Add smaller_degree_permutation_representation, lazy double cosets (o…

…scar-system#3899)

Co-authored-by: ThomasBreuer <sam@math.rwth-aachen.de>
Co-authored-by: Max Horn <max@quendi.de>

docs/src/Groups/permgroup.md

@@ -39,6 +39,7 @@ In OSCAR, every permutation group has a degree `n`, that corresponds to the size
 
 ```@docs
 degree(x::PermGroup)
+smaller_degree_permutation_representation(G::PermGroup)
 ```
 
 

src/GAP/wrappers.jl

@@ -78,6 +78,8 @@ GAP.@wrap DescriptionOfRootOfUnity(x::Any)::GapObj
 GAP.@wrap DeterminantOfCharacter(x::GapObj)::GapObj
 GAP.@wrap Dimension(x::GapObj)::Int
 GAP.@wrap DimensionOfHighestWeightModule(x::GapObj, y::GapObj)::GapInt
+GAP.@wrap DoubleCoset(x::GapObj, y::GapObj, z::GapObj)::GapObj
+GAP.@wrap DoubleCosetRepsAndSizes(x::GapObj, y::GapObj, z::GapObj)::GapObj
 GAP.@wrap DominantCharacter(x::GapObj, y::GapObj)::GapObj
 GAP.@wrap E(x::Any)::GapInt
 GAP.@wrap EigenvaluesChar(x::GapObj, y::GAP.Obj)::GapObj
@@ -138,9 +140,10 @@ GAP.@wrap Indicator(x::GapObj, y::GapObj, z::GapInt)::GapObj
 GAP.@wrap InducedClassFunction(x::GapObj, y::GapObj)::GapObj
 GAP.@wrap InducedClassFunctionsByFusionMap(x::GapObj, y::GapObj, z::GapObj, t::GapObj)::GapObj
 GAP.@wrap InducedCyclic(x::GapObj)::GapObj
-GAP.@wrap InitFusion(x::GapObj, y::GapObj)::GapObj
 GAP.@wrap InducedCyclic(x::GapObj, y::GapObj)::GapObj
+GAP.@wrap InitFusion(x::GapObj, y::GapObj)::GapObj
 GAP.@wrap INT_FFE_DEFAULT(x::Any)::GapInt
+GAP.@wrap Intersection(x::GapObj)::GapObj
 GAP.@wrap IntFFE(x::Any)::GapInt
 GAP.@wrap Inverse(x::GapObj)::GapObj
 GAP.@wrap Irr(x::GapObj)::GapObj
@@ -312,6 +315,7 @@ GAP.@wrap PrimePGroup(x::GapObj)::GapInt
 GAP.@wrap PrimitiveElement(x::GapObj)::GapObj
 GAP.@wrap Projection(x::GapObj)::GapObj
 GAP.@wrap Projection(x::GapObj, i::Int)::GapObj
+GAP.@wrap Random(x::GapObj, y::GapObj)::GAP.Obj
 GAP.@wrap Range(x::GapObj)::GapObj
 GAP.@wrap RecognizeGroup(x::GapObj)::GapObj
 GAP.@wrap ReduceCoeffs(x::GapObj, y::GapObj)
@@ -320,6 +324,8 @@ GAP.@wrap Representative(x::GapObj)::GAP.Obj
 GAP.@wrap RepresentativeTom(x::GapObj, y::Int)::GapObj
 GAP.@wrap RepresentativeAction(x::GapObj, y::GapObj, z::GapObj)::GapObj
 GAP.@wrap RestrictedMapping(x::GapObj, y::GapObj)::GapObj
+GAP.@wrap RightCoset(x::GapObj, y::GapObj)::GapObj
+GAP.@wrap RightTransversal(x::GapObj, y::GapObj)::GapObj
 GAP.@wrap RootSystem(x::GapObj)::GapObj
 GAP.@wrap SLPforElement(x::GapObj, y::GapObj)::GAP.Obj
 GAP.@wrap ScalarProduct(x::GapObj, y::GapObj, z::GapObj)::GAP.Obj

src/Groups/cosets.jl

@@ -24,15 +24,15 @@ function _group_coset(G::GAPGroup, H::GAPGroup, repr::GAPGroupElem, side::Symbol
 end
 
 function ==(x::GroupCoset, y::GroupCoset)
-   return x.X == y.X && x.side == y.side
+   return GapObj(x) == GapObj(y) && x.side == y.side
 end
 
 function Base.show(io::IO, ::MIME"text/plain", x::GroupCoset)
   side = x.side === :left ? "Left" : "Right"
   io = pretty(io)
   println(io, "$side coset of ", Lowercase(), x.H)
   print(io, Indent())
-  println(io, "with representative ", x.repr)
+  println(io, "with representative ", representative(x))
   print(io, "in ", Lowercase(), x.G)
   print(io, Dedent())
 end
@@ -44,7 +44,7 @@ function Base.show(io::IO, x::GroupCoset)
   else
     print(io, "$side coset of ")
     io = pretty(io)
-    print(terse(io), Lowercase(), x.H, " with representative ", x.repr)
+    print(terse(io), Lowercase(), x.H, " with representative ", representative(x))
   end
 end
 
@@ -73,8 +73,8 @@ Right coset of Sym(3)
 ```
 """
 function right_coset(H::GAPGroup, g::GAPGroupElem)
-   @req GAPWrap.IsSubset(parent(g).X, H.X) "H is not a subgroup of parent(g)"
-   return _group_coset(parent(g), H, g, :right, GAP.Globals.RightCoset(H.X,g.X))
+   @req GAPWrap.IsSubset(GapObj(parent(g)), GapObj(H)) "H is not a subgroup of parent(g)"
+   return _group_coset(parent(g), H, g, :right, GAPWrap.RightCoset(GapObj(H), GapObj(g)))
 end
 
 """
@@ -101,8 +101,8 @@ Left coset of Sym(3)
 ```
 """
 function left_coset(H::GAPGroup, g::GAPGroupElem)
-   @req GAPWrap.IsSubset(parent(g).X, H.X) "H is not a subgroup of parent(g)"
-   return _group_coset(parent(g), H, g, :left, GAP.Globals.RightCoset(GAP.Globals.ConjugateSubgroup(H.X,GAP.Globals.Inverse(g.X)),g.X))
+   @req GAPWrap.IsSubset(GapObj(parent(g)), GapObj(H)) "H is not a subgroup of parent(g)"
+   return _group_coset(parent(g), H, g, :left, GAPWrap.RightCoset(GAPWrap.ConjugateSubgroup(GapObj(H), GAPWrap.Inverse(GapObj(g))), GapObj(g)))
 end
 
 
@@ -126,24 +126,24 @@ Base.:*(g::GAPGroupElem, H::GAPGroup) = left_coset(H,g)
 function Base.:*(c::GroupCoset, y::GAPGroupElem)
    @assert y in c.G "element not in the group"
    if c.side == :right
-      return right_coset(c.H, c.repr*y)
+      return right_coset(c.H, representative(c)*y)
    else
-      return left_coset(c.H^y, c.repr*y)
+      return left_coset(c.H^y, representative(c)*y)
    end
 end
 
 function Base.:*(y::GAPGroupElem, c::GroupCoset)
    @assert y in c.G "element not in the group"
    if c.side == :left
-      return left_coset(c.H, y*c.repr)
+      return left_coset(c.H, y*representative(c))
    else
-      return right_coset(c.H^(y^-1), y*c.repr)
+      return right_coset(c.H^(y^-1), y*representative(c))
    end
 end
 
 function Base.:*(c::GroupCoset, d::GroupCoset)
    @req (c.side == :right && d.side == :left) "Wrong input"
-   return double_coset(c.H, c.repr*d.repr, d.H)
+   return double_coset(c.H, representative(c)*representative(d), d.H)
 end
 
 """
@@ -238,7 +238,7 @@ julia> is_bicoset(fH)
 true
 ```
 """
-is_bicoset(C::GroupCoset) = GAPWrap.IsBiCoset(C.X)
+is_bicoset(C::GroupCoset) = GAPWrap.IsBiCoset(GapObj(C))
 
 """
     right_cosets(G::GAPGroup, H::GAPGroup; check::Bool=true)
@@ -317,6 +317,8 @@ struct SubgroupTransversal{T<: GAPGroup, S<: GAPGroup, E<: GAPGroupElem} <: Abst
    X::GapObj               # underlying *right* transversal in GAP
 end
 
+GAP.julia_to_gap(T::SubgroupTransversal) = T.X
+
 function Base.show(io::IO, ::MIME"text/plain", x::SubgroupTransversal)
   side = x.side === :left ? "Left" : "Right"
   println(io, "$side transversal of length $(length(x)) of")
@@ -343,7 +345,7 @@ Base.hash(x::SubgroupTransversal, h::UInt) = h # FIXME
 Base.length(T::SubgroupTransversal) = index(Int, T.G, T.H)
 
 function Base.getindex(T::SubgroupTransversal, i::Int)
-  res = group_element(T.G, T.X[i])
+  res = group_element(T.G, GapObj(T)[i])
   if T.side === :left
     res = inv(res)
   end
@@ -397,7 +399,7 @@ function right_transversal(G::T1, H::T2; check::Bool=true) where T1 <: GAPGroup
      _check_compatible(G, H)
    end
    return SubgroupTransversal{T1, T2, eltype(T1)}(G, H, :right,
-              GAP.Globals.RightTransversal(G.X, H.X))
+              GAPWrap.RightTransversal(GapObj(G), GapObj(H)))
 end
 
 """
@@ -437,11 +439,11 @@ function left_transversal(G::T1, H::T2; check::Bool=true) where T1 <: GAPGroup w
      _check_compatible(G, H)
    end
    return SubgroupTransversal{T1, T2, eltype(T1)}(G, H, :left,
-              GAP.Globals.RightTransversal(G.X, H.X))
+              GAPWrap.RightTransversal(GapObj(G), GapObj(H)))
 end
 
 Base.IteratorSize(::Type{<:GroupCoset}) = Base.SizeUnknown()
-Base.iterate(G::GroupCoset) = iterate(G, GAPWrap.Iterator(G.X))
+Base.iterate(G::GroupCoset) = iterate(G, GAPWrap.Iterator(GapObj(G)))
 
 function Base.iterate(G::GroupCoset, state)
   GAPWrap.IsDoneIterator(state) && return nothing
@@ -463,24 +465,36 @@ struct GroupDoubleCoset{T <: GAPGroup, S <: GAPGroupElem}
    H::GAPGroup
    K::GAPGroup
    repr::S
-   X::GapObj
+   X::Ref{GapObj}
+   size::Ref{ZZRingElem}
+
+   function GroupDoubleCoset(G::T, H::GAPGroup, K::GAPGroup, representative::S) where {T<: GAPGroup, S<:GAPGroupElem}
+     return new{T, S}(G, H, K, representative, Ref{GapObj}(), Ref{ZZRingElem}())
+   end 
 end
 
-GAP.julia_to_gap(obj::GroupDoubleCoset) = obj.X
+function GAP.julia_to_gap(C::GroupDoubleCoset)
+  if !isassigned(C.X)
+    C.X[] = GAPWrap.DoubleCoset(GapObj(C.H), GapObj(representative(C)), GapObj(C.K))
+  end
+  return C.X[]
+end
 
 Base.hash(x::GroupDoubleCoset, h::UInt) = h # FIXME
 Base.eltype(::Type{GroupDoubleCoset{T,S}}) where {T,S} = S
 
 function ==(x::GroupDoubleCoset, y::GroupDoubleCoset)
-   return x.X == y.X
+   # Avoid creating a GAP object if the result is obviously `false`.
+   isassigned(x.size) && isassigned(y.size) && order(x) != order(y) && return false
+   return GapObj(x) == GapObj(y)
 end
 
 function Base.show(io::IO, ::MIME"text/plain", x::GroupDoubleCoset)
   io = pretty(io)
   println(io, "Double coset of ", Lowercase(), x.H)
   print(io, Indent())
   println(io, "and ", Lowercase(), x.K)
-  println(io, "with representative ", x.repr)
+  println(io, "with representative ", representative(x))
   print(io, "in ", Lowercase(), x.G)
   print(io, Dedent())
 end
@@ -492,7 +506,7 @@ function Base.show(io::IO, x::GroupDoubleCoset)
     print(io, "Double coset of ")
     io = pretty(io)
     print(terse(io), Lowercase(), x.H,
-      " and ", Lowercase(), x.K, " with representative ", x.repr)
+      " and ", Lowercase(), x.K, " with representative ", representative(x))
   end
 end
 
@@ -526,9 +540,9 @@ Double coset of Sym(3)
 """
 function double_coset(G::GAPGroup, g::GAPGroupElem, H::GAPGroup)
 #T what if g is in some subgroup of a group of which G, H are also a subgroup?
-   @req GAPWrap.IsSubset(parent(g).X,G.X) "G is not a subgroup of parent(g)"
-   @req GAPWrap.IsSubset(parent(g).X,H.X) "H is not a subgroup of parent(g)"
-   return GroupDoubleCoset(parent(g),G,H,g,GAP.Globals.DoubleCoset(G.X,g.X,H.X))
+   @req GAPWrap.IsSubset(GapObj(parent(g)), GapObj(G)) "G is not a subgroup of parent(g)"
+   @req GAPWrap.IsSubset(GapObj(parent(g)), GapObj(H)) "H is not a subgroup of parent(g)"
+   return GroupDoubleCoset(parent(g), G, H, g)
 end
 
 Base.:*(H::GAPGroup, g::GAPGroupElem, K::GAPGroup) = double_coset(H,g,K)
@@ -558,31 +572,42 @@ julia> double_cosets(G,H,K)
 ```
 """
 function double_cosets(G::T, H::GAPGroup, K::GAPGroup; check::Bool=true) where T <: GAPGroup
-   if !check
-      dcs = GAP.Globals.DoubleCosetsNC(G.X,H.X,K.X)
-   else
+   if check
       @assert is_subset(H, G) "H is not a subgroup of G"
       @assert is_subset(K, G) "K is not a subgroup of G"
-      dcs = GAP.Globals.DoubleCosets(G.X,H.X,K.X)
    end
-   res = Vector{GroupDoubleCoset{T,elem_type(T)}}(undef, length(dcs))
-   for i = 1:length(res)
-     dc = dcs[i]
-     g = group_element(G, GAPWrap.Representative(dc))
-     res[i] = GroupDoubleCoset(G,H,K,g,dc)
+   dcs = GAPWrap.DoubleCosetRepsAndSizes(GapObj(G), GapObj(H), GapObj(K))
+   res = Vector{GroupDoubleCoset{T, elem_type(T)}}(undef, length(dcs))
+   for i in 1:length(res)
+     g = group_element(G, dcs[i][1])
+     C = GroupDoubleCoset(G, H, K, g)
+     n = dcs[i][2]
+     C.size[] = ZZRingElem(n)
+     res[i] = C
    end
-   return res
-   #return [GroupDoubleCoset(G,H,K,group_element(G.X,GAPWrap.Representative(dc)),dc) for dc in dcs]
+   return res 
 end
 
-
 """
-    order(C::Union{GroupCoset,GroupDoubleCoset})
+    order(::Type{T} = ZZRingElem, C::Union{GroupCoset,GroupDoubleCoset})
 
-Return the cardinality of the (double) coset `C`.
+Return the cardinality of the (double) coset `C`,
+as an instance of the type `T`.
 """
-order(C::Union{GroupCoset,GroupDoubleCoset}) = GAPWrap.Size(C.X)
-Base.length(C::Union{GroupCoset,GroupDoubleCoset}) = GAPWrap.Size(C.X)
+order(C::Union{GroupCoset,GroupDoubleCoset}) = order(ZZRingElem, C)
+
+function order(::Type{T}, C::GroupCoset) where T <: IntegerUnion
+  return T(GAPWrap.Size(GapObj(C)))
+end
+
+function order(::Type{T}, C::GroupDoubleCoset) where T <: IntegerUnion
+  if !isassigned(C.size)
+    C.size[] = ZZRingElem(GAPWrap.Size(GapObj(C)))
+  end
+  return T(C.size[])
+end 
+
+Base.length(C::Union{GroupCoset,GroupDoubleCoset}) = order(C)
 
 """
     rand(rng::Random.AbstractRNG = Random.GLOBAL_RNG, C::Union{GroupCoset,GroupDoubleCoset})
@@ -593,7 +618,7 @@ using the random number generator `rng`.
 Base.rand(C::Union{GroupCoset,GroupDoubleCoset}) = Base.rand(Random.GLOBAL_RNG, C)
 
 function Base.rand(rng::Random.AbstractRNG, C::Union{GroupCoset,GroupDoubleCoset})
-  s = GAP.Globals.Random(GAP.wrap_rng(rng), C.X)
+  s = GAPWrap.Random(GAP.wrap_rng(rng), GapObj(C))
   return group_element(C.G, s)
 end
 
@@ -620,7 +645,7 @@ right_acting_group(C::GroupDoubleCoset) = C.K
 
 Base.IteratorSize(::Type{<:GroupDoubleCoset}) = Base.SizeUnknown()
 
-Base.iterate(G::GroupDoubleCoset) = iterate(G, GAPWrap.Iterator(G.X))
+Base.iterate(G::GroupDoubleCoset) = iterate(G, GAPWrap.Iterator(GapObj(G)))
 
 function Base.iterate(G::GroupDoubleCoset, state)
   GAPWrap.IsDoneIterator(state) && return nothing
@@ -634,18 +659,19 @@ end
 Return a vector containing all elements belonging to all groups and cosets
 in `V`.
 """
-function intersect(V::AbstractVector{Union{<: GAPGroup, GroupCoset, GroupDoubleCoset}})
+function Base.intersect(V::AbstractVector{Union{<: GAPGroup, GroupCoset, GroupDoubleCoset}})
    if V[1] isa GAPGroup
       G = V[1]
    else
       G = V[1].G
    end
-   l = GAP.Obj([v.X for v in V])
-   ints = GAP.Globals.Intersection(l)
-   L = Vector{typeof(G)}(undef, length(ints))
+   l = GAP.Obj(V; recursive = true)
+   ints = GAPWrap.Intersection(l)
+   L = Vector{eltype(G)}(undef, length(ints))
    for i in 1:length(ints)
       L[i] = group_element(G,ints[i])
    end
 
    return L
 end
+#TODO:  Can this method get called at all?

src/Groups/perm.jl

@@ -149,6 +149,28 @@ function perm(L::AbstractVector{<:IntegerUnion})
   return PermGroupElem(symmetric_group(length(L)), GAPWrap.PermList(GapObj(L;recursive=true)))
 end
 
+"""
+    smaller_degree_permutation_representation(G::PermGroup) -> PermGroup, map
+  
+Return an isomorphic permutation group of smaller or equal degree
+and the isomorphism from `G` to that group.
+
+# Examples
+```jldoctest
+julia> g = symmetric_group(4);
+
+julia> s, _ = sylow_subgroup(g, 3);
+
+julia> rho = smaller_degree_permutation_representation(s)
+(Permutation group of degree 3 and order 3, Hom: s -> permutation group)
+```
+"""
+function smaller_degree_permutation_representation(G::PermGroup)
+  mp = GAP.Globals.SmallerDegreePermutationRepresentation(GapObj(G))
+  img = PermGroup(GAP.Globals.Image(mp))
+  return img, GAPGroupHomomorphism(G, img, mp)
+end
+
 
 @doc raw"""
     perm(G::PermGroup, L::AbstractVector{<:IntegerUnion})

src/exports.jl

@@ -1395,6 +1395,7 @@ export slpoly_ring
 export small_generating_set, has_small_generating_set, set_small_generating_set
 export small_group
 export small_group_identification, has_small_group_identification
+export smaller_degree_permutation_representation
 export snub_cube
 export snub_dodecahedron
 export socle, has_socle, set_socle

test/Groups/Permutations.jl

@@ -190,3 +190,10 @@ end
     @test cc == [[1, 2, 3], [4, 5], [6, 7], [8, 9, 10], [11, 12, 13, 14, 15]]
   end
 end
+
+@testset "smaller_degree_permuation_group" begin
+  c = cperm(1:3,4:6,7:9)
+  G,_ = sub(symmetric_group(9), [c])
+  H,iso = smaller_degree_permutation_representation(G)
+  @test degree(H)<degree(G)
+end