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#############################################################################
##
#W grppcfp.gi GAP library Bettina Eick
##
#Y Copyright (C) 1997, Lehrstuhl D für Mathematik, RWTH Aachen, Germany
#Y (C) 1998 School Math and Comp. Sci., University of St Andrews, Scotland
#Y Copyright (C) 2002 The GAP Group
##
## This file contains some functions to convert a pc group into an
## fp group and vice versa.
##
#############################################################################
##
#F PcGroupFpGroup( F )
#F PcGroupFpGroupNC( F )
##
InstallGlobalFunction( PcGroupFpGroup, function( F )
return PolycyclicFactorGroup(
FreeGroupOfFpGroup( F ),
RelatorsOfFpGroup( F ) );
end );
InstallGlobalFunction( PcGroupFpGroupNC, function( F )
return PolycyclicFactorGroupNC(
FreeGroupOfFpGroup( F ),
RelatorsOfFpGroup( F ) );
end );
#############################################################################
##
#F IsomorphismFpGroupByPcgs( pcgs, str )
##
InstallGlobalFunction( IsomorphismFpGroupByPcgs, function( pcgs, str )
local n, F, gens, rels, i, pis, exp, t, h, rel, comm, j, H, phi;
n:=Length(pcgs);
if n=0 then
phi:=GroupHomomorphismByImagesNC(GroupOfPcgs(pcgs),
TRIVIAL_FP_GROUP,[],[]);
SetIsBijective( phi, true );
return phi;
fi;
F := FreeGroup( n, str );
gens := GeneratorsOfGroup( F );
pis := RelativeOrders( pcgs );
rels := [ ];
for i in [1..n] do
# the power
exp := ExponentsOfRelativePower( pcgs, i ){[i+1..n]};
t := One( F );
for h in [i+1..n] do
t := t * gens[h]^exp[h-i];
od;
rel := gens[i]^pis[i] / t;
Add( rels, rel );
# the commutators
for j in [i+1..n] do
comm := Comm( pcgs[j], pcgs[i] );
exp := ExponentsOfPcElement( pcgs, comm ){[i+1..n]};
t := One( F );
for h in [i+1..n] do
t := t * gens[h]^exp[h-i];
od;
rel := Comm( gens[j], gens[i] ) / t;
Add( rels, rel );
od;
od;
H := F / rels;
SetSize(H,Product(RelativeOrders(pcgs)));
phi :=
GroupHomomorphismByImagesNC( GroupOfPcgs(pcgs), H, AsList( pcgs ),
GeneratorsOfGroup( H ) );
SetIsBijective( phi, true );
ProcessEpimorphismToNewFpGroup(phi);
return phi;
end );
#############################################################################
##
#M IsomorphismFpGroupByCompositionSeries( G, str )
##
InstallOtherMethod( IsomorphismFpGroupByCompositionSeries, "pc groups",
true, [IsGroup and CanEasilyComputePcgs,IsString], 0,
function( G,nam )
return IsomorphismFpGroupByPcgs( Pcgs(G), nam );
end);
#############################################################################
##
#O IsomorphismFpGroup( G )
##
InstallOtherMethod( IsomorphismFpGroup, "pc groups",
true, [IsGroup and CanEasilyComputePcgs,IsString], 0,
function( G,nam )
return IsomorphismFpGroupByPcgs( Pcgs( G ), nam);
end );
#############################################################################
##
#O IsomorphismFpGroupByGeneratorsNC( G )
##
InstallMethod(IsomorphismFpGroupByGeneratorsNC,"pcgs",
IsFamFamX,[IsGroup,IsPcgs,IsString],0,
function( G,p,nam )
# this test now is obsolete but extremely cheap.
if Product(RelativeOrders(p))<Size(G) then
Error("pcgs does not generate the group");
fi;
return IsomorphismFpGroupByPcgs( p, nam);
end );
#############################################################################
##
#F InitEpimorphismSQ( F )
##
InstallGlobalFunction( InitEpimorphismSQ, function( F )
local g, gens, r, rels, ng, nr, pf, pn, pp, D, P, M, Q, I, A, G, min,
gensA, relsA, gensG, imgs, prei, i, j, k, l, norm, index, diag, n,genu;
if IsFpGroup(F) then
gens := GeneratorsOfGroup( FreeGroupOfFpGroup( F ) );
ng := Length( gens );
genu:=List(gens,i->GeneratorSyllable(i,1));
genu:=List([1..Maximum(genu)],i->Position(genu,i));
rels := RelatorsOfFpGroup( F );
nr := Length( rels );
# build the relation matrix for the commutator quotient group
M := [];
for i in [ 1..Maximum( nr, ng ) ] do
M[i] := List( [ 1..ng ], i->0 );
if i <= nr then
r := rels[i];
for j in [1..NrSyllables(r)] do
g := GeneratorSyllable(r,j);
k:=genu[g];
M[i][k] := M[i][k] + ExponentSyllable(r,j);
od;
fi;
od;
# compute normal form
norm := NormalFormIntMat( M,15 );
D := norm.normal;
P := norm.rowtrans;
Q := norm.coltrans;
I := Q^-1;
min := Minimum( Length(D), Length(D[1]) );
diag := List( [1..min], x -> D[x][x] );
if ForAny( diag, x -> x = 0 ) then
Info(InfoSQ,1,"solvable quotient is infinite");
return false;
fi;
# compute pc presentation for the finite quotient
n := Filtered( diag, x -> x <> 1 );
n := Length( Flat( List( n, x -> Factors(Integers, x ) ) ) );
A := FreeGroup(IsSyllableWordsFamily, n );
gensA := GeneratorsOfGroup( A );
index := [];
relsA := [];
g := 1;
pf := [];
for i in [ 1..ng ] do
if D[i][i] <> 1 then
index[i] := g;
pf[i] := TransposedMat( Collected( Factors(Integers, D[i][i] ) ) );
pf[i] := rec( factors := pf[i][1],
powers := pf[i][2] );
for j in [ 1..Length( pf[i].factors ) ] do
pn := pf[i].factors[j];
pp := pf[i].powers [j];
for k in [ 1..pp ] do
relsA[g] := [];
relsA[g][g] := gensA[g]^pn;
for l in [ 1..g-1 ] do
relsA[g][l] := gensA[g]^gensA[l]/gensA[g];
od;
if j <> 1 or k <> 1 then
relsA[g-1][g-1] := relsA[g-1][g-1]/gensA[g];
fi;
g := g + 1;
od;
od;
fi;
od;
relsA := Flat( relsA );
A := A / relsA;
# compute corresponding pc group
G := PcGroupFpGroup( A );
gensG := Pcgs( G );
# set up epimorphism F -> A -> G
imgs := [];
for i in [ 1..ng ] do
imgs[i] := One( G );
for j in [ 1..ng ] do
if Q[i][j] <> 0 and D[j][j] <> 1 then
imgs[i] := imgs[i] * gensG[index[j]]^( Q[i][j] mod D[j][j] );
fi;
od;
od;
# compute preimages
prei := [];
for i in [ 1..ng ] do
if D[i][i] <> 1 then
r := One( FreeGroupOfFpGroup( F ) );
for j in [ 1..ng ] do
if imgs[j] <> One( G ) then
r := r * gens[j] ^ ( I[i][j] mod Order( imgs[j] ) );
fi;
od;
g := index[i];
for j in [ 1..Length( pf[i].factors ) ] do
pn := pf[i].factors[j];
pp := pf[i].powers [j];
for k in [ 1..pp ] do
prei[g] := r;
g := g + 1;
r := r ^ pn;
od;
od;
fi;
od;
return rec( source := F,
image := G,
imgs := imgs,
prei := prei );
elif IsMapping(F) then
if IsSurjective(F) and IsWholeFamily(Range(F)) then
return rec(source:=Source(F),
image:=Parent(Image(F)), # parent will replace full group
# with other gens.
imgs:=List(GeneratorsOfGroup(Source(F)),
i->Image(F,i)));
else
# ensure the image group is the whole family
gensG:=Pcgs(Image(F));
G:=GroupByPcgs(gensG);
return rec(source:=Source(F),
image:=G,
imgs:=List(GeneratorsOfGroup(Source(F)),
i->PcElementByExponentsNC(FamilyPcgs(G),
ExponentsOfPcElement(gensG,Image(F,i)))));
fi;
fi;
Error("Syntax!");
end );
#############################################################################
##
#F LiftEpimorphismSQ( epi, M, c )
##
InstallGlobalFunction( LiftEpimorphismSQ, function( epi, M, c )
local F, G, pcgsG, n, H, pcgsH, d, gensf, pcgsN, htil, gtil, mtil,mtilinv,
w, e, g, m, i, A, V, rel, l, v, mats, j, t, mat, k, elms, imgs,
lift, null, vec, new, U, sol, sub, elm, r,tval,tvalp,
ex,pos,i1,genid,rels,reln,stopi;
F := epi.source;
gensf := GeneratorsOfGroup( FreeGroupOfFpGroup( F ) );
r := Length( gensf );
genid:=[];
for i in [1..r] do
genid[GeneratorSyllable(gensf[i],1)]:=i;
od;
d := M.dimension;
G := epi.image;
pcgsG := Pcgs( G );
n := Length( pcgsG );
H := ExtensionNC( G, M, c );
pcgsH := Pcgs( H );
pcgsN := InducedPcgsByPcSequence( pcgsH, pcgsH{[n+1..n+d]} );
htil := pcgsH{[1..n]};
gtil := [];
mtil := [];
mtilinv:=[];
for w in epi.imgs do
e := ExponentsOfPcElement( pcgsG, w );
g := PcElementByExponentsNC( pcgsH, htil, e );
Add( gtil, g );
m := ImmutableMatrix(M.field, IdentityMat( d, M.field ) );
for i in [1..n] do
m := m * M.generators[i]^e[i];
od;
Add( mtil,m);
#Add( mtilinv, ImmutableMatrix(M.field,m^-1 ));
od;
mtilinv:=List(mtil,i->i^-1);
# set up inhom eq
A := List( [1..r*d], x -> [] );
V := [];
# for each relator of G add
rels:=RelatorsOfFpGroup(F);
stopi:=[4,8,15,30,200];
AddSet(stopi,Length(rels));
for reln in [1..Length(rels)] do
if IsInt(reln/100) then
Info(InfoSQ,2,reln);
fi;
rel:=rels[reln];
l := NrSyllables( rel );
# right hand side
# was: v := MappedWord( rel, gensf, gtil );
v:=One(gtil[1]);
for i in [1..l] do
j := genid[GeneratorSyllable(rel,i)];
ex:=ExponentSyllable(rel,i);
if ex<0 then
v:=v/gtil[j]^(-ex);
else
v:=v*gtil[j]^ex;
fi;
od;
v := ExponentsOfPcElement( pcgsN, v ) * One( M.field );
Append( V, v );
# left hand side
mats := ListWithIdenticalEntries( r,
Immutable( NullMat( d, d, M.field ) ) );
# ahulpke, 28-feb-00: it seems to be much more clever, to run
# through this loop backwards. Then `MappedWord' can be replaced by
# a multiplication
# Similarly the iterated calls to `Subword' are very expensive - better
# use the internal syllable indexing
# tval is the product from position i on, tvalp the product from
# position i+1 on (the tval of the last round)
tval:=One(mats[1]);
for i in [l,l-1..1] do
j := genid[GeneratorSyllable(rel,i)];
ex:=ExponentSyllable(rel,i);
if ex<0 then
pos:=false;
ex:=-ex;
else
pos:=true;
fi;
for i1 in [1..ex] do
tvalp:=tval;
if pos then
tval:=mtil[j]*tval;
mat:=tvalp;
mats[j] := mats[j] + mat;
else
tval:=mtilinv[j]*tval;
mat := tval;
mats[j] := mats[j] - mat;
fi;
od;
od;
for i in [1..r] do
for j in [1..d] do
k := d * (i-1) + j;
Append( A[k], mats[i][j] );
od;
od;
# do these tests several times earlier to speed up
if reln in stopi then
sol := SolutionMat( A, V );
# if there is no solution, then there is no lift
if sol=fail then
#T return value should be fail?
if reln<Length(rels) then
Info(InfoSQ,3,"early break:",reln);
fi;
return false;
fi;
fi;
od;
# create lift
elms := [];
for i in [1..r] do
sub := - sol{[d*(i-1)+1..d*i]};
elm := PcElementByExponentsNC( pcgsN, sub );
Add( elms, elm );
od;
imgs := List( [1..r], x -> gtil[x] * elms[x] ) ;
lift := rec( source := F,
image := H,
imgs := imgs );
# in non-split case this is it
if IsRowVector( c ) then return lift; fi;
# otherwise check
U := Subgroup( H, imgs );
if Size( U ) = Size( H )
and c=0 then # c=0 is the ordinary case
return lift;
else
lift:=false; # indicate the lift is no good
fi;
# this is not optimal - see Plesken
null := NullspaceMat( A );
Info(InfoSQ,2,"nullspace dimension:",Length(null));
for vec in null do
new := vec + sol;
elms := [];
for i in [1..r] do
sub := new{[d*(i-1)+1..d*i]};
elm := PcElementByExponentsNC( pcgsN, sub );
Add( elms, elm );
od;
imgs := List( [1..r], x -> gtil[x] * elms[x] );
U := Subgroup( H, imgs );
if Size( U ) = Size( H ) then
if lift<>false then
Info(InfoSQ,2,"found one");
lift:=SubdirProdPcGroups(H,imgs,
lift.image,lift.imgs);
H:=lift[1];
imgs:=lift[2];
fi;
lift := rec( source := F,
image := H,
imgs := imgs );
if c=0 then
return lift;
fi;
fi;
od;
# give up
return lift; # if c=0 this is automatically false
end );
#############################################################################
##
#F BlowUpCocycleSQ( v, K, F )
##
InstallGlobalFunction( BlowUpCocycleSQ, function( v, K, F )
local Q, B, vectors, hlp, i, k;
if F = K then return v; fi;
Q := AsField( K, F );
B := Basis( Q );
vectors:= BasisVectors( B );
hlp := [];
for i in [ 1..Length( v ) ] do
for k in [ 1..Length( vectors ) ] do
Add( hlp, Coefficients( B, v[i] * vectors[k] )[1] );
od;
od;
return hlp;
end );
#############################################################################
##
#F TryModuleSQ( epi, M )
##
InstallGlobalFunction( TryModuleSQ, function( epi, M )
local C, lift, co, cb, cc, r, q, ccpos, ccnum, l, v, qi, c;
# first try a split extension
lift := LiftEpimorphismSQ( epi, M, 0 );
if not IsBool( lift ) then return lift; fi;
# get collector
C := CollectorSQ( epi.image, M.absolutelyIrreducible, true );
# compute the two cocycles
co := TwoCocyclesSQ( C, epi.image, M.absolutelyIrreducible );
# if there is one non split extension, try all mod coboundaries
if 0 < Length(co) then
cb := TwoCoboundariesSQ( C, epi.image, M.absolutelyIrreducible );
# use only those coboundaries which lie in <co>
if 0 < Length(C.avoid) then
cb := SumIntersectionMat( co, cb )[2];
fi;
# convert them into row spaces
if 0 < Length(cb) then
cc := BaseSteinitzVectors( co, cb ).factorspace;
else
cc := co;
fi;
# try all non split extensions
if 0 < Length(cc) then
r := PrimitiveRoot( M.absolutelyIrreducible.field );
q := Size( M.absolutelyIrreducible.field );
# loop over all vectors of <cc>
for ccpos in [ 1 .. Length(cc) ] do
for ccnum in [ 0 .. q^(Length(cc)-ccpos)-1 ] do
v := cc[Length(cc)-ccpos+1];
for l in [ 1 .. Length(cc)-ccpos ] do
qi := QuoInt( ccnum, q^(l-1) );
if qi mod q <> q-1 then
v := v + r^(qi mod q) * cc[l];
fi;
od;
# blow cocycle up
c := BlowUpCocycleSQ( v, M.field,
M.absolutelyIrreducible.field );
# try to lift epimorphism
lift := LiftEpimorphismSQ( epi, M, c);
# return if we have found a lift
if not IsBool( lift ) then return lift; fi;
od;
od;
fi;
fi;
# give up
return false;
end );
#############################################################################
##
#F AllModulesSQ( epi, M )
##
InstallGlobalFunction( AllModulesSQ, function( epi, M,onlyact )
local C, lift, co, cb, cc, r, q, ccpos, ccnum, l, v, qi,
c,all,cnt,total,i,j,iter,sel,dim;
iter:=onlyact<Length(Pcgs(epi.image)); # are we running in iteration?
all:=epi;
if not iter then
# first try a split extension
# the -1 indicates we want *all* sdps
lift := LiftEpimorphismSQ( epi, M, -1 );
if not IsBool( lift ) then
all:=lift;
Info(InfoSQ,2,"semidirect ",Size(all.image)/Size(epi.image)," found");
fi;
fi;
# get collector
dim:=M.absolutelyIrreducible.dimension;
C := CollectorSQ( epi.image, M.absolutelyIrreducible, true );
# compute the two cocycles
co := TwoCocyclesSQ( C, epi.image, M.absolutelyIrreducible );
# if there is one non split extension, try all mod coboundaries
if 0 < Length(co) then
cb := TwoCoboundariesSQ( C, epi.image, M.absolutelyIrreducible );
q:=false;
if iter and Length(cb)>0 then
# we only want those cocycles, which are trivial for the extra
# generators
# find those indices which can have nontrivial cocycles
r:=Length(Pcgs(epi.image));
v:=[1..dim];
sel:=[];
for i in [1..r] do
for j in [1..Minimum(i,onlyact)] do
UniteSet(sel,((i^2-i)/2+j-1)*dim+v);
od;
od;
v:=IdentityMat(Length(co[1]),M.absolutelyIrreducible.field){sel};
v:=ImmutableMatrix(M.absolutelyIrreducible.field,v);
r:=SumIntersectionMat(v,co)[2];
if Length(r)<Length(co) then
Info(InfoSQ,1,"don't need all cocycles/reduced cohomology");
co:=r;
q:=true; # use as flag whether it got changed
fi;
fi;
# use only those coboundaries which lie in <co>
if 0 < Length(C.avoid) or q then
cb := SumIntersectionMat( co, cb )[2];
fi;
# representatives for basis for the 2-cohomology
if 0 < Length(cb) then
cc := BaseSteinitzVectors( co, cb ).factorspace;
else
cc := co;
fi;
# try all non split extensions
if 0 < Length(cc) then
r := PrimitiveRoot( M.absolutelyIrreducible.field );
q := Size( M.absolutelyIrreducible.field );
total:=Int(q^Length(cc)/(q-1)); # approximately
cnt:=0;
# loop over all vectors of <cc>
for ccpos in [ 1 .. Length(cc) ] do
for ccnum in [ 0 .. q^(Length(cc)-ccpos)-1 ] do
cnt:=cnt+1;
if cnt mod 10 =0 then
CompletionBar(InfoSQ,2,"cocycle loop: ",cnt/total);
fi;
v := cc[Length(cc)-ccpos+1];
for l in [ 1 .. Length(cc)-ccpos ] do
qi := QuoInt( ccnum, q^(l-1) );
if qi mod q <> q-1 then
v := v + r^(qi mod q) * cc[l];
fi;
od;
# blow cocycle up
c := BlowUpCocycleSQ( v, M.field,
M.absolutelyIrreducible.field );
# try to lift epimorphism
lift := LiftEpimorphismSQ( epi, M, c);
# return if we have found a lift
if not IsBool( lift ) then
lift:=SubdirProdPcGroups(all.image,all.imgs,
lift.image,lift.imgs);
all:=rec(source:=epi.source,
image:=lift[1],
imgs:=lift[2]);
Info(InfoSQ,2,"locally ",Size(all.image)/Size(epi.image),
" found");
fi;
od;
od;
fi;
CompletionBar(InfoSQ,2,"cocycle loop: ",false);
fi;
# return all lifts
return all;
end );
#############################################################################
##
#F TryLayerSQ( epi, layer )
##
InstallGlobalFunction( TryLayerSQ, function( epi, layer )
local field, dim, reps, rep, lift;
# compute modules for prime
field := GF(layer[1]);
dim := layer[2];
reps := IrreducibleModules( epi.image, field, dim );
reps:=reps[2]; # the actual modules
# loop over the representations
for rep in reps do
lift := TryModuleSQ( epi, rep );
if not IsBool( lift ) then
if not layer[3] or rep.dimension = dim then
return lift;
fi;
fi;
od;
# give up
return false;
end );
#############################################################################
##
#F EAPrimeLayerSQ( epi, prime )
##
InstallGlobalFunction( EAPrimeLayerSQ, function( epi, prime )
local field, dim, rep, lift,all,dims,allmo,mo,start,found,genum,genepi;
# compute modules for prime
field := GF(prime);
start:=epi;
dims:=List(CharacterDegrees(epi.image,prime),i->i[1]);
genum:=Length(Pcgs(epi.image)); # number of generators of the starting
# group. (We need to consider nontrivial
# cocycles only for those elements, as we
# only want to get one layer.)
# build all modules
allmo:=[];
for dim in dims do
rep := IrreducibleModules( epi.image, field, dim );
rep:=rep[2]; # the actual modules
rep:=Filtered(rep,i->i.dimension=dim);
Info(InfoSQ,1,"Dimension ",dim,", ",Length(rep)," modules");
allmo[dim]:=rep;
od;
repeat # extend as long as possible
all:=epi;
genepi:=Length(Pcgs(epi.image));
found:=false;
for dim in dims do
# loop over the representations
for rep in [1..Length(allmo[dim])] do
Info(InfoSQ,2,"Module representative ",dim," #",rep);
mo:=allmo[dim][rep];
# inflate to extra generators
if genum<genepi then
mo:=GModuleByMats(Concatenation(mo.generators,
List([1..genepi-genum],
i->One(mo.generators[1]))),field);
if allmo[dim][rep].absolutelyIrreducible=allmo[dim][rep] then
mo.absolutelyIrreducible:=mo;
else
mo.absolutelyIrreducible:=GModuleByMats(
Concatenation(allmo[dim][rep].absolutelyIrreducible.generators,
List([1..genepi-genum],
i->One(allmo[dim][rep].absolutelyIrreducible.generators[1]))),
allmo[dim][rep].absolutelyIrreducible.field);
fi;
fi;
lift := AllModulesSQ( epi, mo,genum);
if Size(lift.image)>Size(epi.image) then
found:=true;
lift:=SubdirProdPcGroups(all.image,all.imgs,
lift.image,lift.imgs);
all:=rec(source:=epi.source,
image:=lift[1],
imgs:=lift[2]);
Info(InfoSQ,1,"globally ",Size(all.image)/Size(start.image)," found");
fi;
od;
od;
epi:=all;
until not found;
return all;
end );
#############################################################################
##
#F SQ( <F>, <...> ) / SolvableQuotient( <F>, <...> )
##
InstallGlobalFunction( SolvableQuotient, function ( F, primes )
local G, epi, tup, lift, i, found, fac, j, p, iso;
# initialise epimorphism
epi := InitEpimorphismSQ(F);
if epi=false then
if 0 in AbelianInvariants(F) then
Error("Group has infinite abelian quotient");
else
Error("initialization failed");
fi;
fi;
iso := IsomorphismSpecialPcGroup( epi.image );
epi.image := Image( iso );
epi.imgs := List( epi.imgs, x -> Image( iso, x ) );
G := epi.image;
Info(InfoSQ,1,"init done, quotient has size ",Size(G));
# if the commutator factor group is trivial return
if Size( G ) = 1 then return epi; fi;
# if <primes> is a list of tuples, it denotes a chief series
if IsList( primes ) and IsList( primes[1] ) then
Info(InfoSQ,2,"have chief series given");
for tup in primes{[2..Length(primes)]} do
Info(InfoSQ,1,"trying ", tup);
tup[3] := true;
lift := TryLayerSQ( epi, tup );
if IsBool( lift ) then
return epi;
else
epi := ShallowCopy( lift );
iso := IsomorphismSpecialPcGroup( epi.image );
epi.image := Image( iso );
epi.imgs := List( epi.imgs, x -> Image( iso, x ) );
G := epi.image;
fi;
Info(InfoSQ,1,"found quotient of size ", Size(G));
od;
fi;
# if <primes> is a list of primes, we have to use try and error
if IsList( primes ) and IsInt( primes[1] ) then
found := true;
i := 1;
while found and i <= Length( primes ) do
p := primes[i];
tup := [p, 0, false];
Info(InfoSQ,1,"trying ", tup);
lift := TryLayerSQ( epi, tup );
if not IsBool( lift ) then
epi := ShallowCopy( lift );
iso := IsomorphismSpecialPcGroup( epi.image );
epi.image := Image( iso );
epi.imgs := List( epi.imgs, x -> Image( iso, x ) );
G := epi.image;
found := true;
i := 1;
else
i := i + 1;
fi;
Info(InfoSQ,1,"found quotient of size ", Size(G));
od;
fi;
# if <primes> is an integer it is size we want
if IsInt(primes) then
if not IsInt(primes/Size(G)) then
i:=Lcm(primes,Size(G));
Info(InfoWarning,1,"Added extra factor ",i/primes,
" to allow for G/G'");
primes:=i;
fi;
i := primes / Size( G );
found := true;
while i > 1 and found do
fac := Collected( Factors(Integers, i ) );
found := false;
j := 1;
while not found and j <= Length( fac ) do
fac[j][3] := false;
Info(InfoSQ,1,"trying ", fac[j]);
lift := TryLayerSQ( epi, fac[j] );
if not IsBool( lift ) then
epi := ShallowCopy( lift );
iso := IsomorphismSpecialPcGroup( epi.image );
epi.image := Image( iso );
epi.imgs := List( epi.imgs, x -> Image( iso, x ) );
G := epi.image;
found := true;
i := primes / Size( G );
else
j := j + 1;
fi;
Info(InfoSQ,1,"found quotient of size ", Size(G));
od;
od;
fi;
# this is the result - should be G only with setted epimorphism
return epi;
end );
InstallGlobalFunction(EpimorphismSolvableQuotient,function(arg)
local g, sq, hom;
g:=arg[1];
sq:=CallFuncList(SQ,arg);
hom:=GroupHomomorphismByImages(g,sq.image,GeneratorsOfGroup(g),sq.imgs);
return hom;
end);