> from sympy.combinatorics import free_group, Permutation, PermutationGroup >>> from sympy.combinatorics.homomorphisms import homomorphism >>> from sympy.combinatorics.fp_groups import FpGroup >>> H = PermutationGroup(Permutation(0, 2), Permutation (1, 5)) >>> K = PermutationGroup(Permutation(5)(0, 2)) >>> F = free_group("x_0 x_1")[0] >>> gens = F.generators >>> phi = homomorphism(F, H, F.generators, H.generators) >>> rels_k = [gens[0]**2] # relators for presentation of K >>> z= Permutation(1, 5) >>> check, rels_h = H._verify(K, phi, z, 1) >>> check True >>> rels = rels_k + rels_h >>> G = FpGroup(F, rels) # presentation of H >>> G.order() == H.order() True See also ======== strong_presentation, presentation, stabilizer ''' orbit = H.orbit(alpha) beta = alpha^(z**-1) K_beta = K.stabilizer(beta) # orbit representatives of K_beta gammas = [alpha, beta] orbits = list({tuple(K_beta.orbit(o)) for o in orbit}) orbit_reps = [orb[0] for orb in orbits] for rep in orbit_reps: if rep not in gammas: gammas.append(rep) # orbit transversal of K betas = [alpha, beta] transversal = {alpha: phi.invert(H.identity), beta: phi.invert(z**-1)} for s, g in K.orbit_transversal(beta, pairs=True): if s not in transversal: transversal[s] = transversal[beta]*phi.invert(g) union = K.orbit(alpha).union(K.orbit(beta)) while (len(union) < len(orbit)): for gamma in gammas: if gamma in union: r = gamma^z if r not in union: betas.append(r) transversal[r] = transversal[gamma]*phi.invert(z) for s, g in K.orbit_transversal(r, pairs=True): if s not in transversal: transversal[s] = transversal[r]*phi.invert(g) union = union.union(K.orbit(r)) break # compute relators rels = [] for b in betas: k_gens = K.stabilizer(b).generators for y in k_gens: new_rel = transversal[b] gens = K.generator_product(y, original=True) for g in gens[::-1]: new_rel = new_rel*phi.invert(g) new_rel = new_rel*transversal[b]**-1 perm = phi(new_rel) try: gens = K.generator_product(perm, original=True) except ValueError: return False, perm for g in gens: new_rel = new_rel*phi.invert(g)**-1 if new_rel not in rels: rels.append(new_rel) for gamma in gammas: new_rel = transversal[gamma]*phi.invert(z)*transversal[gamma^z]**-1 perm = phi(new_rel) try: gens = K.generator_product(perm, original=True) except ValueError: return False, perm for g in gens: new_rel = new_rel*phi.invert(g)**-1 if new_rel not in rels: rels.append(new_rel) return True, rels def strong_presentation(self): ''' Return a strong finite presentation of group. The generators of the returned group are in the same order as the strong generators of group. The algorithm is based on Sims' Verify algorithm described in [1], Chapter 6. Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> P = DihedralGroup(4) >>> G = P.strong_presentation() >>> P.order() == G.order() True See Also ======== presentation, _verify ''' from sympy.combinatorics.fp_groups import (FpGroup, simplify_presentation) from sympy.combinatorics.free_groups import free_group from sympy.combinatorics.homomorphisms import (block_homomorphism, homomorphism, GroupHomomorphism) strong_gens = self.strong_gens[:] stabs = self.basic_stabilizers[:] base = self.base[:] # injection from a free group on len(strong_gens) # generators into G gen_syms = [('x_%d'%i) for i in range(len(strong_gens))] F = free_group(', '.join(gen_syms))[0] phi = homomorphism(F, self, F.generators, strong_gens) H = PermutationGroup(self.identity) while stabs: alpha = base.pop() K = H H = stabs.pop() new_gens = [g for g in H.generators if g not in K] if K.order() == 1: z = new_gens.pop() rels = [F.generators[-1]**z.order()] intermediate_gens = [z] K = PermutationGroup(intermediate_gens) # add generators one at a time building up from K to H while new_gens: z = new_gens.pop() intermediate_gens = [z] + intermediate_gens K_s = PermutationGroup(intermediate_gens) orbit = K_s.orbit(alpha) orbit_k = K.orbit(alpha) # split into cases based on the orbit of K_s if orbit_k == orbit: if z in K: rel = phi.invert(z) perm = z else: t = K.orbit_rep(alpha, alpha^z) rel = phi.invert(z)*phi.invert(t)**-1 perm = z*t**-1 for g in K.generator_product(perm, original=True): rel = rel*phi.invert(g)**-1 new_rels = [rel] elif len(orbit_k) == 1: # `success` is always true because `strong_gens` # and `base` are already a verified BSGS. Later # this could be changed to start with a randomly # generated (potential) BSGS, and then new elements # would have to be appended to it when `success` # is false. success, new_rels = K_s._verify(K, phi, z, alpha) else: # K.orbit(alpha) should be a block # under the action of K_s on K_s.orbit(alpha) check, block = K_s._block_verify(K, alpha) if check: # apply _verify to the action of K_s # on the block system; for convenience, # add the blocks as additional points # that K_s should act on t = block_homomorphism(K_s, block) m = t.codomain.degree # number of blocks d = K_s.degree # conjugating with p will shift # permutations in t.image() to # higher numbers, e.g. # p*(0 1)*p = (m m+1) p = Permutation() for i in range(m): p *= Permutation(i, i+d) t_img = t.images # combine generators of K_s with their # action on the block system images = {g: g*p*t_img[g]*p for g in t_img} for g in self.strong_gens[:-len(K_s.generators)]: images[g] = g K_s_act = PermutationGroup(list(images.values())) f = GroupHomomorphism(self, K_s_act, images) K_act = PermutationGroup([f(g) for g in K.generators]) success, new_rels = K_s_act._verify(K_act, f.compose(phi), f(z), d) for n in new_rels: if n not in rels: rels.append(n) K = K_s group = FpGroup(F, rels) return simplify_presentation(group) def presentation(self, eliminate_gens=True): ''' Return an `FpGroup` presentation of the group. The algorithm is described in [1], Chapter 6.1. ''' from sympy.combinatorics.fp_groups import (FpGroup, simplify_presentation) from sympy.combinatorics.coset_table import CosetTable from sympy.combinatorics.free_groups import free_group from sympy.combinatorics.homomorphisms import homomorphism if self._fp_presentation: return self._fp_presentation def _factor_group_by_rels(G, rels): if isinstance(G, FpGroup): rels.extend(G.relators) return FpGroup(G.free_group, list(set(rels))) return FpGroup(G, rels) gens = self.generators len_g = len(gens) if len_g == 1: order = gens[0].order() # handle the trivial group if order == 1: return free_group([])[0] F, x = free_group('x') return FpGroup(F, [x**order]) if self.order() > 20: half_gens = self.generators[0:(len_g+1)//2] else: half_gens = [] H = PermutationGroup(half_gens) H_p = H.presentation() len_h = len(H_p.generators) C = self.coset_table(H) n = len(C) # subgroup index gen_syms = [('x_%d'%i) for i in range(len(gens))] F = free_group(', '.join(gen_syms))[0] # mapping generators of H_p to those of F images = [F.generators[i] for i in range(len_h)] R = homomorphism(H_p, F, H_p.generators, images, check=False) # rewrite relators rels = R(H_p.relators) G_p = FpGroup(F, rels) # injective homomorphism from G_p into self T = homomorphism(G_p, self, G_p.generators, gens) C_p = CosetTable(G_p, []) C_p.table = [[None]*(2*len_g) for i in range(n)] # initiate the coset transversal transversal = [None]*n transversal[0] = G_p.identity # fill in the coset table as much as possible for i in range(2*len_h): C_p.table[0][i] = 0 gamma = 1 for alpha, x in product(range(n), range(2*len_g)): beta = C[alpha][x] if beta == gamma: gen = G_p.generators[x//2]**((-1)**(x % 2)) transversal[beta] = transversal[alpha]*gen C_p.table[alpha][x] = beta C_p.table[beta][x + (-1)**(x % 2)] = alpha gamma += 1 if gamma == n: break C_p.p = list(range(n)) beta = x = 0 while not C_p.is_complete(): # find the first undefined entry while C_p.table[beta][x] == C[beta][x]: x = (x + 1) % (2*len_g) if x == 0: beta = (beta + 1) % n # define a new relator gen = G_p.generators[x//2]**((-1)**(x % 2)) new_rel = transversal[beta]*gen*transversal[C[beta][x]]**-1 perm = T(new_rel) nxt = G_p.identity for s in H.generator_product(perm, original=True): nxt = nxt*T.invert(s)**-1 new_rel = new_rel*nxt # continue coset enumeration G_p = _factor_group_by_rels(G_p, [new_rel]) C_p.scan_and_fill(0, new_rel) C_p = G_p.coset_enumeration([], strategy="coset_table", draft=C_p, max_cosets=n, incomplete=True) self._fp_presentation = simplify_presentation(G_p) return self._fp_presentation def polycyclic_group(self): """ Return the PolycyclicGroup instance with below parameters: Explanation =========== * pc_sequence : Polycyclic sequence is formed by collecting all the missing generators between the adjacent groups in the derived series of given permutation group. * pc_series : Polycyclic series is formed by adding all the missing generators of ``der[i+1]`` in ``der[i]``, where ``der`` represents the derived series. * relative_order : A list, computed by the ratio of adjacent groups in pc_series. """ from sympy.combinatorics.pc_groups import PolycyclicGroup if not self.is_polycyclic: raise ValueError("The group must be solvable") der = self.derived_series() pc_series = [] pc_sequence = [] relative_order = [] pc_series.append(der[-1]) der.reverse() for i in range(len(der)-1): H = der[i] for g in der[i+1].generators: if g not in H: H = PermutationGroup([g] + H.generators) pc_series.insert(0, H) pc_sequence.insert(0, g) G1 = pc_series[0].order() G2 = pc_series[1].order() relative_order.insert(0, G1 // G2) return PolycyclicGroup(pc_sequence, pc_series, relative_order, collector=None) def _orbit(degree, generators, alpha, action='tuples'): r"""Compute the orbit of alpha `\{g(\alpha) | g \in G\}` as a set. Explanation =========== The time complexity of the algorithm used here is `O(|Orb|*r)` where `|Orb|` is the size of the orbit and ``r`` is the number of generators of the group. For a more detailed analysis, see [1], p.78, [2], pp. 19-21. Here alpha can be a single point, or a list of points. If alpha is a single point, the ordinary orbit is computed. if alpha is a list of points, there are three available options: 'union' - computes the union of the orbits of the points in the list 'tuples' - computes the orbit of the list interpreted as an ordered tuple under the group action ( i.e., g((1, 2, 3)) = (g(1), g(2), g(3)) ) 'sets' - computes the orbit of the list interpreted as a sets Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup >>> from sympy.combinatorics.perm_groups import _orbit >>> a = Permutation([1, 2, 0, 4, 5, 6, 3]) >>> G = PermutationGroup([a]) >>> _orbit(G.degree, G.generators, 0) {0, 1, 2} >>> _orbit(G.degree, G.generators, [0, 4], 'union') {0, 1, 2, 3, 4, 5, 6} See Also ======== orbit, orbit_transversal """ if not hasattr(alpha, '__getitem__'): alpha = [alpha] gens = [x._array_form for x in generators] if len(alpha) == 1 or action == 'union': orb = alpha used = [False]*degree for el in alpha: used[el] = True for b in orb: for gen in gens: temp = gen[b] if used[temp] == False: orb.append(temp) used[temp] = True return set(orb) elif action == 'tuples': alpha = tuple(alpha) orb = [alpha] used = {alpha} for b in orb: for gen in gens: temp = tuple([gen[x] for x in b]) if temp not in used: orb.append(temp) used.add(temp) return set(orb) elif action == 'sets': alpha = frozenset(alpha) orb = [alpha] used = {alpha} for b in orb: for gen in gens: temp = frozenset([gen[x] for x in b]) if temp not in used: orb.append(temp) used.add(temp) return {tuple(x) for x in orb} def _orbits(degree, generators): """Compute the orbits of G. If ``rep=False`` it returns a list of sets else it returns a list of representatives of the orbits Examples ======== >>> from sympy.combinatorics import Permutation >>> from sympy.combinatorics.perm_groups import _orbits >>> a = Permutation([0, 2, 1]) >>> b = Permutation([1, 0, 2]) >>> _orbits(a.size, [a, b]) [{0, 1, 2}] """ orbs = [] sorted_I = list(range(degree)) I = set(sorted_I) while I: i = sorted_I[0] orb = _orbit(degree, generators, i) orbs.append(orb) # remove all indices that are in this orbit I -= orb sorted_I = [i for i in sorted_I if i not in orb] return orbs def _orbit_transversal(degree, generators, alpha, pairs, af=False, slp=False): r"""Computes a transversal for the orbit of ``alpha`` as a set. Explanation =========== generators generators of the group ``G`` For a permutation group ``G``, a transversal for the orbit `Orb = \{g(\alpha) | g \in G\}` is a set `\{g_\beta | g_\beta(\alpha) = \beta\}` for `\beta \in Orb`. Note that there may be more than one possible transversal. If ``pairs`` is set to ``True``, it returns the list of pairs `(\beta, g_\beta)`. For a proof of correctness, see [1], p.79 if ``af`` is ``True``, the transversal elements are given in array form. If `slp` is `True`, a dictionary `{beta: slp_beta}` is returned for `\beta \in Orb` where `slp_beta` is a list of indices of the generators in `generators` s.t. if `slp_beta = [i_1 \dots i_n]` `g_\beta = generators[i_n] \times \dots \times generators[i_1]`. Examples ======== >>> from sympy.combinatorics.named_groups import DihedralGroup >>> from sympy.combinatorics.perm_groups import _orbit_transversal >>> G = DihedralGroup(6) >>> _orbit_transversal(G.degree, G.generators, 0, False) [(5), (0 1 2 3 4 5), (0 5)(1 4)(2 3), (0 2 4)(1 3 5), (5)(0 4)(1 3), (0 3)(1 4)(2 5)] """ tr = [(alpha, list(range(degree)))] slp_dict = {alpha: []} used = [False]*degree used[alpha] = True gens = [x._array_form for x in generators] for x, px in tr: px_slp = slp_dict[x] for gen in gens: temp = gen[x] if used[temp] == False: slp_dict[temp] = [gens.index(gen)] + px_slp tr.append((temp, _af_rmul(gen, px))) used[temp] = True if pairs: if not af: tr = [(x, _af_new(y)) for x, y in tr] if not slp: return tr return tr, slp_dict if af: tr = [y for _, y in tr] if not slp: return tr return tr, slp_dict tr = [_af_new(y) for _, y in tr] if not slp: return tr return tr, slp_dict def _stabilizer(degree, generators, alpha): r"""Return the stabilizer subgroup of ``alpha``. Explanation =========== The stabilizer of `\alpha` is the group `G_\alpha = \{g \in G | g(\alpha) = \alpha\}`. For a proof of correctness, see [1], p.79. degree : degree of G generators : generators of G Examples ======== >>> from sympy.combinatorics.perm_groups import _stabilizer >>> from sympy.combinatorics.named_groups import DihedralGroup >>> G = DihedralGroup(6) >>> _stabilizer(G.degree, G.generators, 5) [(5)(0 4)(1 3), (5)] See Also ======== orbit """ orb = [alpha] table = {alpha: list(range(degree))} table_inv = {alpha: list(range(degree))} used = [False]*degree used[alpha] = True gens = [x._array_form for x in generators] stab_gens = [] for b in orb: for gen in gens: temp = gen[b] if used[temp] is False: gen_temp = _af_rmul(gen, table[b]) orb.append(temp) table[temp] = gen_temp table_inv[temp] = _af_invert(gen_temp) used[temp] = True else: schreier_gen = _af_rmuln(table_inv[temp], gen, table[b]) if schreier_gen not in stab_gens: stab_gens.append(schreier_gen) return [_af_new(x) for x in stab_gens] PermGroup = PermutationGroup class SymmetricPermutationGroup(Basic): """ The class defining the lazy form of SymmetricGroup. deg : int """ def __new__(cls, deg): deg = _sympify(deg) obj = Basic.__new__(cls, deg) return obj def __init__(self, *args, **kwargs): self._deg = self.args[0] self._order = None def __contains__(self, i): """Return ``True`` if *i* is contained in SymmetricPermutationGroup. Examples ======== >>> from sympy.combinatorics import Permutation, SymmetricPermutationGroup >>> G = SymmetricPermutationGroup(4) >>> Permutation(1, 2, 3) in G True """ if not isinstance(i, Permutation): raise TypeError("A SymmetricPermutationGroup contains only Permutations as " "elements, not elements of type %s" % type(i)) return i.size == self.degree def order(self): """ Return the order of the SymmetricPermutationGroup. Examples ======== >>> from sympy.combinatorics import SymmetricPermutationGroup >>> G = SymmetricPermutationGroup(4) >>> G.order() 24 """ if self._order is not None: return self._order n = self._deg self._order = factorial(n) return self._order @property def degree(self): """ Return the degree of the SymmetricPermutationGroup. Examples ======== >>> from sympy.combinatorics import SymmetricPermutationGroup >>> G = SymmetricPermutationGroup(4) >>> G.degree 4 """ return self._deg @property def identity(self): ''' Return the identity element of the SymmetricPermutationGroup. Examples ======== >>> from sympy.combinatorics import SymmetricPermutationGroup >>> G = SymmetricPermutationGroup(4) >>> G.identity() (3) ''' return _af_new(list(range(self._deg))) class Coset(Basic): """A left coset of a permutation group with respect to an element. Parameters ========== g : Permutation H : PermutationGroup dir : "+" or "-", If not specified by default it will be "+" here ``dir`` specified the type of coset "+" represent the right coset and "-" represent the left coset. G : PermutationGroup, optional The group which contains *H* as its subgroup and *g* as its element. If not specified, it would automatically become a symmetric group ``SymmetricPermutationGroup(g.size)`` and ``SymmetricPermutationGroup(H.degree)`` if ``g.size`` and ``H.degree`` are matching.``SymmetricPermutationGroup`` is a lazy form of SymmetricGroup used for representation purpose. """ def __new__(cls, g, H, G=None, dir="+"): g = _sympify(g) if not isinstance(g, Permutation): raise NotImplementedError H = _sympify(H) if not isinstance(H, PermutationGroup): raise NotImplementedError if G is not None: G = _sympify(G) if not isinstance(G, (PermutationGroup, SymmetricPermutationGroup)): raise NotImplementedError if not H.is_subgroup(G): raise ValueError("{} must be a subgroup of {}.".format(H, G)) if g not in G: raise ValueError("{} must be an element of {}.".format(g, G)) else: g_size = g.size h_degree = H.degree if g_size != h_degree: raise ValueError( "The size of the permutation {} and the degree of " "the permutation group {} should be matching " .format(g, H)) G = SymmetricPermutationGroup(g.size) if isinstance(dir, str): dir = Symbol(dir) elif not isinstance(dir, Symbol): raise TypeError("dir must be of type basestring or " "Symbol, not %s" % type(dir)) if str(dir) not in ('+', '-'): raise ValueError("dir must be one of '+' or '-' not %s" % dir) obj = Basic.__new__(cls, g, H, G, dir) return obj def __init__(self, *args, **kwargs): self._dir = self.args[3] @property def is_left_coset(self): """ Check if the coset is left coset that is ``gH``. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup, Coset >>> a = Permutation(1, 2) >>> b = Permutation(0, 1) >>> G = PermutationGroup([a, b]) >>> cst = Coset(a, G, dir="-") >>> cst.is_left_coset True """ return str(self._dir) == '-' @property def is_right_coset(self): """ Check if the coset is right coset that is ``Hg``. Examples ======== >>> from sympy.combinatorics import Permutation, PermutationGroup, Coset >>> a = Permutation(1, 2) >>> b = Permutation(0, 1) >>> G = PermutationGroup([a, b]) >>> cst = Coset(a, G, dir="+") >>> cst.is_right_coset True """ return str(self._dir) == '+' def as_list(self): """ Return all the elements of coset in the form of list. """ g = self.args[0] H = self.args[1] cst = [] if str(self._dir) == '+': for h in H.elements: cst.append(h*g) else: for h in H.elements: cst.append(g*h) return cst