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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Alexander Hulpke.
##
## Copyright of GAP belongs to its developers, whose names are too numerous
## to list here. Please refer to the COPYRIGHT file for details.
##
## SPDX-License-Identifier: GPL-2.0-or-later
##
## This file contains the operations for cosets of pc groups
##
#############################################################################
##
#M CanonicalRightCosetElement( <U>, <g> ) . . . . . . . . cce for pcgroups
##
## Main part of the computation of a canonical coset representative in a
## PcGroup. This is done by factoring with the canonical generators of the
## subgroup to set the appropriate exponents to zero. Since the
## representation as an PcWord is "from left to right", we can multiply with
## subgroup elements from _right_, without changing exponents of the
## generators with lower depth (that are supposedly in canonical form yet).
## Since we want _right_ cosets, everything is done with the _inverse_
## elements, which are representatives for the left cosets. The routine
## supposes, that an Cgs has been set up and the relative orders of the
## generators have been computed by the calling routine.
##
InstallMethod(CanonicalRightCosetElement,"Pc",IsCollsElms,
[IsPcGroup,IsObject],0,
function(U,g)
local p,ro,a,d1,d,u,e;
p:=HomePcgs(U);
ro:=RelativeOrders(p);
a:=g^(-1);
d1:=DepthOfPcElement(p,a);
for u in CanonicalPcgsWrtHomePcgs(U) do
d:=DepthOfPcElement(p,u);
if d>=d1 then
e:=ExponentOfPcElement(p,a,d);
a:=a*u^(ro[d]-e);
d1:=DepthOfPcElement(p,a);
fi;
od;
return a^(-1);
end);
#############################################################################
##
#F DoubleCosetsPcGroup( <G>, <L>, <R> ) .. . . . double cosets for Pcgroups
##
## Double Coset calculation for PcGroups, inductive scheme, according to
## Mike Slattery
##
BindGlobal("DoubleCosetsPcGroup",function(G,A,B)
local r,st,nr,nst,ind,sff,f,m,i,j,ao,Npcgs,v,isi,
wbase,neubas,wproj,wg,W,x,mats,U,flip,dr,en,sf,u,
Hpcgs,Upcgs,prime,dim,one,zero,affsp,
wgr,sp,lgf,ll,Aind;
Info(InfoCoset,1,"Affine version");
# if a is small and b large, compute cosets b\G/a and take inverses of the
# representatives: Since we compute stabilizers in B and a chain down to
# A, this is remarkable faster
if 3*Size(A)<2*Size(B) then
m:=B;
B:=A;
A:=m;
flip:=true;
Info(InfoCoset,1,"DoubleCosetFlip");
else
flip:=false;
fi;
# force elementary abelian Series
sp:=PcgsElementaryAbelianSeries(G);
lgf:=IndicesEANormalSteps(sp);
ll:=Length(lgf);
#eas:=[];
#for i in [1..Length(lgf)] do
# Add(eas,Subgroup(G,sp{[lgf[i]..Length(sp)]}));
#od;
r:=[One(G)];
st:=[B];
Aind:=InducedPcgs(sp,A);
for ind in [2..ll] do
Info(InfoCoset,2,"step ",ind);
#kpcgs:=InducedPcgsByPcSequenceNC(sp,sp{[lgf[ind]..Length(sp)]});
#Npcgs:=InducedPcgsByPcSequenceNC(sp,sp{[lgf[ind-1]..Length(sp)]}) mod kpcgs;
Npcgs:=ModuloTailPcgsByList(sp,sp{[lgf[ind-1]..lgf[ind]-1]},
[lgf[ind]..Length(sp)]);
#Hpcgs:=InducedPcgsByGenerators(sp,Concatenation(GeneratorsOfGroup(A),
# kpcgs));
#Hpcgs:=CanonicalPcgs(Hpcgs) mod kpcgs;
Hpcgs:=Filtered(Aind,i->DepthOfPcElement(sp,i)<lgf[ind]);
sff:=SumFactorizationFunctionPcgs(sp,Hpcgs,Npcgs,
#negative depth: clean out
-lgf[ind]);
#fsn:=Factors(Index(eas[ind-1],eas[ind]));
dim:=lgf[ind]-lgf[ind-1];
prime:=RelativeOrders(sp)[lgf[ind-1]];
f:=GF(prime);
one:=One(f);
zero:=Zero(f);
v:= Immutable( IdentityMat(dim,one) );
# compute complement W
if Length(sff.intersection)=0 then
isi:=[];
wbase:=v;
else
isi:=List(sff.intersection,
i->ExponentsOfPcElement(Npcgs,i)*one);
wbase:=BaseSteinitzVectors(v,isi).factorspace;
fi;
if Length(wbase)>0 then
dr:=[1..Length(wbase)]; # 3 for stripping the affine 1
neubas:=Concatenation(wbase, isi );
wproj:=List(neubas^(-1), i -> i{[1..Length(wbase)]} );
wg:=[];
for i in wbase do
Add(wg,PcElementByExponentsNC(Npcgs,i));
od;
W:=false;
nr:=[];
nst:=[];
for i in [1..Length(r)] do
x:=r[i];#FactorAgWord(r[i],fgi);
U:=ConjugateGroup(st[i],x^(-1));
# build matrices
mats:=[];
Upcgs:=InducedPcgs(sp,U);
for u in Upcgs do
m:=[];
for j in wg do
Add(m,Concatenation((ExponentsConjugateLayer(Npcgs,j,u)*one)*wproj,
[zero]));
od;
Add(m,Concatenation((ExponentsOfPcElement(Npcgs,
sff.factorization(u).n)*one)*wproj,[one]));
m:=ImmutableMatrix(prime,m);
Add(mats,m);
od;
# modify later: if U trivial
if Length(mats)>0 then
affsp:=ExtendedVectors(FullRowSpace(f,Length(wg)));
ao:=ExternalSet(U,affsp,Upcgs,mats);
ao:=ExternalOrbits(ao);
ao:=rec(representatives:=List(ao,i->
PcElementByExponentsNC(Npcgs,(Representative(i){dr})*wbase)),
stabilizers:=List(ao,StabilizerOfExternalSet));
else
if W=false then
if Length(wg)=0 then
W:=[One(G)];
else
en:=Enumerator(FullRowSpace(f,Length(wg)));
W:=[];
wgr:=[1..Length(wg)];
for u in en do
Add(W,Product(wgr,j->wg[j]^IntFFE(u[j])));
od;
fi;
fi;
ao:=rec(
representatives:=W,
stabilizers:=List(W,i->U)
);
fi;
for j in [1..Length(ao.representatives)] do
Add(nr,ao.representatives[j]*x);
# we will finally just need the stabilizers size and not the
# stabilizer
if ind<ll then
Add(nst,ConjugateGroup(ao.stabilizers[j],x));
else
Add(nst,ao.stabilizers[j]);
fi;
od;
od;
r:=nr;
st:=nst;
#else
# Print(ind,"\n");
fi;
od;
sf:=Size(A)*Size(B);
for i in [1..Length(r)] do
if flip then
f:=[r[i]^(-1),sf/Size(st[i])];
else
f:=[r[i],sf/Size(st[i])];
fi;
r[i]:=f;
od;
return r;
end);
InstallMethod(DoubleCosetRepsAndSizes,"Pc",true,
[IsPcGroup,IsPcGroup,IsPcGroup],0,
function(G,U,V)
if Size(G)<=500 then
TryNextMethod();
else
return DoubleCosetsPcGroup(G,U,V);
fi;
end);
#############################################################################
##
#R IsRightTransversalPcGroupRep . . . . . . . right transversal of pc group
##
DeclareRepresentation( "IsRightTransversalPcGroupRep", IsRightTransversalRep,
[ "transversal", "canonReps" ] );
#############################################################################
##
#M RightTransversal( <G>, <U> ) . . . . . . . . . for pc groups
##
BindGlobal( "DoRightTransversalPc", function( G, U )
local elements, g, u, e, i,t,depths,gens,p;
t := Objectify( NewType( FamilyObj( G ),
IsList and IsDuplicateFreeList
and IsRightTransversalPcGroupRep ),
rec( group :=G,
subgroup :=U,
canonReps:=[]));
elements := [One(G)];
p := Pcgs( G );
depths:=List( InducedPcgs( p, U ),
i->DepthOfPcElement(p,i));
gens:=Filtered(p, i->not DepthOfPcElement(p,i) in depths);
for g in Reversed(gens ) do
u := One(G);
e := ShallowCopy( elements );
for i in [1..RelativeOrderOfPcElement(p,g)-1] do
u := u * g;
UniteSet( elements, e * u );
od;
od;
Assert(1,Length(elements)=Index(G,U));
t!.transversal:=elements;
return t;
end );
InstallMethod(RightTransversalOp,"pc groups",IsIdenticalObj,
[ IsPcGroup, IsGroup ],0,DoRightTransversalPc);
InstallMethod(RightTransversalOp,"pc groups",IsIdenticalObj,
[ CanEasilyComputePcgs and HasPcgs, IsGroup ],0,DoRightTransversalPc);
InstallMethod(\[\],"for Pc transversals",true,
[ IsList and IsRightTransversalPcGroupRep, IsPosInt ],0,
function(t,num)
return t!.transversal[num];
end );
InstallMethod(AsList,"for Pc transversals",true,
[ IsList and IsRightTransversalPcGroupRep ],0,
function(t)
return t!.transversal;
end );
InstallMethod(PositionCanonical,"RT",IsCollsElms,
[ IsList and IsRightTransversalPcGroupRep,
IsMultiplicativeElementWithInverse ],0,
function(t,elm)
local i;
elm:=CanonicalRightCosetElement(t!.subgroup,elm);
i:=1;
while i<=Length(t) do
if not IsBound(t!.canonReps[i]) then
t!.canonReps[i]:=
CanonicalRightCosetElement(t!.subgroup,t!.transversal[i]);
fi;
if elm=t!.canonReps[i] then
return i;
fi;
i:=i+1;
od;
return fail;
end);
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