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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 functions that compute the conjugacy classes of a
## finite group by homomorphic images.
## Literature: A.H: Conjugacy classes in finite permutation groups via
## homomorphic images, MathComp
##
# get classes from classical group if possible.
BindGlobal("ClassesFromClassical",function(G)
local hom,d,cl;
if IsPermGroup(G) and (IsNaturalAlternatingGroup(G) or
IsNaturalSymmetricGroup(G)) then
return ConjugacyClasses(G); # there is a method for this
fi;
if not IsNonabelianSimpleGroup(PerfectResiduum(G)) then
return fail;
fi;
d:=DataAboutSimpleGroup(PerfectResiduum(G));
if d.idSimple.series<>"L" then
return fail;
fi;
hom:=EpimorphismFromClassical(G);
if hom=fail then
return fail;
fi;
# so far matrix classes only implemented for GL/SL
if not (IsNaturalGL(Source(hom)) or IsNaturalSL(Source(hom))) then
return fail;
fi;
cl:=ClassesProjectiveImage(hom);
if HasConjugacyClasses(G) then
cl:=ConjugacyClasses(G); # will have been set
elif G=Image(hom) then
cl:=ConjugacyClasses(Image(hom)); # will have been set
else
Info(InfoWarning,1,"Weird class storage");
return fail;
fi;
return cl;
end);
#############################################################################
##
#F ClassRepsPermutedTuples(<g>,<ran>)
##
## computes representatives of the colourbars with colours selected from
## <ran>.
BindGlobal("ClassRepsPermutedTuples",function(g,ran)
local anz,erg,pat,pat2,sym,nrcomp,coldist,stab,dc,i,j,k,sum,schieb,lstab,
stabs,p;
anz:=NrMovedPoints(g);
sym:=SymmetricGroup(anz);
erg:=[];
stabs:=[];
for nrcomp in [1..anz] do
# all sorted colour distributions
coldist:=Combinations(ran,nrcomp);
for pat in OrderedPartitions(anz,nrcomp) do
Info(InfoHomClass,3,"Pattern: ",pat);
# compute the partition stabilizer
stab:=[];
sum:=0;
for i in pat do
schieb:=MappingPermListList([1..i],[sum+1..sum+i]);
sum:=sum+i;
stab:=Concatenation(stab,
List(GeneratorsOfGroup(SymmetricGroup(i)),j->j^schieb));
od;
stab:=Subgroup(sym,stab);
dc:=List(DoubleCosetRepsAndSizes(sym,stab,g),i->i[1]);
# compute expanded pattern
pat2:=[];
for i in [1..nrcomp] do
for j in [1..pat[i]] do
Add(pat2,i);
od;
od;
for j in dc do
# the new bar's stabilizer
lstab:=Intersection(g,ConjugateSubgroup(stab,j));
p:=Position(stabs,lstab);
if p=fail then
Add(stabs,lstab);
else
lstab:=stabs[p];
fi;
# the new bar
j:=Permuted(pat2,j);
for k in coldist do
Add(erg,[List(j,i->k[i]),lstab]);
od;
od;
od;
od;
return erg;
end);
#############################################################################
##
#F ConjugacyClassesSubwreath(<F>,<M>,<n>,<autT>,<T>,<Lloc>,<comp>,<emb>,<proj>)
##
InstallGlobalFunction(ConjugacyClassesSubwreath,
function(F,M,n,autT,T,Lloc,components,embeddings,projections)
local clT, # classes T
lcl, # Length(clT)
clTR, # classes under other group (autT,centralizer)
fus, # class fusion
sci, # |centralizer_i|
oci, # |reps_i|
i,j,k,l, # loop
pfus, # potential fusion
op, # operation of F on components
ophom, # F -> op
clF, # classes of F
clop, # classes of op
bars, # colour bars
barsi, # partial bars
lallcolors, # |all colors|
reps,Mproj,centralizers,centindex,emb,pi,varpi,newreps,newcent,
newcentindex,centimages,centimgindex,C,p,P,selectcen,select,
cen,eta,newcentlocal,newcentlocalindex,d,dc,s,t,elm,newcen,shift,
cengen,b1,ore,
# as in paper
colourbar,newcolourbar,possiblecolours,potentialbars,bar,colofclass,
clin,clout,
etas, # list of etas
opfun, # operation function
r,rp, # op-element complement in F
cnt,
brp,bcen,
centralizers_r, # centralizers of r
newcent_r, # new list to build
centrhom, # projection \rest{centralizer of r}
localcent_r,# image
cr,
isdirprod, # is just M a direct product
genpos, # generator index
genpos2,
gen, # generator
stab, # stabilizer
stgen, # local stabilizer generators
trans,
repres,
img,
limg,
con,
pf,
orb, # orbit
orpo, # orbit position
minlen, # minimum orbit length
remainlen, #list of remaining lengths
gcd, # gcd of remaining orbit lengths
stabtrue,
diff,
possible,
combl,
smacla,
smare,
ppos,
maxdiff,
cystr,
again, # run orbit again to get all
trymap, # operation to try
skip, # skip (if u=ug)
ug, # u\cap u^{gen^-1}
scj, # size(centralizers[j])
dsz, # Divisors(scj);
pper,
cbs,cbi,
faillim,
failcnt;
Info(InfoHomClass,1,
"ConjugacyClassesSubwreath called for almost simple group of size ",
Size(T));
faillim:=Maximum(100,Size(F)/Size(M));
isdirprod:=Size(M)=Size(autT)^n;
# classes of T
clT:=ClassesFromClassical(T);
if clT=fail then
clT:=ConjugacyClassesByRandomSearch(T);
fi;
clT:=List(clT,i->[Representative(i),Centralizer(i)]);
lcl:=Length(clT);
Info(InfoHomClass,1,"found ",lcl," classes in almost simple");
clTR:=List(clT,i->ConjugacyClass(autT,i[1]));
# possible fusion under autT
fus:=List([1..lcl],i->[i]);
for i in [1..lcl] do
sci:=Size(clT[i][2]);
# we have taken a permutation representation that prolongates to autT!
oci:=CycleStructurePerm(clT[i][1]);
# we have tested already the smaller-# classes
pfus:=Filtered([i+1..lcl],j->CycleStructurePerm(clT[j][1])=oci and
Size(clT[j][2])=sci);
pfus:=Difference(pfus,fus[i]);
if Length(pfus)>0 then
Info(InfoHomClass,3,"possible fusion ",pfus);
for j in pfus do
if clT[j][1] in clTR[i] then
fus[i]:=Union(fus[i],fus[j]);
# fuse the entries
for k in fus[i] do
fus[k]:=fus[i];
od;
fi;
od;
fi;
od;
fus:=Set(fus); # throw out duplicates
colofclass:=List([1..lcl],i->PositionProperty(fus,j->i in j));
Info(InfoHomClass,2,"fused to ",Length(fus)," colours");
# get the allowed colour bars
ophom:=ActionHomomorphism(F,components,OnSets,"surjective");
op:=Image(ophom);
lallcolors:=Length(fus);
bars:=ClassRepsPermutedTuples(op,[1..lallcolors]);
Info(InfoHomClass,1,"classes in normal subgroup");
# inner classes
reps:=[One(M)];
centralizers:=[M];
centindex:=[1];
colourbar:=[[]];
Mproj:=[];
varpi:=[];
for i in [1..n] do
Info(InfoHomClass,1,"component ",i);
barsi:=Set(Immutable(List(bars,j->j[1]{[1..i]})));
emb:=embeddings[i];
pi:=projections[i];
Add(varpi,ActionHomomorphism(M,Union(components{[1..i]}),"surjective"));
Add(Mproj,Image(varpi[i],M));
newreps:=[];
newcent:=[];
newcentindex:=[];
centimages:=[];
centimgindex:=[];
newcolourbar:=[];
etas:=[]; # etas for the centralizers
# fuse centralizers that become the same
for j in [1..Length(centralizers)] do
C:=Image(pi,centralizers[j]);
p:=Position(centimages,C);
if p=fail then
Add(centimages,C);
p:=Length(centimages);
fi;
Add(centimgindex,p);
# #force 'centralizers[j]' to have its base appropriate to the component
# # (this will speed up preimages)
# cen:=centralizers[j];
# d:=Size(cen);
# cen:=Group(GeneratorsOfGroup(cen),());
# StabChain(cen,rec(base:=components[i],size:=d));
# centralizers[j]:=cen;
# etas[j]:=ActionHomomorphism(cen,components[i],"surjective");
od;
Info(InfoHomClass,2,Length(centimages)," centralizer images");
# consider previous centralizers
for j in [1..Length(centimages)] do
# determine all reps belonging to this centralizer
C:=centimages[j];
selectcen:=Filtered([1..Length(centimgindex)],k->centimgindex[k]=j);
Info(InfoHomClass,2,"Number ",j,": ",Length(selectcen),
" previous centralizers to consider");
# 7'
select:=Filtered([1..Length(centindex)],k->centindex[k] in selectcen);
# Determine the addable colours
if i=1 then
possiblecolours:=[1..Length(fus)];
else
possiblecolours:=[];
#for k in select do
# bar:=colourbar[k];
k:=1;
while k<=Length(select)
and Length(possiblecolours)<lallcolors do
bar:=colourbar[select[k]];
potentialbars:=Filtered(bars,j->j[1]{[1..i-1]}=bar);
UniteSet(possiblecolours,
potentialbars{[1..Length(potentialbars)]}[1][i]);
k:=k+1;
od;
fi;
for k in Union(fus{possiblecolours}) do
# double cosets
if Size(C)=Size(T) then
dc:=[One(T)];
else
Assert(2,IsSubgroup(T,clT[k][2]));
Assert(2,IsSubgroup(T,C));
dc:=List(DoubleCosetRepsAndSizes(T,clT[k][2],C),i->i[1]);
fi;
for t in selectcen do
# continue partial rep.
# #force 'centralizers[j]' to have its base appropriate to the component
# # (this will speed up preimages)
# if not (HasStabChainMutable(cen)
# and i<=Length(centralizers)
# and BaseStabChain(StabChainMutable(cen))[1] in centralizers[i])
# then
# d:=Size(cen);
# cen:= Group( GeneratorsOfGroup( cen ), One( cen ) );
# StabChain(cen,rec(base:=components[i],size:=d));
# #centralizers[t]:=cen;
# fi;
cen:=centralizers[t];
if not IsBound(etas[t]) then
if Number(etas)>500 then
for d in
Filtered([1..Length(etas)],i->IsBound(etas[i])){[1..500]} do
Unbind(etas[d]);
od;
fi;
etas[t]:=ActionHomomorphism(cen,components[i],"surjective");
fi;
eta:=etas[t];
select:=Filtered([1..Length(centindex)],l->centindex[l]=t);
Info(InfoHomClass,3,"centralizer nr.",t,", ",
Length(select)," previous classes");
newcentlocal:=[];
newcentlocalindex:=[];
for d in dc do
for s in select do
# test whether colour may be added here
bar:=Concatenation(colourbar[s],[colofclass[k]]);
bar:=ShallowCopy(colourbar[s]);
Add(bar,colofclass[k]);
MakeImmutable(bar);
#if ForAny(bars,j->j[1]{[1..i]}=bar) then
if bar in barsi then
# new representative
elm:=reps[s]*Image(emb,clT[k][1]^d);
if elm in Mproj[i] then
# store the new element
Add(newreps,elm);
Add(newcolourbar,bar);
if i<n then # we only need the centralizer for further
# components
newcen:=ClosureGroup(Lloc,
List(GeneratorsOfGroup(clT[k][2]),g->g^d));
p:=Position(newcentlocal,newcen);
if p=fail then
Add(newcentlocal,newcen);
p:=Length(newcentlocal);
fi;
Add(newcentlocalindex,p);
else
Add(newcentlocalindex,1); # dummy, just for counting
fi;
#else
# Info(InfoHomClass,5,"not in");
fi;
#else
# Info(InfoHomClass,5,bar,"not minimal");
fi;
# end the loops from step 9
od;
od;
Info(InfoHomClass,2,Length(newcentlocalindex),
" new representatives");
if i<n then # we only need the centralizer for further components
# Centralizer preimages
shift:=[];
for l in [1..Length(newcentlocal)] do
P:=PreImage(eta,Intersection(Image(eta),newcentlocal[l]));
p:=Position(newcent,P);
if p=fail then
Add(newcent,P);
p:=Length(newcent);
fi;
shift[l]:=p;
od;
# move centralizer indices to global
for l in newcentlocalindex do
Add(newcentindex,shift[l]);
od;
fi;
# end the loops from step 6,7 and 8
od;
od;
od;
centralizers:=newcent;
centindex:=newcentindex;
reps:=newreps;
colourbar:=newcolourbar;
# end the loop of step 2.
od;
Info(InfoHomClass,1,Length(reps)," classreps constructed");
# allow for faster sorting through color bars
cbs:=ShallowCopy(colourbar);
cbi:=[1..Length(cbs)];
SortParallel(cbs,cbi);
# further fusion among bars
newreps:=[];
Info(InfoHomClass,2,"computing centralizers");
k:=0;
for bar in bars do
k:=k+1;
#Info(InfoHomClass,4,"k-",k);
CompletionBar(InfoHomClass,3,"Color Bars ",k/Length(bars));
b1:=Immutable(bar[1]);
select:=[];
i:=PositionSorted(cbs,b1);
if i<>fail and i<=Length(cbs) and cbs[i]=b1 then
AddSet(select,cbi[i]);
while i<Length(cbs) and cbs[i+1]=b1 do
i:=i+1;
AddSet(select,cbi[i]);
od;
fi;
#Assert(1,select=Filtered([1..Length(reps)],i->colourbar[i]=b1));
if Length(select)>1 then
Info(InfoHomClass,2,"test ",Length(select)," classes for fusion");
fi;
newcentlocal:=[];
for i in [1..Length(select)] do
if not ForAny(newcentlocal,j->reps[select[i]] in j) then
#AH we could also compute the centralizer
C:=Centralizer(F,reps[select[i]]);
Add(newreps,[reps[select[i]],C]);
if i<Length(select) and Size(bar[2])>1 then
# there are other reps with the same bar left and the bar
# stabilizer is bigger than M
if not IsBound(bar[2]!.colstabprimg) then
# identical stabilizers have the same link. Therefore store the
# preimage in them
bar[2]!.colstabprimg:=PreImage(ophom,bar[2]);
fi;
# any fusion would take place in the stabilizer preimage
# we know that C must fix the bar, so it is the centralizer there.
r:=ConjugacyClass(bar[2]!.colstabprimg,reps[select[i]],C);
Add(newcentlocal,r);
fi;
fi;
od;
od;
CompletionBar(InfoHomClass,3,"Color Bars ",false);
Info(InfoHomClass,1,"fused to ",Length(newreps)," inner classes");
clF:=newreps;
clin:=ShallowCopy(clF);
Assert(2,Sum(clin,i->Index(F,i[2]))=Size(M));
clout:=[];
# outer classes
clop:=Filtered(ConjugacyClasses(op),i->Order(Representative(i))>1);
for k in clop do
Info(InfoHomClass,1,"lifting class ",Representative(k));
r:=PreImagesRepresentative(ophom,Representative(k));
# try to make r of small order
rp:=r^Order(Representative(k));
rp:=RepresentativeAction(M,Concatenation(components),
Concatenation(OnTuples(components[1],rp^-1),
Concatenation(components{[2..n]})),OnTuples);
if rp<>fail then
r:=r*rp;
else
Info(InfoHomClass,2,
"trying random modification to get large centralizer");
cnt:=LogInt(Size(autT),2)*10;
brp:=();
bcen:=Size(Centralizer(F,r));
repeat
rp:=Random(M);
cengen:=Size(Centralizer(M,r*rp));
if cengen>bcen then
bcen:=cengen;
brp:=rp;
cnt:=LogInt(Size(autT),2)*10;
else
cnt:=cnt-1;
fi;
until cnt<0;
r:=r*brp;
Info(InfoHomClass,2,"achieved centralizer size ",bcen);
fi;
Info(InfoHomClass,2,"representative ",r);
cr:=Centralizer(M,r);
# first look at M-action
reps:=[One(M)];
centralizers:=[M];
centralizers_r:=[cr];
for i in [1..n] do;
failcnt:=0;
newreps:=[];
newcent:=[];
newcent_r:=[];
opfun:=function(a,m)
return Comm(r,m)*a^m;
end;
for j in [1..Length(reps)] do
scj:=Size(centralizers[j]);
dsz:=0;
centrhom:=ActionHomomorphism(centralizers_r[j],components[i],
"surjective");
localcent_r:=Image(centrhom);
Info(InfoHomClass,4,i,":",j);
Info(InfoHomClass,3,"acting: ",Size(centralizers[j])," minimum ",
Int(Size(Image(projections[i]))/Size(centralizers[j])),
" orbits.");
# compute C(r)-classes
clTR:=[];
for l in clT do
Info(InfoHomClass,4,"DC",Index(T,l[2])," ",Index(T,localcent_r));
dc:=DoubleCosetRepsAndSizes(T,l[2],localcent_r);
clTR:=Concatenation(clTR,List(dc,i->l[1]^i[1]));
od;
orb:=[];
for p in [1..Length(clTR)] do
repres:=PreImagesRepresentative(projections[i],clTR[p]);
if i=1 or isdirprod
or reps[j]*RestrictedPermNC(repres,components[i])
in Mproj[i] then
stab:=Centralizer(localcent_r,clTR[p]);
if Index(localcent_r,stab)<Length(clTR)/10 then
img:=Orbit(localcent_r,clTR[p]);
#ensure Representative is in first position
if img[1]<>clTR[p] then
genpos:=Position(img,clTR[p]);
img:=Permuted(img,(1,genpos));
fi;
else
img:=ConjugacyClass(localcent_r,clTR[p],stab);
fi;
Add(orb,[repres,PreImage(centrhom,stab),img,localcent_r]);
fi;
od;
clTR:=orb;
#was:
#clTR:=List(clTR,i->ConjugacyClass(localcent_r,i));
#clTR:=List(clTR,j->[PreImagesRepresentative(projections[i],
# Representative(j)),
# PreImage(centrhom,Centralizer(j)),
# j]);
# put small classes to the top (to be sure to hit them and make
# large local stabilizers)
SortBy(clTR,x->Size(x[3]));
Info(InfoHomClass,3,Length(clTR)," local classes");
cystr:=[];
for p in [1..Length(clTR)] do
repres:=Immutable(CycleStructurePerm(Representative(clTR[p][3])));
select:=First(cystr,x->x[1]=repres);
if select=fail then
Add(cystr,[repres,[p]]);
else
AddSet(select[2],p);
fi;
od;
cengen:=GeneratorsOfGroup(centralizers[j]);
if Length(cengen)>10 then
cengen:=SmallGeneratingSet(centralizers[j]);
fi;
#cengen:=Filtered(cengen,i->not i in localcent_r);
while Length(clTR)>0 do
# orbit algorithm on classes
stab:=clTR[1][2];
orb:=[clTR[1]];
#repres:=RestrictedPermNC(clTR[1][1],components[i]);
repres:=clTR[1][1];
trans:=[One(M)];
select:=[2..Length(clTR)];
orpo:=1;
minlen:=Size(orb[1][3]);
possible:=false;
stabtrue:=false;
pf:=infinity;
maxdiff:=Size(T);
again:=0;
trymap:=false;
ug:=[];
# test whether we have full orbit and full stabilizer
while Size(centralizers[j])>(Sum(orb,i->Size(i[3]))*Size(stab)) do
genpos:=1;
while genpos<=Length(cengen) and
Size(centralizers[j])>(Sum(orb,i->Size(i[3]))*Size(stab)) do
gen:=cengen[genpos];
skip:=false;
if trymap<>false then
orpo:=trymap[1];
gen:=trymap[2];
trymap:=false;
elif again>0 then
if not IsBound(ug[genpos]) then
ug[genpos]:=Intersection(centralizers_r[j],
ConjugateSubgroup(centralizers_r[j],gen^-1));
fi;
if again<500 and ForAll(GeneratorsOfGroup(centralizers_r[j]),
i->i in ug[genpos])
then
# the random elements will give us nothing new
skip:=true;
else
# get an element not in ug[genpos]
repeat
img:=Random(centralizers_r[j]);
until not img in ug[genpos] or again>=500;
gen:=img*gen;
fi;
fi;
if not skip then
img:=Image(projections[i],opfun(orb[orpo][1],gen));
smacla:=select;
if not stabtrue then
p:=PositionProperty(orb,i->img in i[3]);
ppos:=fail;
else
# we have the stabilizer and thus are only interested in
# getting new elements.
p:=CycleStructurePerm(img);
ppos:=First(First(cystr,x->x[1]=p)[2],
i->i in select and
Size(clTR[i][3])<=maxdiff and img in clTR[i][3]);
if ppos=fail then
p:="ignore"; #to avoid the first case
else
p:=fail; # go to first case
fi;
fi;
if p=fail then
if ppos=fail then
p:=First(select,
i->Size(clTR[i][3])<=maxdiff and img in clTR[i][3]);
if p=fail then
return fail;
fi;
else
p:=ppos;
fi;
RemoveSet(select,p);
Add(orb,clTR[p]);
if trans[orpo]=false then
Add(trans,false);
else
#change the transversal element to map to the representative
con:=trans[orpo]*gen;
limg:=opfun(repres,con);
con:=con*PreImagesRepresentative(centrhom,
RepresentativeAction(localcent_r,
Image(projections[i],limg),
Representative(clTR[p][3])));
Assert(2,Image(projections[i],opfun(repres,con))
=Representative(clTR[p][3]));
Add(trans,con);
for stgen in GeneratorsOfGroup(clTR[p][2]) do
Assert( 2, IsOne( Image( projections[i],
opfun(repres,con*stgen/con)/repres ) ) );
stab:=ClosureGroup(stab,con*stgen/con);
od;
fi;
# compute new minimum length
if Length(select)>0 then
remainlen:=List(clTR{select},i->Size(i[3]));
gcd:=Gcd(remainlen);
diff:=minlen-Sum(orb,i->Size(i[3]));
if diff<0 then
# only go through this if the orbit actually grew
# larger
minlen:=Sum(orb,i->Size(i[3]));
repeat
if dsz=0 then
dsz:=DivisorsInt(scj);
fi;
while not minlen in dsz do
# workaround rare problem -- try again
if First(dsz,i->i>=minlen)=fail then
return ConjugacyClassesSubwreath(
F,M,n,autT,T,Lloc,components,embeddings,projections);
fi;
# minimum gcd multiple to get at least the
# smallest divisor
minlen:=minlen+
(QuoInt((First(dsz,i->i>=minlen)-minlen-1),
gcd)+1)*gcd;
od;
# now try whether we actually can add orbits to make up
# that length
diff:=minlen-Sum(orb,i->Size(i[3]));
Assert(2,diff>=0);
# filter those remaining classes small enough to make
# up the length
smacla:=Filtered(select,i->Size(clTR[i][3])<=diff);
remainlen:=List(clTR{smacla},i->Size(i[3]));
combl:=1;
possible:=false;
if diff=0 then
possible:=fail;
fi;
while gcd*combl<=diff
and combl<=Length(remainlen) and possible=false do
if NrCombinations(remainlen,combl)<100 then
possible:=ForAny(Combinations(remainlen,combl),
i->Sum(i)=diff);
else
possible:=fail;
fi;
combl:=combl+1;
od;
if possible=false then
minlen:=minlen+gcd;
fi;
until possible<>false;
fi; # if minimal orbit length grew
Info(InfoHomClass,5,"Minimum length of this orbit ",
minlen," (",diff," missing)");
fi;
if minlen*Size(stab)=Size(centralizers[j]) then
#Assert(1,Length(smacla)>0);
maxdiff:=diff;
stabtrue:=true;
fi;
elif not stabtrue then
# we have an element that stabilizes the conjugacy class.
# correct this to an element that fixes the representative.
# (As we have taken already the centralizer in
# centralizers_r, it is sufficient to correct by
# centralizers_r-conjugation.)
con:=trans[orpo]*gen;
limg:=opfun(repres,con);
con:=con*PreImagesRepresentative(centrhom,
RepresentativeAction(localcent_r,
Image(projections[i],limg),
Representative(orb[p][3])));
stab:=ClosureGroup(stab,con/trans[p]);
if Size(stab)*2*minlen>Size(centralizers[j]) then
Info(InfoHomClass,3,
"true stabilizer found (cannot grow)");
minlen:=Size(centralizers[j])/Size(stab);
maxdiff:=minlen-Sum(orb,i->Size(i[3]));
stabtrue:=true;
fi;
fi;
if stabtrue then
smacla:=Filtered(select,i->Size(clTR[i][3])<=maxdiff);
if Length(smacla)<pf then
pf:=Length(smacla);
remainlen:=List(clTR{smacla},i->Size(i[3]));
CompletionBar(InfoHomClass,3,"trueorb ",1-maxdiff/minlen);
#Info(InfoHomClass,3,
# "This is the true orbit length (missing ",
# maxdiff,")");
if Size(stab)*Sum(orb,i->Size(i[3]))
=Size(centralizers[j]) then
maxdiff:=0;
elif Sum(remainlen)=maxdiff then
Info(InfoHomClass,2,
"Full possible remainder must fuse");
orb:=Concatenation(orb,clTR{smacla});
select:=Difference(select,smacla);
else
# test whether there is only one possibility to get
# this length
if Length(smacla)<20 and
Sum(List([1..Minimum(Length(smacla),
Int(maxdiff/gcd+1))],
x-> NrCombinations(smacla,x)))<10000 then
# get all reasonable combinations
smare:=[1..Length(smacla)]; #range for smacla
combl:=Concatenation(List([1..Int(maxdiff/gcd+1)],
i->Combinations(smare,i)));
# pick those that have the correct length
combl:=Filtered(combl,i->Sum(remainlen{i})=maxdiff);
if Length(combl)>1 then
Info(InfoHomClass,3,"Addendum not unique (",
Length(combl)," possibilities)");
if (maxdiff<10 or again>0)
and ForAll(combl,i->Length(i)<=5) then
# we have tried often enough, now try to pick the
# right ones
possible:=false;
combl:=Union(combl);
combl:=smacla{combl};
genpos2:=1;
smacla:=[];
while possible=false and Length(combl)>0 do
img:=Image(projections[i],
opfun(clTR[combl[1]][1],cengen[genpos2]));
p:=PositionProperty(orb,i->img in i[3]);
if p<>fail then
# it is!
Info(InfoHomClass,4,"got one!");
# remember the element to try
trymap:=[p,(cengen[genpos2]*
PreImagesRepresentative(
RestrictedMapping(projections[i],
centralizers[j]),
RepresentativeAction(
orb[p][4],
img,Representative(orb[p][3])) ))^-1];
Add(smacla,combl[1]);
combl:=combl{[2..Length(combl)]};
if Sum(clTR{smacla},i->Size(i[3]))=maxdiff then
# bingo!
possible:=true;
fi;
fi;
genpos2:=genpos2+1;
if genpos2>Length(cengen) then
genpos2:=1;
combl:=combl{[2..Length(combl)]};
fi;
od;
if possible=false then
Info(InfoHomClass,4,"Even test failed!");
else
orb:=Concatenation(orb,clTR{smacla});
select:=Difference(select,smacla);
Info(InfoHomClass,3,"Completed orbit (hard)");
fi;
fi;
elif Length(combl)>0 then
combl:=combl[1];
orb:=Concatenation(orb,clTR{smacla{combl}});
select:=Difference(select,smacla{combl});
Info(InfoHomClass,3,"Completed orbit");
fi;
fi;
fi;
fi;
fi;
else
Info(InfoHomClass,5,"skip");
fi; # if not skip
genpos:=genpos+1;
od;
orpo:=orpo+1;
if orpo>Length(orb) then
Info(InfoHomClass,3,"Size factor:",EvalF(
(Sum(orb,i->Size(i[3]))*Size(stab))/Size(centralizers[j])),
" orbit consists of ",Length(orb)," suborbits, iterating");
if stabtrue then
pper:=false;
# we know stabilizer, just need to find orbit. As these are
# likely small additions, search in reverse.
for p in select do
for genpos in [1..Length(cengen)] do
gen:=Random(centralizers_r[j])*cengen[genpos];
img:=Image(projections[i],opfun(clTR[p][1],gen));
orpo:=CycleStructurePerm(img);
ppos:=First(First(cystr,x->x[1]=orpo)[2],
i->not i in select and
img in clTR[i][3]);
if ppos<>fail and p in select then
# so the image is in clTR[ppos] which must be in orb
ppos:=Position(orb,clTR[ppos]);
Info(InfoHomClass,3,"found new orbit addition ",p);
Add(orb,clTR[p]);
# #change the transversal element to map to the representative
# con:=trans[ppos]*RepresentativeAction(localcent_r,
# Representative(orb[ppos][3]),img)/gen;
# if not Image(projections[i],opfun(repres,con))
# =Representative(clTR[p][3]) then
# Error("wrong rep");
# fi;
# Add(trans,con);
# cannot easily do transversal this way.
Add(trans,false);
RemoveSet(select,p);
elif ppos=fail then
pper:=true;
fi;
od;
od;
if pper then
# trap some weird setup where it does not terminate
failcnt:=failcnt+1;
if failcnt>=1000*faillim then
#Error("fail4");
return fail;
fi;
fi;
fi;
orpo:=1;
again:=again+1;
if again>1000*faillim then
return fail;
fi;
fi;
od;
Info(InfoHomClass,2,"Stabsize = ",Size(stab),
", centstabsize = ",Size(orb[1][2]));
clTR:=clTR{select};
# fix index positions
for p in cystr do
p[2]:=Filtered(List(p[2],x->Position(select,x)),IsInt);
od;
Info(InfoHomClass,2,"orbit consists of ",Length(orb)," suborbits,",
Length(clTR)," classes left.");
Info(InfoHomClass,3,List(orb,i->Size(i[2])));
Info(InfoHomClass,4,List(orb,i->Size(i[3])));
# select the orbit element with the largest local centralizer
orpo:=1;
p:=2;
while p<=Length(orb) do
if IsBound(trans[p]) and Size(orb[p][2])>Size(orb[orpo][2]) then
orpo:=p;
fi;
p:=p+1;
od;
if orpo<>1 then
Info(InfoHomClass,3,"switching to orbit position ",orpo);
repres:=opfun(repres,trans[orpo]);
#repres:=RestrictedPermNC(clTR[1][1],repres);
stab:=stab^trans[orpo];
fi;
Assert(2,ForAll(GeneratorsOfGroup(stab),
j -> IsOne( Image(projections[i],opfun(repres,j)/repres) )));
# correct stabilizer to element stabilizer
Add(newreps,reps[j]*RestrictedPermNC(repres,components[i]));
Add(newcent,stab);
Add(newcent_r,orb[orpo][2]);
od;
od;
reps:=newreps;
centralizers:=newcent;
centralizers_r:=newcent_r;
Info(InfoHomClass,2,Length(reps)," representatives");
od;
select:=Filtered([1..Length(reps)],i->reps[i] in M);
reps:=reps{select};
reps:=List(reps,i->r*i);
centralizers:=centralizers{select};
centralizers_r:=centralizers_r{select};
Info(InfoHomClass,1,Length(reps)," in M");
# fuse reps if necessary
cen:=PreImage(ophom,Centralizer(k));
newreps:=[];
newcentlocal:=[];
for i in [1..Length(reps)] do
bar:=CycleStructurePerm(reps[i]);
ore:=Order(reps[i]);
newcentlocal:=Filtered(newreps,
i->Order(Representative(i))=ore and
i!.elmcyc=bar);
if not ForAny(newcentlocal,j->reps[i] in j) then
C:=Centralizer(cen,reps[i]);
# AH can we use centralizers[i] here ?
Add(clF,[reps[i],C]);
Add(clout,[reps[i],C]);
bar:=ConjugacyClass(cen,reps[i],C);
bar!.elmcyc:=CycleStructurePerm(reps[i]);
Add(newreps,bar);
fi;
od;
Info(InfoHomClass,1,"fused to ",Length(newreps)," classes");
od;
if Sum(clout,i->Index(F,i[2]))<>Size(F)-Size(M) then return fail;fi;
Info(InfoHomClass,2,Length(clin)," inner classes, total size =",
Sum(clin,i->Index(F,i[2])));
Info(InfoHomClass,2,Length(clout)," outer classes, total size =",
Sum(clout,i->Index(F,i[2])));
Info(InfoHomClass,3," Minimal ration for outer classes =",
EvalF(Minimum(List(clout,i->Index(F,i[2])/(Size(F)-Size(M)))),30));
Info(InfoHomClass,1,"returning ",Length(clF)," classes");
Assert(2,Sum(clF,i->Index(F,i[2]))=Size(F));
return clF;
end);
InstallGlobalFunction(ConjugacyClassesFittingFreeGroup,function(G)
local cs, # chief series of G
i, # index cs
cl, # list [classrep,centralizer]
hom, # G->G/cs[i]
M, # cs[i-1]
N, # cs[i]
subN, # maximan normal in M over N
csM, # orbit of nt in M under G
n, # Length(csM)
T, # List of T_i
Q, # Action(G,T)
Qhom, # G->Q and F->Q
S, # PreImage(Qhom,Stab_Q(1))
S1, # Action of S on T[1]
deg1, # deg (s1)
autos, # automorphism for action
arhom, # autom permrep list
Thom, # S->S1
T1, # T[1] Thom
w, # S1\wrQ
wbas, # base of w
emb, # embeddings of w
proj, # projections of wbas
components, # components of w
reps, # List reps in G for 1->i in Q
F, # action of G on M/N
Fhom, # G -> F
FQhom, # Fhom*Qhom
genimages,# G.generators Fhom
img, # gQhom
gimg, # gFhom
act, # component permcation to 1
j,k, # loop
clF, # classes of F
ncl, # new classes
FM, # normal subgroup in F, Fhom(M)
FMhom, # M->FM
dc, # double cosets
jim, # image of j
Cim,
CimCl,
p,
l,lj,
l1,
elm,
zentr,
onlysizes,
good,bad,
lastM;
onlysizes:=ValueOption("onlysizes");
# we assume the group has no solvable normal subgroup. Thus we get all
# classes by lifts via nonabelian factors and can disregard all abelian
# factors.
# we will give classes always by their representatives in G and
# centralizers by their full preimages in G.
cs:= ChiefSeriesThrough( G,[Socle(G)] );
# First do socle factor
if Size(Socle(G))=Size(G) then
cl:=[One(G),G];
lastM:=G;
else
lastM:=Socle(G);
# compute the classes of the simple nonabelian factor by random search
hom:=NaturalHomomorphismByNormalSubgroupNC(G,lastM);
cl:=ConjugacyClasses(Image(hom));
cl:=List(cl,i->[PreImagesRepresentative(hom,Representative(i)),
PreImage(hom,StabilizerOfExternalSet(i))]);
cs:=Concatenation([G],Filtered(cs,x->IsSubset(lastM,x)));
fi;
for i in [2..Length(cs)] do
# we assume that cl contains classreps/centralizers for G/cs[i-1]
# we want to lift to G/cs[i]
M:=cs[i-1];
N:=cs[i];
Info(InfoHomClass,1,i,":",Index(M,N),"; ",Size(N));
if HasAbelianFactorGroup(M,N) then
Info(InfoHomClass,2,"abelian factor ignored");
else
# nonabelian factor. Now it means real work.
# 1) compute the action for the factor
# first, we obtain the simple factors T_i/N.
# we get these as intersections of the conjugates of the subnormal
# subgroup
csM:=CompositionSeries(M); # stored attribute
if not IsSubset(csM[2],N) then
# the composition series goes the wrong way. Now take closures of
# its steps with N to get a composition series for M/N, take the
# first proper factor for subN.
n:=3;
subN:=fail;
while n<=Length(csM) and subN=fail do
subN:=ClosureGroup(N,csM[n]);
if Index(M,subN)=1 then
subN:=fail; # still wrong
fi;
n:=n+1;
od;
else
subN:=csM[2];
fi;
if IsNormal(G,subN) then
# only one -> Call standard process
Fhom:=fail;
# is this an almost top factor?
if Index(G,M)<10 then
Thom:=NaturalHomomorphismByNormalSubgroupNC(G,subN);
T1:=Image(Thom,M);
S1:=Image(Thom);
if Size(Centralizer(S1,T1))=1 then
deg1:=NrMovedPoints(S1);
Info(InfoHomClass,2,
"top factor gives conjugating representation, deg ",deg1);
Fhom:=Thom;
fi;
else
Thom:=NaturalHomomorphismByNormalSubgroupNC(M,subN);
T1:=Image(Thom,M);
fi;
if Fhom=fail then
autos:=List(GeneratorsOfGroup(G),
i->GroupHomomorphismByImagesNC(T1,T1,GeneratorsOfGroup(T1),
List(GeneratorsOfGroup(T1),
j->Image(Thom,PreImagesRepresentative(Thom,j)^i))));
# find (probably another) permutation rep for T1 for which all
# automorphisms can be represented by permutations
arhom:=AutomorphismRepresentingGroup(T1,autos);
S1:=arhom[1];
deg1:=NrMovedPoints(S1);
Fhom:=GroupHomomorphismByImagesNC(G,S1,GeneratorsOfGroup(G),arhom[3]);
fi;
F:=Image(Fhom,G);
clF:=ClassesFromClassical(F);
if clF=fail then
clF:=ConjugacyClassesByRandomSearch(F);
fi;
clF:=List(clF,j->[Representative(j),StabilizerOfExternalSet(j)]);
else
csM:=Orbit(G,subN); # all conjugates
n:=Length(csM);
if n=1 then
Error("this cannot happen");
T:=M;
fi;
T:=Intersection(csM{[2..Length(csM)]}); # one T_i
if Length(GeneratorsOfGroup(T))>5 then
T:=Group(SmallGeneratingSet(T));
fi;
T:=Orbit(G,T); # get all the t's
# now T[1] is a complement to csM[1] in G/N.
# now compute the operation of G on M/N
Qhom:=ActionHomomorphism(G,T,"surjective");
Q:=Image(Qhom,G);
S:=PreImage(Qhom,Stabilizer(Q,1));
# find a permutation rep. for S-action on T[1]
Thom:=NaturalHomomorphismByNormalSubgroupNC(T[1],N);
T1:=Image(Thom);
if not IsSubset([1..NrMovedPoints(T1)],
MovedPoints(T1)) then
Thom:=Thom*ActionHomomorphism(T1,MovedPoints(T1),"surjective");
fi;
T1:=Image(Thom,T[1]);
if IsPermGroup(T1) and
NrMovedPoints(T1)>SufficientlySmallDegreeSimpleGroupOrder(Size(T1)) then
Thom:=Thom*SmallerDegreePermutationRepresentation(T1:cheap);
Info(InfoHomClass,1,"reduced simple degree ",NrMovedPoints(T1),
" ",NrMovedPoints(Image(Thom)));
T1:=Image(Thom,T[1]);
fi;
autos:=List(GeneratorsOfGroup(S),
i->GroupHomomorphismByImagesNC(T1,T1,GeneratorsOfGroup(T1),
List(GeneratorsOfGroup(T1),
j->Image(Thom,PreImagesRepresentative(Thom,j)^i))));
# find (probably another) permutation rep for T1 for which all
# automorphisms can be represented by permutations
arhom:=AutomorphismRepresentingGroup(T1,autos);
S1:=arhom[1];
deg1:=NrMovedPoints(S1);
Thom:=GroupHomomorphismByImagesNC(S,S1,GeneratorsOfGroup(S),arhom[3]);
T1:=Image(Thom,T[1]);
# now embed into wreath
w:=WreathProduct(S1,Q);
wbas:=DirectProduct(List([1..n],i->S1));
emb:=List([1..n+1],i->Embedding(w,i));
proj:=List([1..n],i->Projection(wbas,i));
components:=WreathProductInfo(w).components;
# define isomorphisms between the components
reps:=List([1..n],i->
PreImagesRepresentative(Qhom,RepresentativeAction(Q,1,i)));
genimages:=[];
for j in GeneratorsOfGroup(G) do
img:=Image(Qhom,j);
gimg:=Image(emb[n+1],img);
for k in [1..n] do
# look at part of j's action on the k-th factor.
# we get this by looking at the action of
# reps[k] * j * reps[k^img]^-1
# 1 -> k -> k^img -> 1
# on the first component.
act:=reps[k]*j*(reps[k^img]^-1);
# this must be multiplied *before* permuting
gimg:=ImageElm(emb[k],ImageElm(Thom,act))*gimg;
gimg:=RestrictedPermNC(gimg,MovedPoints(w));
od;
Add(genimages,gimg);
od;
F:=Subgroup(w,genimages);
if AssertionLevel()>=2 then
Fhom:=GroupHomomorphismByImages(G,F,GeneratorsOfGroup(G),genimages);
Assert(1,fail<>Fhom);
else
Fhom:=GroupHomomorphismByImagesNC(G,F,GeneratorsOfGroup(G),genimages);
fi;
Info(InfoHomClass,1,"constructed Fhom");
# 2) compute the classes for F
if n>1 then
#if IsPermGroup(F) and NrMovedPoints(F)<18 then
# # the old Butler/Theissen approach is still OK
# clF:=[];
# for j in
# Concatenation(List(RationalClasses(F),DecomposedRationalClass)) do
# Add(clF,[Representative(j),StabilizerOfExternalSet(j)]);
# od;
#else
FM:=F;
for j in components do
FM:=Stabilizer(FM,j,OnSets);
od;
clF:=ConjugacyClassesSubwreath(F,FM,n,S1,
Action(FM,components[1]),T1,components,emb,proj);
if clF=fail then
#Error("failure");
# weird error happened -- redo
j:=Random(SymmetricGroup(MovedPoints(G)));
FM:=List(GeneratorsOfGroup(G),x->x^j);
F:=Group(FM);
SetSize(F,Size(G));
FM:=GroupHomomorphismByImagesNC(G,F,GeneratorsOfGroup(G),FM);
clF:=ConjugacyClassesFittingFreeGroup(F);
clF:=List(clF,x->[PreImagesRepresentative(FM,x[1]),PreImage(FM,x[2])]);
return clF;
fi;
#fi;
else
FM:=Image(Fhom,M);
Info(InfoHomClass,1,
"classes by random search in almost simple group");
clF:=ClassesFromClassical(F);
if clF=fail then
clF:=ConjugacyClassesByRandomSearch(F);
fi;
clF:=List(clF,j->[Representative(j),StabilizerOfExternalSet(j)]);
fi;
fi; # true orbit of T.
Assert(2,Sum(clF,i->Index(F,i[2]))=Size(F));
Assert(2,ForAll(clF,i->Centralizer(F,i[1])=i[2]));
# 3) combine to form classes of sdp
# the length(cl)=1 gets rid of solvable stuff on the top we got ``too
# early''.
if IsSubgroup(N,KernelOfMultiplicativeGeneralMapping(Fhom)) then
Info(InfoHomClass,1,
"homomorphism is faithful for relevant factor, take preimages");
if Size(N)=1 and onlysizes=true then
cl:=List(clF,i->[PreImagesRepresentative(Fhom,i[1]),Size(i[2])]);
else
cl:=List(clF,i->[PreImagesRepresentative(Fhom,i[1]),
PreImage(Fhom,i[2])]);
fi;
else
Info(InfoHomClass,1,"forming subdirect products");
FM:=Image(Fhom,lastM);
FMhom:=RestrictedMapping(Fhom,lastM);
if Index(F,FM)=1 then
Info(InfoHomClass,1,"degenerated to direct product");
ncl:=[];
for j in cl do
for k in clF do
# modify the representative with a kernel elm. to project
# correctly on the second component
elm:=j[1]*PreImagesRepresentative(FMhom,
LeftQuotient(Image(Fhom,j[1]),k[1]));
zentr:=Intersection(j[2],PreImage(Fhom,k[2]));
Assert(3,ForAll(GeneratorsOfGroup(zentr),
i->Comm(i,elm) in N));
Add(ncl,[elm,zentr]);
od;
od;
cl:=ncl;
else
# first we add the centralizer closures and sort by them
# (this allows to reduce the number of double coset calculations)
ncl:=[];
for j in cl do
Cim:=Image(Fhom,j[2]);
CimCl:=Cim;
#CimCl:=ClosureGroup(FM,Cim); # should be unnecessary, as we took
# the full preimage
p:=PositionProperty(ncl,i->i[1]=CimCl);
if p=fail then
Add(ncl,[CimCl,[j]]);
else
Add(ncl[p][2],j);
fi;
od;
Qhom:=NaturalHomomorphismByNormalSubgroupNC(F,FM);
Q:=Image(Qhom);
FQhom:=Fhom*Qhom;
# now construct the sdp's
cl:=[];
for j in ncl do
lj:=List(j[2],i->Image(FQhom,i[1]));
for k in clF do
# test whether the classes are potential mates
elm:=Image(Qhom,k[1]);
if not ForAll(lj,i->RepresentativeAction(Q,i,elm)=fail) then
#l:=Image(Fhom,j[1]);
if Index(F,j[1])=1 then
dc:=[()];
else
dc:=List(DoubleCosetRepsAndSizes(F,k[2],j[1]),i->i[1]);
fi;
good:=0;
bad:=0;
for l in j[2] do
jim:=Image(FQhom,l[1]);
for l1 in dc do
elm:=k[1]^l1;
if Image(Qhom,elm)=jim then
# modify the representative with a kernel elm. to project
# correctly on the second component
elm:=l[1]*PreImagesRepresentative(FMhom,
LeftQuotient(Image(Fhom,l[1]),elm));
zentr:=PreImage(Fhom,k[2]^l1);
zentr:=Intersection(zentr,l[2]);
Assert(3,ForAll(GeneratorsOfGroup(zentr),
i->Comm(i,elm) in N));
Info(InfoHomClass,4,"new class, order ",Order(elm),
", size=",Index(G,zentr));
Add(cl,[elm,zentr]);
good:=good+1;
else
Info(InfoHomClass,5,"not in");
bad:=bad+1;
fi;
od;
od;
Info(InfoHomClass,4,good," good, ",bad," bad of ",Length(dc));
fi;
od;
od;
fi; # real subdirect product
fi; # else Fhom not faithful on factor
# uff. That was hard work. We're finally done with this layer.
lastM:=N;
fi; # else nonabelian
Info(InfoHomClass,1,"so far ",Length(cl)," classes computed");
od;
if Length(cs)<3 then
Info(InfoHomClass,1,"Fitting free factor returns ",Length(cl)," classes");
fi;
Assert( 2, Sum( List( cl, pair -> Size(G) / Size( pair[2] ) ) ) = Size(G) );
return cl;
end);
## Lifting code, using new format and compatible with matrix groups
#############################################################################
##
#F FFClassesVectorSpaceComplement( <N>, <p>, <gens>, <howmuch> )
##
## This function creates a record containing information about a complement
## in <N> to the span of <gens>.
##
# field, dimension, subgenerators (as vectors),howmuch
BindGlobal("FFClassesVectorSpaceComplement",function(field,r, Q,howmuch )
local zero, one, ran, n, nan, cg, pos, i, j, v;
one:=One( field); zero:=Zero(field);
ran:=[ 1 .. r ];
n:=Length( Q ); nan:=[ 1 .. n ];
cg:=rec( matrix :=[ ],
one :=one,
baseComplement:=ShallowCopy( ran ),
commutator :=0,
centralizer :=0,
dimensionN :=r,
dimensionC :=n );
if n = 0 or r = 0 then
cg.inverse:=NullMapMatrix;
cg.projection :=IdentityMat( r, one );
cg.needed :=[];
return cg;
fi;
for i in nan do
cg.matrix[ i ]:=Concatenation( Q[ i ], zero * nan );
cg.matrix[ i ][ r + i ]:=one;
od;
TriangulizeMat( cg.matrix );
pos:=1;
for v in cg.matrix do
while v[ pos ] = zero do
pos:=pos + 1;
od;
RemoveSet( cg.baseComplement, pos );
if pos <= r then cg.commutator :=cg.commutator + 1;
else cg.centralizer:=cg.centralizer + 1; fi;
od;
if howmuch=1 then
return Immutable(cg);
fi;
cg.needed :=[ ];
cg.projection :=IdentityMat( r, one );
# Find a right pseudo inverse for <Q>.
Append( Q, cg.projection );
Q:=MutableTransposedMat( Q );
TriangulizeMat( Q );
Q:=TransposedMat( Q );
i:=1;
j:=1;
while i <= r do
while j <= n and Q[ j ][ i ] = zero do
j:=j + 1;
od;
if j <= n and Q[ j ][ i ] <> zero then
cg.needed[ i ]:=j;
else
# If <Q> does not have full rank, terminate when the bottom row
# is reached.
i:=r;
fi;
i:=i + 1;
od;
if IsEmpty( cg.needed ) then
cg.inverse:=NullMapMatrix;
else
cg.inverse:=Q{ n + ran }
{ [ 1 .. Length( cg.needed ) ] };
cg.inverse:=ImmutableMatrix(field,cg.inverse,true);
fi;
if IsEmpty( cg.baseComplement ) then
cg.projection:=NullMapMatrix;
else
# Find a base change matrix for the projection onto the complement.
for i in [ 1 .. cg.commutator ] do
cg.projection[ i ][ i ]:=zero;
od;
Q:=[ ];
for i in [ 1 .. cg.commutator ] do
Q[ i ]:=cg.matrix[ i ]{ ran };
od;
for i in [ cg.commutator + 1 .. r ] do
Q[ i ]:=ListWithIdenticalEntries( r, zero );
Q[ i ][ cg.baseComplement[ i-r+Length(cg.baseComplement) ] ]
:=one;
od;
cg.projection:=cg.projection ^ Q;
cg.projection:=cg.projection{ ran }{ cg.baseComplement };
cg.projection:=ImmutableMatrix(field,cg.projection,true);
fi;
return cg;
end);
#############################################################################
##
#F VSDecompCentAction( <N>, <h>, <C>, <howmuch> )
##
## Given a homomorphism C -> N, c |-> [h,c], this function determines (a) a
## vector space decomposition N = [h,C] + K with projection onto K and (b)
## the ``kernel'' S < C which plays the role of C_G(h) in lemma 3.1 of
## [Mecky, Neub\"user, Bull. Aust. Math. Soc. 40].
##
BindGlobal("VSDecompCentAction",function( pcgs, h, C, field,howmuch )
local i, tmp, v,x,cg;
i:=One(field);
x:=List( C, c -> ExponentsOfPcElement(pcgs,Comm( h, c ) )*i);
#Print(Number(x,IsZero)," from ",Length(x),"\n");
cg:=FFClassesVectorSpaceComplement(field,Length(pcgs),x,howmuch);
tmp:=[ ];
for i in [ cg.commutator + 1 ..
cg.commutator + cg.centralizer ] do
v:=cg.matrix[ i ];
tmp[ i - cg.commutator ]:=PcElementByExponentsNC( C,
v{ [ cg.dimensionN + 1 ..
cg.dimensionN + cg.dimensionC ] } );
od;
Unbind(cg.matrix);
cg.cNh:=tmp;
return cg;
end);
#############################################################################
##
#F LiftClassesEANonsolvGeneral( <H>,<N>,<NT>,<cl> )
##
BindGlobal("LiftClassesEANonsolvGeneral",
function(Npcgs, cl, hom, pcisom,solvtriv)
local classes, # classes to be constructed, the result
correctingelement,
field, # field over which <N> is a vector space
one,
h, # preimage `cl.representative' under <hom>
cg,
cNh, # centralizer of <h> in <N>
gens, # preimage `Centralizer( cl )' under <hom>
r, # dimension of <N>
ran, # constant range `[ 1 .. r ]'
aff, # <N> as affine space
imgs, M, # generating matrices for affine operation
orb, # orbit of affine operation
rep,# set of classes with canonical representatives
c, i, # loop variables
PPcgs,denomdepths,
correctionfactor,
stabfacgens,
stabfacimg,
stabrad,
gpsz,subsz,solvsz,
b,
fe,
radidx,
comm;# for class correction
correctingelement:=function(h,rep,fe)
local comm;
comm:=Comm( h, fe ) * Comm( rep, fe );
comm:= ExponentsOfPcElement(Npcgs,comm)*one;
ConvertToVectorRep(comm,field);
comm := List(comm * cg.inverse,Int);
comm:=PcElementByExponentsNC
( Npcgs, Npcgs{ cg.needed }, -comm );
fe:=fe*comm;
return fe;
end;
h := cl[1];
field := GF( RelativeOrders( Npcgs )[ 1 ] );
one:=One(field);
PPcgs:=ParentPcgs(NumeratorOfModuloPcgs(Npcgs));
denomdepths:=ShallowCopy(DenominatorOfModuloPcgs(Npcgs)!.depthsInParent);
Add(denomdepths,Length(PPcgs)+1); # one
# Determine the subspace $[h,N]$ and calculate the centralizer of <h>.
#cNh := ExtendedPcgs( DenominatorOfModuloPcgs( N!.capH ),
# VSDecompCentAction( N, h, N!.capH ) );
#oldcg:=KernelHcommaC(Npcgs,h,NumeratorOfModuloPcgs(Npcgs),2);
#cg:=VSDecompCentAction( Npcgs, h, NumeratorOfModuloPcgs(Npcgs),field,2 );
cg:=VSDecompCentAction( Npcgs, h, Npcgs,field,2 );
#Print("complen =",Length(cg.baseComplement)," of ",cg.dimensionN,"\n");
#if Length(Npcgs)>5 then Error("cb"); fi;
cNh:=cg.cNh;
r := Length( cg.baseComplement );
ran := [ 1 .. r ];
# Construct matrices for the affine operation on $N/[h,N]$.
Info(InfoHomClass,4,"space=",Size(field),"^",r);
gens:=Concatenation(cl[2],Npcgs,cl[3]); # all generators
gpsz:=cl[5];
solvsz:=cl[6];
radidx:=Length(Npcgs)+Length(cl[2]);
imgs := [ ];
for c in gens do
M := [ ];
for i in [ 1 .. r ] do
M[ i ] := Concatenation( ExponentsConjugateLayer( Npcgs,
Npcgs[ cg.baseComplement[ i ] ] , c )
* cg.projection, [ Zero( field ) ] );
od;
M[ r + 1 ] := Concatenation( ExponentsOfPcElement
( Npcgs, Comm( h, c ) ) * cg.projection,
[ One( field ) ] );
M:=ImmutableMatrix(field,M);
Add( imgs, M );
od;
if Size(field)^r>3*10^8 then Error("too large");fi;
aff := ExtendedVectors( field ^ r );
# now compute orbits, being careful to get stabilizers in steps
#orbreps:=[];
#stabs:=[];
orb:=OrbitsRepsAndStabsVectorsMultistage(gens{[1..radidx]},
imgs{[1..radidx]},pcisom,solvsz,solvtriv,
gens{[radidx+1..Length(gens)]},
imgs{[radidx+1..Length(gens)]},cl[4],hom,gpsz,OnRight,aff);
classes:=[];
for b in orb do
rep := PcElementByExponentsNC( Npcgs, Npcgs{ cg.baseComplement },
b.rep{ ran } );
#Assert(3,ForAll(GeneratorsOfGroup(stabsub),i->Comm(i,h*rep) in NT));
stabrad:=ShallowCopy(b.stabradgens);
#Print("startdep=",List(stabrad,x->DepthOfPcElement(PPcgs,x)),"\n");
#if ForAny(stabrad,x->Order(x)=1) then Error("HUH3"); fi;
stabfacgens:=b.stabfacgens;
stabfacimg:=b.stabfacimgs;
# correct generators. Partially in Pc Image
if Length(cg.needed)>0 then
stabrad:=List(stabrad,x->correctingelement(h,rep,x));
# must make proper pcgs -- correction does not preserve that
stabrad:=TFMakeInducedPcgsModulo(PPcgs,stabrad,denomdepths);
# we change by radical elements, so the images in the factor don't
# change
stabfacgens:=List(stabfacgens,x->correctingelement(h,rep,x));
fi;
correctionfactor:=Characteristic(field)^Length(cg.needed);
subsz:=b.subsz/correctionfactor;
c := [h * rep,stabrad,stabfacgens,stabfacimg,subsz,
b.stabrsubsz/correctionfactor];
Assert(3,Size(Group(Concatenation(DenominatorOfModuloPcgs(Npcgs),
stabrad,stabfacgens)))=subsz);
Add(classes,c);
od;
return classes;
end);
#############################################################################
##
#F LiftClassesEANonsolvCentral( <H>, <N>, <cl> )
##
# the version for pc groups implicitly uses a pc-group orbit-stabilizer
# algorithm. We can't do this but have to use a more simple-minded
# orbit/stabilizer approach.
BindGlobal("LiftClassesEANonsolvCentral",
function( Npcgs, cl,hom,pcisom,solvtriv )
local classes, # classes to be constructed, the result
field, # field over which <Npcgs> is a vector space
o,
n,r, # dimensions
space,
com,
comms,
mats,
decomp,
gens,
radidx,
stabrad,stabfacgens,stabfacimg,stabrsubsz,relo,orblock,fe,st,
orb,rep,reps,repword,repwords,p,stabfac,img,vp,genum,gpsz,
subsz,solvsz,i,j,
v,
h, # preimage `cl.representative' under <hom>
w, # coefficient vectors for projection along $[h,N]$
c; # loop variable
field := GF( RelativeOrders( Npcgs )[ 1 ] );
h := cl[1];
#reduce:=ReducedPermdegree(C);
#if reduce<>fail then
# C:=Image(reduce,C);
# Info(InfoHomClass,4,"reduced to deg:",NrMovedPoints(C));
# h:=Image(reduce,h);
# N:=ModuloPcgs(SubgroupNC(C,Image(reduce,NumeratorOfModuloPcgs(N))),
# SubgroupNC(C,Image(reduce,DenominatorOfModuloPcgs(N))));
# fi;
# centrality still means that conjugation by c is multiplication with
# [h,c] and that the complement space is generated by commutators [h,c]
# for a generating set {c|...} of C.
n:=Length(Npcgs);
o:=One(field);
stabrad:=Concatenation(cl[2],Npcgs);
radidx:=Length(stabrad);
stabfacgens:=cl[3];
stabfacimg:=cl[4];
gpsz:=cl[5];
subsz:=gpsz;
solvsz:=cl[6];
stabfac:=TrivialSubgroup(Image(hom));
gens:=Concatenation(stabrad,stabfacgens); # all generators
# commutator space basis
comms:=List(gens,c->o*ExponentsOfPcElement(Npcgs,Comm(h,c)));
List(comms,x->ConvertToVectorRep(x,field));
space:=List(comms,ShallowCopy);
TriangulizeMat(space);
space:=Filtered(space,i->not IsZero(i)); # remove spurious columns
com:=BaseSteinitzVectors(IdentityMat(n,field),space);
# decomposition of vectors into the subspace basis
r:=Length(com.subspace);
if r>0 then
# if the subspace is trivial, everything stabilizes
decomp:=Concatenation(com.subspace,com.factorspace)^-1;
decomp:=decomp{[1..Length(decomp)]}{[1..r]};
decomp:=ImmutableMatrix(field,decomp);
# build matrices for the affine action
mats:=[];
for w in comms do
c:=IdentityMat(r+1,o);
c[r+1]{[1..r]}:=w*decomp; # translation bit
c:=ImmutableMatrix(field,c);
Add(mats,c);
od;
#subspace affine enumerator
v:=ExtendedVectors(field^r);
# orbit-stabilizer algorithm solv/nonsolv version
#C := Stabilizer( C, v, v[1],GeneratorsOfGroup(C), mats,OnPoints );
# assume small domain -- so no bother with bitlist
orb:= [v[1]];
reps:=[One(gens[1])];
repwords:=[[]];
stabrad:=[];
stabrsubsz:=Size(solvtriv);
vp:=1;
for genum in [radidx,radidx-1..1] do
relo:=RelativeOrders(pcisom!.sourcePcgs)[
DepthOfPcElement(pcisom!.sourcePcgs,gens[genum])];
img:=orb[1]*mats[genum];
repword:=repwords[vp];
p:=Position(orb,img);
if p=fail then
for j in [1..Length(orb)*(relo-1)] do
img:=orb[j]*mats[genum];
Add(orb,img);
Add(reps,reps[j]*gens[genum]);
Add(repwords,repword);
od;
else
rep:=gens[genum]/reps[p];
Add(stabrad,rep);
stabrsubsz:=stabrsubsz*relo;
fi;
od;
stabrad:=Reversed(stabrad);
Assert(1,solvsz=stabrsubsz*Length(orb));
#nosolvable part
orblock:=Length(orb);
vp:=1;
stabfacgens:=[];
stabfacimg:=[];
while vp<=Length(orb) do
for genum in [radidx+1..Length(gens)] do
img:=orb[vp]*mats[genum];
rep:=reps[vp]*gens[genum];
repword:=Concatenation(repwords[vp],[genum-radidx]);
--> --------------------
--> maximum size reached
--> --------------------
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