mirror of
https://github.com/kalmarek/Groups.jl.git
synced 2024-10-31 18:00:35 +01:00
209 lines
6.4 KiB
Julia
209 lines
6.4 KiB
Julia
export WreathProduct, WreathProductElem
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###############################################################################
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#
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# WreathProduct / WreathProductElem
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#
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###############################################################################
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doc"""
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WreathProduct(N, P) <: Group
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> Implements Wreath product of a group `N` by permutation group $P = S_n$,
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> usually written as $N \wr P$.
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> The multiplication inside wreath product is defined as
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> > `(n, σ) * (m, τ) = (n*σ(m), στ)`
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> where `σ(m)` denotes the action (from the right) of the permutation group on
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> `n-tuples` of elements from `N`
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# Arguments:
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* `N::Group` : the single factor of group $N$
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* `P::Generic.PermGroup` : full `PermutationGroup`
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"""
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struct WreathProduct{T<:Group, I<:Integer} <: Group
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N::DirectProductGroup{T}
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P::Generic.PermGroup{I}
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function WreathProduct{T, I}(Gr::T, P::Generic.PermGroup{I}) where {T, I}
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N = DirectProductGroup(Gr, Int(P.n))
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return new(N, P)
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end
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end
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struct WreathProductElem{T<:GroupElem, I<:Integer} <: GroupElem
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n::DirectProductGroupElem{T}
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p::Generic.perm{I}
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# parent::WreathProduct
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function WreathProductElem{T, I}(n::DirectProductGroupElem{T}, p::Generic.perm{I},
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check::Bool=true) where {T, I}
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if check
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length(n.elts) == length(p) || throw(DomainError(
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"Can't form WreathProductElem: lengths differ"))
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end
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return new(n, p)
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end
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end
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###############################################################################
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#
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# Type and parent object methods
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#
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###############################################################################
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elem_type(::Type{WreathProduct{T, I}}) where {T, I} = WreathProductElem{elem_type(T), I}
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parent_type(::Type{WreathProductElem{T, I}}) where {T, I} =
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WreathProduct{parent_type(T), I}
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parent(g::WreathProductElem) = WreathProduct(parent(g.n[1]), parent(g.p))
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###############################################################################
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#
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# WreathProduct / WreathProductElem constructors
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#
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###############################################################################
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WreathProduct(G::T, P::Generic.PermGroup{I}) where {T, I} = WreathProduct{T, I}(G, P)
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WreathProduct(G::T, P::Generic.PermGroup{I}) where {T<:AbstractAlgebra.Ring, I} = WreathProduct(AddGrp(G), P)
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WreathProductElem(n::DirectProductGroupElem{T}, p::Generic.perm{I}, check=true) where {T,I} = WreathProductElem{T,I}(n, p, check)
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WreathProductElem(n::DirectProductGroupElem{T}, p::Generic.perm{I}, check=true) where {T<:AbstractAlgebra.RingElem, I} = WreathProductElem(DirectProductGroupElem(AddGrpElem.(n.elts)), p, check)
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###############################################################################
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#
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# Parent object call overloads
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#
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###############################################################################
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function (G::WreathProduct)(g::WreathProductElem)
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n = try
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G.N(g.n)
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catch
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throw(DomainError("Can't coerce $(g.n) to $(G.N) factor of $G"))
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end
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p = try
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G.P(g.p)
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catch
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throw(DomainError("Can't coerce $(g.p) to $(G.P) factor of $G"))
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end
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return WreathProductElem(n, p)
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end
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doc"""
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(G::WreathProduct)(n::DirectProductGroupElem, p::Generic.perm)
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> Creates an element of wreath product `G` by coercing `n` and `p` to `G.N` and
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> `G.P`, respectively.
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"""
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(G::WreathProduct)(n::DirectProductGroupElem, p::Generic.perm) = WreathProductElem(n,p)
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(G::WreathProduct)() = WreathProductElem(G.N(), G.P(), false)
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doc"""
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(G::WreathProduct)(p::Generic.perm)
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> Returns the image of permutation `p` in `G` via embedding `p -> (id,p)`.
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"""
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(G::WreathProduct)(p::Generic.perm) = G(G.N(), p)
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doc"""
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(G::WreathProduct)(n::DirectProductGroupElem)
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> Returns the image of `n` in `G` via embedding `n -> (n,())`. This is the
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> embedding that makes sequence `1 -> N -> G -> P -> 1` exact.
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"""
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(G::WreathProduct)(n::DirectProductGroupElem) = G(n, G.P())
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(G::WreathProduct)(n,p) = G(G.N(n), G.P(p))
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###############################################################################
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#
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# Basic manipulation
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#
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###############################################################################
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function hash(G::WreathProduct, h::UInt)
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return hash(G.N, hash(G.P, hash(WreathProduct, h)))
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end
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function hash(g::WreathProductElem, h::UInt)
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return hash(g.n, hash(g.p, hash(WreathProductElem, h)))
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end
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###############################################################################
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#
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# String I/O
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#
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###############################################################################
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function show(io::IO, G::WreathProduct)
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print(io, "Wreath Product of $(G.N.group) by $(G.P)")
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end
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function show(io::IO, g::WreathProductElem)
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print(io, "($(g.n)≀$(g.p))")
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end
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###############################################################################
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#
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# Comparison
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#
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###############################################################################
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function (==)(G::WreathProduct, H::WreathProduct)
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G.N == H.N || return false
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G.P == H.P || return false
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return true
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end
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function (==)(g::WreathProductElem, h::WreathProductElem)
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g.n == h.n || return false
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g.p == h.p || return false
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return true
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end
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###############################################################################
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#
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# Group operations
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#
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###############################################################################
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(p::perm)(n::DirectProductGroupElem) = DirectProductGroupElem(n.elts[p.d])
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doc"""
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*(g::WreathProductElem, h::WreathProductElem)
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> Return the wreath product group operation of elements, i.e.
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>
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> `g*h = (g.n*g.p(h.n), g.p*h.p)`,
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>
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> where `g.p(h.n)` denotes the action of `g.p::Generic.perm` on
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> `h.n::DirectProductGroupElem` via standard permutation of coordinates.
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"""
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function *(g::WreathProductElem, h::WreathProductElem)
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return WreathProductElem(g.n*g.p(h.n), g.p*h.p, false)
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end
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doc"""
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inv(g::WreathProductElem)
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> Returns the inverse of element of a wreath product, according to the formula
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> `g^-1 = (g.n, g.p)^-1 = (g.p^-1(g.n^-1), g.p^-1)`.
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"""
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function inv(g::WreathProductElem)
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pinv = inv(g.p)
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return WreathProductElem(pinv(inv(g.n)), pinv, false)
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end
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###############################################################################
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#
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# Misc
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#
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###############################################################################
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matrix_repr(g::WreathProductElem) = Any[matrix_repr(g.p) g.n]
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function elements(G::WreathProduct)
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iter = Base.product(collect(elements(G.N)), collect(elements(G.P)))
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return (WreathProductElem(n, p, false) for (n,p) in iter)
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end
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order(G::WreathProduct) = order(G.P)*order(G.N)
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