GroupsWithPropertyT/groups/speciallinear.jl

struct SpecialLinearGroup <: SymmetrizedGroup
    args::Dict{String,Any}
    group::AbstractAlgebra.Group
    N::Int

    function SpecialLinearGroup(args::Dict)
        n = args["SL"]
        p = args["p"]
        X = args["X"]

        if p == 0
            G = MatrixSpace(Nemo.ZZ, n, n)
        else
            R = Nemo.NmodRing(UInt(p))
            G = MatrixSpace(R, n, n)
        end
        return new(args, G, n)
    end
end

function name(G::SpecialLinearGroup)
    p = G.args["p"]
    X = G.args["X"]

    if p == 0
       R = (X ? "Z[x]" : "Z")
    else
       R = "F$p"
    end
    if G.args["nosymmetry"]
        return "SL($(G.N),$R)"
    else
        return "oSL($(G.N),$R)"
    end
end

group(G::SpecialLinearGroup) = G.group

function E(i::Int, j::Int, M::MatSpace, val=one(M.base_ring))
    @assert i≠j
    m = one(M)
    m[i,j] = val
    return m
end

function generatingset(G::SpecialLinearGroup)
    p = G.args["p"]
    X = G.args["X"]
    p > 0 && X && throw("SL(n, F_p[x]) not implemented")
    SL = group(G)
    r = G.args["radius"]
    return generatingset(SL, r, X)
end


function generatingset(SL::MatSpace, radius::Integer, X::Bool=false)

    n = SL.cols
    indexing = [(i,j) for i in 1:n for j in 1:n if i≠j]

    if !X
        S = [E(idx[1],idx[2],SL) for idx in indexing]
    else
        S = [E(i,j,SL,v) for (i,j) in indexing for v in [1, 100*r]]
    end
    return unique([S; inv.(S)])
end

function autS(G::SpecialLinearGroup)
    return WreathProduct(PermutationGroup(2), PermutationGroup(G.N))
end

###############################################################################
#
#  Action of WreathProductElems on Nemo.MatElem
#
###############################################################################

function matrix_emb(n::DirectProductGroupElem, p::perm)
    Id = parent(n.elts[1])()
    elt = diagm([(-1)^(el == Id ? 0 : 1) for el in n.elts])
    return elt[:, p.d]
end

function (g::WreathProductElem)(A::MatElem)
    g_inv = inv(g)
    G = matrix_emb(g.n, g_inv.p)
    G_inv = matrix_emb(g_inv.n, g.p)
    M = parent(A)
    return M(G)*A*M(G_inv)
end

import Base.*

doc"""
    *(x::AbstractAlgebra.MatElem, P::Generic.perm)
> Apply the pemutation $P$ to the rows of the matrix $x$ and return the result.
"""
function *(x::AbstractAlgebra.MatElem, P::Generic.perm)
   z = similar(x)
   m = rows(x)
   n = cols(x)
   for i = 1:m
      for j = 1:n
         z[i, j] = x[i,P[j]]
      end
   end
   return z
end

function (p::perm)(A::MatElem)
    length(p.d) == A.r == A.c || throw("Can't act via $p on matrix of size ($(A.r), $(A.c))")
    return p*A*inv(p)
end