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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Bettina Eick and 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
##
#############################################################################
##
#F CollectedWordSQ( C, u, v )
##
## The tail of a conjugate i^j (i>j) or a power i^p (i=j) is stored at
## posiition (i^2-i)/2+j
##
InstallGlobalFunction( CollectedWordSQ, function( C, u, v )
local w, p, c, m, g, n, i, j, x, mx, l1, l2, l;
# convert lists in to word/module pair
if IsList(v) then
v := rec( word := v, tail := [] );
fi;
if IsList(u) then
u := rec( word := u, tail := [] );
fi;
# if <v> is trivial return <u>
if 0 = Length(v.word) and 0 = Length(v.tail) then
return u;
fi;
# if <u> is trivial return <v>
if 0 = Length(u.word) and 0 = Length(u.tail) then
return v;
fi;
# if <v> has trivial word but a nontrivial tail add tails
if 0 = Length(v.word) then
u := ShallowCopy(u);
for i in [ 1 .. Length(v.tail) ] do
if IsBound(v.tail[i]) then
if IsBound(u.tail[i]) then
u.tail[i] := u.tail[i] + v.tail[i];
else
u.tail[i] := v.tail[i];
fi;
fi;
od;
return u;
fi;
# unpack <u> into <x>
x := C.list;
n := Length(x);
for i in [ 1 .. n ] do
x[i] := 0;
od;
for i in [ 1, 3 .. Length(u.word)-1 ] do
x[u.word[i]] := u.word[i+1];
od;
# <mx> contains the tail of <x>
mx := ShallowCopy(u.tail);
# get stacks
w := C.wstack;
p := C.pstack;
c := C.cstack;
m := C.mstack;
# put <v> onto the stack
w[1] := v.word;
p[1] := 1;
c[1] := 1;
m[1] := ShallowCopy(v.tail);
# run until the stack is empty
l := 1;
while 0 < l do
# remove next generator from stack
g := w[l][p[l]];
# apply generator to <mx>
for i in [ 1 .. Length(mx) ] do
if IsBound(mx[i]) then
# we use the transposed for technical reasons
mx[i] := C.module[g] * mx[i];
fi;
od;
# raise current exponent
c[l] := c[l]+1;
# if exponent is too big
if w[l][p[l]+1] < c[l] then
# reset exponent
c[l] := 1;
# move position
p[l] := p[l] + 2;
# if position is too big
if Length(w[l]) < p[l] then
# modify tail (add both tails)
l1 := Length(mx);
l2 := Length(m[l]);
for i in [ 1 .. Minimum(l1,l2) ] do
if IsBound(mx[i]) then
if IsBound(m[l][i]) then
mx[i] := mx[i]+m[l][i];
fi;
elif IsBound(m[l][i]) then
mx[i] := m[l][i];
fi;
od;
if l1 < l2 then
for i in [ l1+1 .. l2 ] do
if IsBound(m[l][i]) then
mx[i] := m[l][i];
fi;
od;
fi;
# and unbind word
m[l] := 0;
l := l - 1;
fi;
fi;
# now move generator to correct position
for i in [ n, n-1 .. g+1 ] do
if x[i] <> 0 then
l := l+1;
w[l] := C.relators[i][g];
c[l] := 1;
p[l] := 1;
l1 := (i^2-i)/2+g;
if not l1 in C.avoid then
if IsBound(mx[l1]) then
mx[l1] := C.mone + mx[l1];
else
mx[l1] := C.mone;
fi;
fi;
m[l] := mx;
mx := [];
l2 := [];
if not l1 in C.avoid then
l2[l1] := C.mone;
fi;
for j in [ 2 .. x[i] ] do
l := l+1;
w[l] := C.relators[i][g];
c[l] := 1;
p[l] := 1;
m[l] := l2;
od;
x[i] := 0;
fi;
od;
# raise exponent
x[g] := x[g] + 1;
# and check for overflow
if x[g] = C.orders[g] then
x[g] := 0;
if C.relators[g][g] <> 0 then
l1 := C.relators[g][g];
for i in [ 1, 3 .. Length(l1)-1 ] do
x[l1[i]] := l1[i+1];
od;
fi;
l1 := (g^2+g)/2;
if not l1 in C.avoid then
if IsBound(mx[l1]) then
mx[l1] := C.mone + mx[l1];
else
mx[l1] := C.mone;
fi;
fi;
fi;
od;
# and return result
w := [];
for i in [ 1 .. Length(x) ] do
if x[i] <> 0 then
Add( w, i );
Add( w, x[i] );
fi;
od;
return rec( word := w, tail := mx );
end );
#############################################################################
##
#F CollectorSQ( G, M, isSplit )
##
InstallGlobalFunction( CollectorSQ, function( G, M, isSplit )
local r, pcgs, o, i, j, word, k, Gcoll;
# convert word into gen/exp form
word := function( pcgs, w )
local r, l, k;
if OneOfPcgs( pcgs ) = w then
r := 0;
else
l := ExponentsOfPcElement( pcgs, w );
r := [];
for k in [ 1 .. Length(l) ] do
if l[k] <> 0 then
Add( r, k );
Add( r, l[k] );
fi;
od;
fi;
return r;
end;
# convert relators into list of lists
if IsPcgs( G ) then
pcgs := G;
else
pcgs := Pcgs( G );
fi;
r := [];
o := RelativeOrders( pcgs );
for i in [ 1 .. Length(pcgs) ] do
r[i] := [];
for j in [ 1 .. i-1 ] do
r[i][j] := word( pcgs, pcgs[i]^pcgs[j]);
od;
r[i][i] := word( pcgs, pcgs[i]^o[i] );
od;
# create collector for G
Gcoll := rec( );
Gcoll.relators := r;
Gcoll.orders := o;
# create stacks
Gcoll.wstack := [];
Gcoll.estack := [];
Gcoll.pstack := [];
Gcoll.cstack := [];
Gcoll.mstack := [];
# create collector list
Gcoll.list := List( pcgs, x -> 0 );
# in case we are not interested in the module
if IsBool( M ) then return Gcoll; fi;
# copy collector and add module generators
r := ShallowCopy(Gcoll);
# create module gens (the transposed is a technical detail)
r.module:=List(M.generators,i->ImmutableMatrix(M.field,TransposedMat(i)));
r.mone :=ImmutableMatrix(M.field,(IdentityMat(M.dimension, M.field)));
r.mzero :=ImmutableMatrix(M.field,Zero(r.mone));
# add avoid
r.avoid := [];
if isSplit then
k := Characteristic( M.field );
for i in [ 1 .. Length(r.orders) ] do
for j in [ 1 .. i ] do
# we can avoid
if Order(pcgs[i]) mod k <>0 and Order(pcgs[j]) mod k <>0 then
# was: r.orders[i] <> k and r.orders[j] <> k then
AddSet( r.avoid, (i^2-i)/2 + j );
fi;
od;
od;
fi;
# and return collector
return r;
end );
#############################################################################
##
#F AddEquationsSQ( eq, t1, t2 )
##
InstallGlobalFunction( AddEquationsSQ, function( eq, t1, t2 )
local i, j, l, v, w, x, n, c;
# if <t1> = <t2> return
if t1 = t2 then return; fi;
# compute <t1> - <t2>
t1 := ShallowCopy(t1);
for i in [ 1 .. Length(t2) ] do
if IsBound(t2[i]) then
if IsBound(t1[i]) then
t1[i] := t1[i] - t2[i];
else
t1[i] := -t2[i];
fi;
fi;
od;
# make lines
l := List( eq.vzero, x -> [] );
v := [];
for i in [ 1 .. Length(t1) ] do
if IsBound(t1[i]) then
for j in [ 1 .. eq.dimension ] do
if t1[i][j] <> eq.vzero then
l[j][i] := ShallowCopy(t1[i][j]);
AddCoeffs( l[j][i], v );
ShrinkRowVector(l[j][i]);
fi;
od;
fi;
od;
# and reduce lines
n := eq.dimension;
for j in [ 1 .. n ] do
x := l[j];
v := Length(x);
if 0 < v then w := (v-1)*n + Length(x[v]); fi;
while 0 < v and IsBound(eq.system[w]) do
c := -Last(x[v]);
for i in eq.spos[w] do
if IsBound(x[i]) then
x[i] := ShallowCopy( x[i] );
AddCoeffs( x[i], eq.system[w][i], c );
ShrinkRowVector(x[i]);
if 0 = Length(x[i]) then
Unbind(x[i]);
fi;
else
x[i] := c * eq.system[w][i];
fi;
od;
v := Length(x);
if 0 < v then w := (v-1)*n + Length(x[v]); fi;
od;
if 0 < v then
eq.system[w] := x * (1/Last(x[v]));
eq.spos[w] := Filtered( [1..eq.nrels], t -> IsBound(x[t]) );
fi;
od;
end );
#############################################################################
##
#F SolutionSQ( C, eq )
##
InstallGlobalFunction( SolutionSQ, function( C, eq )
local x, e, d, t, j, v, i, n, p, w;
# construct null vector
n := [];
for i in [ 1 .. eq.nrels-Length(C.avoid) ] do
Append( n, eq.vzero );
od;
# generated position
C.unavoidable := [];
j := 1;
for i in [ 1 .. eq.nrels ] do
if not i in C.avoid then
C.unavoidable[i] := j;
j := j+1;
fi;
od;
# blow up vectors
t := [];
w := [];
for j in [ 1 .. Length(eq.system) ] do
if IsBound(eq.system[j]) then
v := ShallowCopy(n);
for i in eq.spos[j] do
if not i in C.avoid then
p := eq.dimension*(C.unavoidable[i]-1);
v{[p+1..p+Length(eq.system[j][i])]} := eq.system[j][i];
fi;
od;
ShrinkRowVector(v);
t[Length(v)] := v;
AddSet( w, Length(v) );
fi;
od;
# normalize system
v := 0*eq.vzero[1];
for i in w do
for j in w do
if j > i then
p := t[j][i];
if p <> v then
t[j] := ShallowCopy( t[j] );
AddCoeffs( t[j], t[i], -p );
ShrinkRowVector(t[j]);
fi;
fi;
od;
od;
# compute homogeneous solution
d := Difference( [ 1 .. (eq.nrels-Length(C.avoid))*eq.dimension ], w );
v := [];
e := eq.vzero[1]^0;
for i in d do
x := ShallowCopy(n);
x[i] := e;
for j in w do
if j >= i then
x[j] := -t[j][i];
fi;
od;
Add( v, x );
od;
if 0 = Length(C.avoid) then
return v;
fi;
# construct null vector
n := [];
for i in [ 1 .. eq.nrels ] do
Append( n, eq.vzero );
od;
# construct a blow up matrix
i := [];
for j in [ 1 .. eq.nrels ] do
if not j in C.avoid then
Append( i, (j-1)*eq.dimension + [ 1 .. eq.dimension ] );
fi;
od;
# blowup the vectors
e := [];
for x in v do
d := ShallowCopy(n);
d{i} := x;
Add( e, d );
od;
# and return
return e;
end );
#############################################################################
##
#F TwoCocyclesSQ( C, G, M )
##
InstallGlobalFunction( TwoCocyclesSQ, function( C, G, M )
local pairs, i, j, k, w1, w2, eq, p, n;
# get number of generators
n := Length(Pcgs(G));
# collect equations in <eq>
eq := rec( vzero := C.mzero[1],
mzero := C.mzero,
dimension := Length(C.mzero),
nrels := (n^2+n)/2,
spos := [],
system := [] );
# precalculate (ij) for i > j
pairs := List( [1..n], x -> [] );
for i in [ 2 .. n ] do
for j in [ 1 .. i-1 ] do
pairs[i][j] := CollectedWordSQ( C, [i,1], [j,1] );
od;
od;
# consistency 1: k(ji) = (kj)i
for i in [ n, n-1 .. 1 ] do
for j in [ n, n-1 .. i+1 ] do
for k in [ n, n-1 .. j+1 ] do
w1 := CollectedWordSQ( C, [k,1], pairs[j][i] );
w2 := CollectedWordSQ( C, pairs[k][j], [i,1] );
if w1.word <> w2.word then
Error( "k(ji) <> (kj)i" );
else
AddEquationsSQ( eq, w1.tail, w2.tail );
fi;
od;
od;
od;
# consistency 2: j^(p-1) (ji) = j^p i
for i in [ n, n-1 .. 1 ] do
for j in [ n, n-1 .. i+1 ] do
p := C.orders[j];
w1 := CollectedWordSQ( C, [j,p-1],
CollectedWordSQ( C, [j,1], [i,1] ) );
w2 := CollectedWordSQ( C, CollectedWordSQ( C, [j,p-1], [j,1] ),
[i,1] );
if w1.word <> w2.word then
Error( "j^(p-1) (ji) <> j^p i" );
else
AddEquationsSQ( eq, w1.tail, w2.tail );
fi;
od;
od;
# consistency 3: k (i i^(p-1)) = (ki) i^p-1
for i in [ n, n-1 .. 1 ] do
p := C.orders[i];
for k in [ n, n-1 .. i+1 ] do
w1 := CollectedWordSQ( C, [k,1],
CollectedWordSQ( C, [i,1], [i,p-1] ) );
w2 := CollectedWordSQ( C, CollectedWordSQ( C, [k,1], [i,1] ),
[i,p-1] );
if w1.word <> w2.word then
Error( "k i^p <> (ki) i^(p-1)" );
else
AddEquationsSQ( eq, w1.tail, w2.tail );
fi;
od;
od;
# consistency 4: (i i^(p-1)) i = i (i^(p-1) i)
for i in [ n, n-1 .. 1 ] do
p := C.orders[i];
w1 := CollectedWordSQ( C, CollectedWordSQ( C, [i,1], [i,p-1] ),
[i,1] );
w2 := CollectedWordSQ( C, [i,1],
CollectedWordSQ( C, [i,p-1], [i,1] ) );
if w1.word <> w2.word then
Error( "i i^p-1 <> i^p" );
else
AddEquationsSQ( eq, w1.tail, w2.tail );
fi;
od;
# and return solution
return SolutionSQ( C, eq );
end );
#############################################################################
##
#F TwoCoboundariesSQ( C, G, M )
##
InstallGlobalFunction( TwoCoboundariesSQ, function( C, G, M )
local n, R, MI, j, i, x, m, e, k, r, d;
# start with zero matrix
n := Length(Pcgs( G ));
R := [];
r := n*(n+1)/2;
for i in [ 1 .. n ] do
R[i] := [];
for j in [ 1 .. r ] do
R[i][j] := C.mzero;
od;
od;
# compute inverse generators
M := M.generators;
MI := List( M, x -> x^-1 );
d := Length(M[1]);
# loop over all relators
for j in [ 1 .. n ] do
for i in [ j .. n ] do
x := (i^2-i)/2 + j;
# power relator
if i = j then
m := C.mone;
for e in [ 1 .. C.orders[i] ] do
R[i][x] := R[i][x] - m; m := M[i] * m;
od;
# conjugate
else
R[i][x] := R[i][x] - M[j];
R[j][x] := R[j][x] + MI[j]*M[i]*M[j] - C.mone;
fi;
# compute fox derivatives
m := C.mone;
r := C.relators[i][j];
if r <> 0 then
for k in [ Length(r)-1, Length(r)-3 .. 1 ] do
for e in [ 1 .. r[k+1] ] do
R[r[k]][x] := R[r[k]][x] + m;
m := M[r[k]] * m;
od;
od;
fi;
od;
od;
# make one list
m := [];
r := n*(n+1)/2;
for i in [ 1 .. n ] do
for k in [ 1 .. d ] do
e := [];
for j in [ 1 .. r ] do
Append( e, R[i][j][k] );
od;
Add( m, e );
od;
od;
# compute a base for <m>
return BaseMat(m);
end );
#############################################################################
##
#F TwoCohomologySQ( C, G, M )
##
InstallGlobalFunction( TwoCohomologySQ, function( C, G, M )
local cc, cb;
cc := TwoCocyclesSQ( C, G, M );
if Length( cc ) > 0 then
cb := TwoCoboundariesSQ( C, G, M );
if Length( C.avoid ) > 0 then
cb := SumIntersectionMat( cc, cb )[2];
fi;
if Length( cb ) > 0 then
cc := BaseSteinitzVectors( cc, cb ).factorspace;
fi;
fi;
return cc;
end );
#############################################################################
##
#M TwoCocycles( G, M )
##
InstallMethod( TwoCocycles,
"generic method for pc groups",
true,
[ IsPcGroup, IsObject ],
0,
function( G, M )
local C;
C := CollectorSQ( G, M, false );
return TwoCocyclesSQ( C, G, M );
end );
#############################################################################
##
#M TwoCoboundaries( G, M )
##
InstallMethod( TwoCoboundaries,
"generic method for pc groups",
true,
[ IsPcGroup, IsObject ],
0,
function( G, M )
local C;
C := CollectorSQ( G, M, false );
return TwoCoboundariesSQ( C, G, M );
end );
# non-solvable cohomology based on rewriting
#############################################################################
##
#M TwoCohomology( G, M )
##
InstallMethod( TwoCohomology,
"generic method for pc groups",
true,
[ IsPcGroup, IsObject ],
0,
function( G, M )
local C, d, z, co, cb;
C := CollectorSQ( G, M, false );
d := Length( C.orders );
d := d * (d+1) / 2;
z := Flat( List( [1..d], x -> C.mzero[1] ) );
co := TwoCocyclesSQ( C, G, M );
co := VectorSpace( M.field, co, z );
cb := TwoCoboundariesSQ( C, G, M );
cb := SubspaceNC( co, cb );
return rec( group := G,
module := M,
collector := C,
isPcCohomology:=true,
cohom :=
NaturalHomomorphismBySubspaceOntoFullRowSpace(co,cb),
presentation := FpGroupPcGroupSQ( G ) );
end );
# code for generic 2-cohomology (solvable not required)
InstallMethod( TwoCohomologyGeneric,"generic, using rewriting system",true,
[IsGroup and IsFinite,IsObject],0,
function(G,mo)
local field,fp,fpg,gens,hom,mats,fm,mon,tzrules,dim,rules,eqs,i,j,k,l,o,l1,
len1,l2,m,start,formalinverse,hastail,one,zero,new,v1,v2,collectail,
findtail,colltz,mapped,mapped2,onemat,zerovec,mal,p,genkill,
c,nvars,htpos,zeroq,r,ogens,bds,model,q,pre,pcgs,miso,ker,solvec,rulpos,
nonone,olen,dag;
# collect the word in factor group
colltz:=function(a)
local i,p;
# collect from left
i:=1;
while i<=Length(a) do
# does a rule apply at position i?
p:=RuleAtPosKBDAG(dag,a,i);
if IsInt(p) then
a:=Concatenation(a{[1..i-1]},tzrules[p][2],
a{[i+Length(tzrules[p][1])..Length(a)]});
i:=Maximum(0,i-mal); # earliest which could be affected
fi;
i:=i+1;
od;
return a;
end;
# matrix corresponding to monoid word
mapped:=function(list)
local a,i;
a:=onemat;
for i in list do
if i in nonone then
a:=a*mats[i];
fi;
od;
return a;
end;
# normalform word and collect the tails
collectail:=function(wrd)
local v,tail,i,p;
v:=List(rules,x->zero);
# collect from left
i:=1;
while i<=Length(wrd) do
# does a rule apply at position i?
p:=RuleAtPosKBDAG(dag,wrd,i);
if IsInt(p) and rulpos[p]<>fail then
p:=rulpos[p];
tail:=wrd{[i+Length(rules[p][1])..Length(wrd)]};
wrd:=Concatenation(wrd{[1..i-1]},rules[p][2],tail);
if p in hastail then
if IsIdenticalObj(v[p],zero) then
v[p]:=mapped(tail);
else
v[p]:=v[p]+mapped(tail);
fi;
fi;
i:=Maximum(0,i-mal); # earliest which could be affected
fi;
i:=i+1;
od;
return [wrd,v];
end;
field:=mo.field;
ogens:=GeneratorsOfGroup(G);
# if false then
# # old general KB code, left for debugging
# fp:=IsomorphismFpGroup(G);
# fpg:=Range(fp);
# fm:=IsomorphismFpMonoid(fpg);
# mon:=Range(fm);
#
# if IsBound(mon!.confl) then
# tzrules:=mon!.confl;
# else
# kb:=KnuthBendixRewritingSystem(mon);
# MakeConfluent(kb);
# tzrules:=kb!.tzrules;
# mon!.confl:=tzrules;
# fi;
# else
# new approach with RWS from chief series
mon:=ConfluentMonoidPresentationForGroup(G);
fp:=mon.fphom;
fpg:=Range(fp);
fm:=mon.monhom;
mon:=Range(fm);
tzrules:=List(RelationsOfFpMonoid(mon),x->List(x,LetterRepAssocWord));
# fi;
dag:=EmptyKBDAG(Union(List(GeneratorsOfMonoid(FreeMonoidOfFpMonoid(mon)),
LetterRepAssocWord)));
mal:=Maximum(List(tzrules,x->Length(x[1])));
for i in [1..Length(tzrules)] do
AddRuleKBDAG(dag,tzrules[i][1],i);
od;
gens:=List(GeneratorsOfGroup(FamilyObj(fpg)!.wholeGroup),
x->PreImagesRepresentative(fp,x));
hom:=GroupHomomorphismByImagesNC(G,Group(mo.generators),
GeneratorsOfGroup(G),mo.generators);
mo:=GModuleByMats(List(gens,x->ImagesRepresentative(hom,x)),mo.field); # new gens
l1:=GeneratorsOfGroup(fpg);
l1:=Concatenation(l1,List(l1,Inverse));
formalinverse:=[];
for i in l1 do
j:=LetterRepAssocWord(UnderlyingElement(ImagesRepresentative(fm,i)));
o:=LetterRepAssocWord(UnderlyingElement(ImagesRepresentative(fm,i^-1)));
if Length(j)<>1 or Length(o)<>1 then Error("length!");fi;
formalinverse[j[1]]:=o[1];
od;
# rules that describe formal inverses, or delete generators of order 2 get no
# tails.
hastail:=[];
rules:=[];
genkill:=[]; # relations that kill generators. Needed for presenation.
for r in tzrules do
if Length(r[1])>=2 then
Add(rules,r);
if Length(r[1])>2 or
(Length(r[1])=2 and (Length(r[2])>0 or formalinverse[r[1][1]]<>r[1][2]))
then
AddSet(hastail,Length(rules));
fi;
else
# Length of r[1] is 1. That is, this generator is not used!
m:=First(RelationsOfFpMonoid(mon),x->List(x,LetterRepAssocWord)=r);
m:=List(m,x->PreImagesRepresentative(fm,ElementOfFpMonoid(FamilyObj(One(mon)),x)));
m:=List(m,UnderlyingElement); # free group elements/words
if not IsOne(m[1]*Subword(m[2],1,1)) then
# Does the relation make a generator redundant (by expressing it in the
# other gens)? If so, remember relation as needed to kill this generator, but
# no influence on Cohomology calculation (which just uses the rest)
if m[1] in GeneratorsOfGroup(FreeGroupOfFpGroup(FamilyObj(fpg)!.wholeGroup))
then Add(genkill,r);
fi;
fi;
fi;
od;
rulpos:=List(tzrules,x->PositionProperty(rules,y->y[1]=x[1]));
htpos:=List([1..Length(rules)],x->Position(hastail,x));
model:=ValueOption("model");
if model<>fail then
q:=GQuotients(model,G)[1];
pre:=List(gens,x->PreImagesRepresentative(q,x));
ker:=KernelOfMultiplicativeGeneralMapping(q);
pcgs:=Pcgs(ker);
l1:=GModuleByMats(LinearActionLayer(Group(pre),pcgs),mo.field);
MTX.IsIrreducible(mo);
miso:=MTX.Isomorphism(mo,l1);
new:=List(miso,x->PcElementByExponents(pcgs,x));
pcgs:=PcgsByPcSequence(FamilyObj(One(model)),new);
# now calculate the vector
solvec:=[];
one:=One(model);
m:=GroupGeneralMappingByImagesNC(fpg,model,GeneratorsOfGroup(fpg),pre);
mats:=List(GeneratorsOfMonoid(mon),
x->ImagesRepresentative(m,
PreImagesRepresentative(fm,x))); #Elements for monoid generators
nonone:=[1..Length(mats)];
pre:=mats;
onemat:=One(G);
for i in [1..Length(hastail)] do
r:=rules[hastail[i]];
m:=LeftQuotient(mapped(r[2]),mapped(r[1]));
m:=ExponentsOfPcElement(pcgs,m);
solvec:=Concatenation(solvec,m*One(field));
od;
fi;
onemat:=One(mo.generators[1]);
zerovec:=Zero(onemat[1]);
mats:=List(GeneratorsOfMonoid(mon),
x->ImagesRepresentative(hom,PreImagesRepresentative(fp,
PreImagesRepresentative(fm,x)))); # matrices for monoid generators
one:=One(mats[1]);
nonone:=Filtered([1..Length(mats)],x->not IsOne(mats[x]));
zero:=zerovec;
dim:=Length(zero);
nvars:=dim*Length(hastail); #Number of variables
#rk:=0;
zeroq:=ImmutableVector(field,ListWithIdenticalEntries(nvars,Zero(field)));
eqs:=MutableBasis(field,[],zeroq);
olen:=[-1,0];
for i in [1..Length(rules)] do
Info(InfoCoh,1,"First rule ",i,", ",Length(BasisVectors(eqs))," equations");
l1:=rules[i][1];
len1:=Length(l1);
for j in [1..Length(rules)] do
l2:=rules[j][1];
m:=Minimum(len1,Length(l2));
for o in [1..m-1] do # possible overlap Length
start:=len1-o;
if ForAll([1..o],k->l1[start+k]=l2[k]) then
# apply l1 first
new:=Concatenation(rules[i][2],l2{[o+1..Length(l2)]});
c:=collectail(new);
v1:=c[2];c:=c[1];
if i in hastail then
v1[i]:=v1[i]+mapped(l2{[o+1..Length(l2)]});
fi;
# apply l2 first
new:=Concatenation(l1{[1..len1-o]},rules[j][2]);
v2:=collectail(new);
if c<>v2[1] then Error("different in factor");
#else Print("Both reduce to ",c,"\n");
fi;
v2:=v2[2];
if j in hastail then
v2[j]:=v2[j]+one;
fi;
if v1<>v2 then # If entries stay zero they are identical (and thus test cheaply)
c:=List([1..dim],x->ShallowCopy(zeroq));
for k in hastail do
if v1[k]<>v2[k] then
new:=TransposedMat(v1[k]-v2[k]);
r:=(htpos[k]-1)*dim+[1..dim];
for l in [1..dim] do
if not IsZero(new[l]) then
c[l]{r}:=new[l];
fi;
od;
fi;
od;
for k in c do
if model<>fail and not IsZero(solvec*k) then
Error("model does not fit");
fi;
#AddSet(eqs,ImmutableVector(field,k));
#k:=SiftedVector(eqs,ImmutableVector(field,k));
#Add(alleeqs,ImmutableVector(field,k));
k:=SiftedVector(eqs,k);
if not IsZero(k) then
CloseMutableBasis(eqs,ImmutableVector(field,k));
fi;
od;
fi;
fi;
od;
od;
Add(olen,Length(BasisVectors(eqs)));
if Length(olen)>3 and
# if twice stayed the same after increase
olen[Length(olen)]=olen[Length(olen)-2] and
olen[Length(olen)]<>olen[Length(olen)-3] then
k:=List(BasisVectors(eqs),ShallowCopy);;
TriangulizeMat(k);
eqs:=MutableBasis(field,
List(k,x->ImmutableVector(field,x)),zeroq);
fi;
od;
#eqs:=Filtered(TriangulizedMat(eqs),x->not IsZero(x));
eqs:=ShallowCopy(BasisVectors(eqs));
if Length(eqs)=0 then
eqs:=IdentityMat(Length(rules),field);
else
eqs:=ImmutableMatrix(field,eqs);
eqs:=NullspaceMat(TransposedMat(eqs)); # basis of cocycles
fi;
# Now get Coboundaries
# different collection function: Sum up changes for generator i
findtail:=function(wrd,nums,vecs)
local a,i,j,p,v;
a:=zerovec;
for i in [1..Length(wrd)] do
p:=Position(nums,wrd[i]);
if p<>fail then
v:=vecs[p];
for j in [i+1..Length(wrd)] do
v:=v*mats[wrd[j]];
od;
a:=a+v;
fi;
od;
return a;
end;
bds:=[];
for i in [1..Length(mats)] do
if formalinverse[i]>i then
c:=[i,formalinverse[i]];
for k in one do
r:=[k,-k*mats[formalinverse[i]]];
new:=[];
for j in hastail do
v1:=findtail(rules[j][1],c,r);
v2:=findtail(rules[j][2],c,r);
Append(new,v1-v2);
od;
new:=ImmutableVector(field,new);
Assert(0,(Length(eqs)>0 and SolutionMat(eqs,new)<>fail) or IsZero(new));
Add(bds,new);
od;
fi;
od;
bds:=Filtered(TriangulizedMat(bds),x->not IsZero(x));
bds:=List(bds,Immutable);
if gens<>GeneratorsOfGroup(G) then
G:=GroupWithGenerators(gens);
fi;
r:=rec(group:=G,module:=mo,cocycles:=eqs,coboundaries:=bds,zero:=zeroq,
prime:=Size(field));
new:=List(BaseSteinitzVectors(eqs,bds).factorspace,x->ImmutableVector(field,x));
r.cohomology:=new;
new:=[];
one:=One(FreeGroupOfFpGroup(fpg));
k:=List(GeneratorsOfMonoid(mon),
x->UnderlyingElement(PreImagesRepresentative(fm,x)));
# matrix corresponding to monoid word
mapped2:=function(list)
local a,i;
a:=one;
for i in list do
a:=a*k[i];
od;
return a;
end;
for i in hastail do
# rule that would get the tail
Add(new,LeftQuotient(mapped2(rules[i][1]),mapped2(rules[i][2])));
od;
r.presentation:=rec(group:=FreeGroupOfFpGroup(fpg),relators:=new,
# relators to kill superfluous generators
killrelators:=List(genkill,i->LeftQuotient(mapped2(i[1]),mapped2(i[2]))),
# position of relators with tails in tzrules
monrulpos:=List(rules{hastail},x->Position(tzrules,x)),
prewords:=List(ogens,x->UnderlyingElement(ImagesRepresentative(fp,x))));
# normalform word and collect the tails
r.tailforword:=function(wrd,zy)
local v,i,j,p,w,tail;
v:=zerovec;
# collect from left
i:=1;
while i<=Length(wrd) do
# does a rule apply at position i?
p:=RuleAtPosKBDAG(dag,wrd,i);
if IsInt(p) and rulpos[p]<>fail then
p:=rulpos[p];
tail:=wrd{[i+Length(rules[p][1])..Length(wrd)]};
wrd:=Concatenation(wrd{[1..i-1]},rules[p][2],tail);
if p in hastail then
w:=zy{dim*(htpos[p]-1)+[1..dim]};
for j in tail do
w:=w*mats[j];
od;
v:=v+w;
fi;
i:=Maximum(0,i-mal); # earliest which could be affected
fi;
i:=i+1;
od;
return [wrd,v];
end;
r.fphom:=fp;
r.monhom:=fm;
r.colltz:=colltz;
# inverses of generators
r.myinvers:=function(wrd,zy)
return [colltz(formalinverse{Reversed(wrd)}),
-r.tailforword(Concatenation(wrd,colltz(formalinverse{Reversed(wrd)})),zy)
[2]];
end;
r.pairact:=function(zy,pair)
local autom,mat,i,imagemonwords,imgwrd,left,right,extim,prdout,myinvers,v;
autom:=InverseGeneralMapping(pair[1]);
# cache words for images of monoid generators
if not IsBound(autom!.imagemonwords) then autom!.imagemonwords:=[]; fi;
imagemonwords:=autom!.imagemonwords;
imgwrd:=function(nr)
local a;
if not IsBound(imagemonwords[nr]) then
# apply automorphism
a:=PreImagesRepresentative(fm,GeneratorsOfMonoid(mon)[nr]);
a:=PreImagesRepresentative(fp,a);
a:=ImagesRepresentative(autom,a);
a:=ImagesRepresentative(fp,a);
a:=ImagesRepresentative(fm,a);
a:=LetterRepAssocWord(UnderlyingElement(a));
a:=colltz(a);
imagemonwords[nr]:=a;
fi;
return imagemonwords[nr];
end;
# normalform word and collect the tails
collectail:=function(wrd)
local v,i,j,p,w,tail;
v:=zerovec;
# collect from left
i:=1;
while i<=Length(wrd) do
# does a rule apply at position i?
p:=RuleAtPosKBDAG(dag,wrd,i);
if IsInt(p) and rulpos[p]<>fail then
p:=rulpos[p];
tail:=wrd{[i+Length(rules[p][1])..Length(wrd)]};
wrd:=Concatenation(wrd{[1..i-1]},rules[p][2],tail);
if p in hastail then
w:=zy{dim*(htpos[p]-1)+[1..dim]};
for j in tail do
w:=w*mats[j];
od;
v:=v+w;
fi;
i:=Maximum(0,i-mal); # earliest which could be affected
fi;
i:=i+1;
od;
return [wrd,v];
end;
prdout:=function(l)
local w,v,j;
w:=[];
v:=zerovec;
for j in l do
v:=v*mapped(j[1]); # move right
w:=collectail(Concatenation(w,j[1]));
v:=v+w[2]+j[2];
w:=w[1];
od;
return [w,v];
end;
mat:=pair[2];
new:=[];
# inverses of generators
myinvers:=wrd->[colltz(formalinverse{Reversed(wrd)}),
-collectail(Concatenation(wrd,colltz(formalinverse{Reversed(wrd)})))[2]];
# images of generators
extim:=List([1..Length(mats)/2],x->imgwrd(2*x-1));
# collect inverses and interleave generators/inverses
extim:=Concatenation(List(extim,x->[[x,zerovec],myinvers(x)]));
for i in [1..Length(hastail)] do
# evaluate rules in images
left:=prdout(extim{rules[hastail[i]][1]});
# invert left
v:=myinvers(left[1]);
left:=[v[1],v[2]-left[2]/mapped(left[1])];
right:=prdout(extim{rules[hastail[i]][2]});
# quotient left^-1*right
left:=prdout(Concatenation([left],[right]));
if left[1]<>[] then Error("not rule");fi;
Append(new,-left[2]*mat);
od;
# debugging code: Construct group and work there
# new2:=[];
# ext:=FpGroupCocycle(r,zy,true);
# hom:=IsomorphismPermGroup(ext);
# ext:=Image(hom);
# epcgs:=PcgsByPcSequence(FamilyObj(One(ext)),
# GeneratorsOfGroup(ext){[Length(mats)/2+1..Length(GeneratorsOfGroup(ext))]});
#
# exta:=Concatenation(List([1..Length(mats)/2],x->[ext.(x),ext.(x)^-1]));
# exte:=exta;
#
# myprd:=function(wrd)
# local i,w;
# w:=One(ext);
# for i in wrd do
# w:=w*exte[i];
# od;
# return w;
# end;
#
# v:=List([1..Length(mats)/2],x->myprd(imgwrd(2*x-1))); # new generators
# v:=Concatenation(List([1..Length(mats)/2],x->[v[x],v[x]^-1]));
# exte:=v;
#
# for i in [1..Length(hastail)] do
# left:=rules[hastail[i]][1];
# right:=rules[hastail[i]][2];
# left:=myprd(left)^-1*myprd(right);
# v:=-ExponentsOfPcElement(epcgs,left)*One(mo.field);
#
# Append(new2,v*mat);
# od;
# if new2<>new then Error("wrong ",pair);fi;
# if IsOne(pair[1]) and IsOne(pair[2]) then
# if new<>zy then Error("one");else Print("onegood\n");fi;
# fi;
return ImmutableVector(mo.field,new);
end;
return r;
end);
BindGlobal( "MatricesStabilizerOneDim", function(field,mats)
local e,one,m,r,a,c,is,i;
e:=[];
one:=One(mats[1]);
for m in mats do
r:=Set(RootsOfUPol(field,CharacteristicPolynomial(m)));
a:=List(r,x->NullspaceMat(m-x*one));
if Length(a)=0 then return false;fi;
Add(e,a);
od;
for c in Cartesian(e) do
is:=c[1];
for i in [2..Length(c)] do
is:=SumIntersectionMat(is,c[i])[2];
od;
if Length(is)>0 then
return is;
fi;
od;
return false;
end );
BindGlobal("WreathElm",function(b,l,m)
local n,ran,r,d,p,i,j;
n:=Length(l);
ran:=[1..b];
r:=0;
d:=[];
p:=[];
# base bit
for i in [1..n] do
for j in ran do
p[r+j]:=r+j^l[i];
od;
Add(d,ran+r);
r:=r+b;
od;
# permuter bit
p:=PermList(p)/PermList(Concatenation(Permuted(d,m)));
return p;
end);
BindGlobal("PermrepSemidirectModule",function(G,module)
local hom,mats,m,i,j,mo,bas,a,l,ugens,gi,r,cy,act,k,it,p;
if not MTX.IsIrreducible(module) then Error("reducible");fi;
p:=Size(module.field);
if not IsPrime(p) then Error("must be over prime field");fi;
k:=Length(module.generators);
hom:=GroupHomomorphismByImagesNC(G,Group(module.generators),
GeneratorsOfGroup(G),module.generators);
# we allow do go immediately to normal subgroup of index up to 4.
# This reduces search space
it:=DescSubgroupIterator(G:skip:=LogInt(Size(G),2));
repeat
m:=NextIterator(it);
if Index(G,m)*p>p^module.dimension then
Info(InfoExtReps,2,"Index reached ",Index(G,m),
", write out module action");
# alternative, boring version
mo:=AbelianGroup(ListWithIdenticalEntries(module.dimension,p));
bas:=Pcgs(mo);
act:=List(module.generators,m->
GroupHomomorphismByImagesNC(mo,mo,bas,
List(m,x->PcElementByExponents(bas,x)):noassert));
Assert(3,ForAll(act,IsBijective));
a:=Group(act);
SetIsGroupOfAutomorphismsFiniteGroup(a,true);
gi:=SemidirectProduct(G,GroupHomomorphismByImages(G,a,
GeneratorsOfGroup(G),act),mo);
r:=rec(group:=gi,
ggens:=List(GeneratorsOfGroup(G),x->
ImagesRepresentative(Embedding(gi,1),x)),
#vector:=ImagesRepresentative(Embedding(gi,2),bas[1]),
basis:=List(bas,x->ImagesRepresentative(Embedding(gi,2),x)));
Assert(0,Size(gi)=Size(G)*p^module.dimension);
return r;
elif Size(Core(G,m))>1 then
Info(InfoExtReps,4,"Index ",Index(G,m)," has nontrivial core");
else
Info(InfoExtReps,2,"Trying index ",Index(G,m));
mats:=List(GeneratorsOfGroup(m),
x->TransposedMat(ImagesRepresentative(hom,x^-1)));
a:=MatricesStabilizerOneDim(module.field,mats);
if a<>false then
# quotient module
# basis: supplemental vector and submodule basis
bas:=BaseSteinitzVectors(IdentityMat(module.dimension,GF(p)),
NullspaceMat(TransposedMat(a{[1]})));
bas:=Concatenation(bas.factorspace,bas.subspace);
# assume we have a generating set for the SDP consisting of the
# complement gens, and one element of the module.
cy:=CyclicGroup(IsPermGroup,p).1; # p-cycle
ugens:=[];
# also transversal chosen in complement
r:=RightTransversal(G,m);
act:=ActionHomomorphism(G,r,OnRight);
act:=List(GeneratorsOfGroup(G),x->ImagesRepresentative(act,x));
#l:=ListWithIdenticalEntries(Index(G,m),());
for gi in [1..k] do
l:=[];
for j in [1..Length(r)] do
# matrix for Schreier gen
a:=ImagesRepresentative(hom,
r[j]*G.(gi)/r[PositionCanonical(r,r[j]*G.(gi))]);
# how does it act on bas[1]-span, get factor
a:=Int(SolutionMat(bas,bas[1]*a)[1]);
# write down permutation that acts as mult by a
if IsZero(a) then
l[j]:=();
else
l[j]:=MappingPermListList([1..p],Cycle(cy^a,1));
fi;
od;
Add(ugens,WreathElm(p,l,act[gi]) );
od;
# module generator
for j in [1..Length(r)] do
#r[j]*g/r[j^g]); Note j^g=j here as kernel of permrep
l[j]:=
cy^Int(SolutionMat(bas,bas[1]/ImagesRepresentative(hom,r[j]))[1]);
od;
Add(ugens,WreathElm(p,l,()) );
gi:=Group(ugens);
Assert(0,Size(gi)=p^module.dimension*Size(G));
if ValueOption("cheap")<>true then
a:=SmallerDegreePermutationRepresentation(gi:cheap);
if NrMovedPoints(Range(a))<NrMovedPoints(gi) then
gi:=Image(a,gi);
ugens:=List(ugens,x->ImagesRepresentative(a,x));
fi;
fi;
r:=rec(group:=gi,ggens:=ugens{[1..k]});
# compute basis
bas:=bas{[1]};
gi:=ugens{[1..k]};
ugens:=ugens{[k+1]};
i:=1;
while Length(bas)<module.dimension do
for j in [1..k] do
a:=bas[i]*module.generators[j];
if SolutionMat(bas,a)=fail then
Add(bas,a);
Add(ugens,ugens[i]^gi[j]);
fi;
od;
i:=i+1;
od;
# convert back to standard basis
r.basis:=List(Inverse(bas),x->LinearCombinationPcgs(ugens,x));
return r;
fi;
fi;
until false;
end);
BindGlobal("RandomSubgroupNotIncluding",function(g,n,time)
local best,u,v,start,cnt,runtime;
runtime:=GET_TIMER_FROM_ReproducibleBehaviour();
start:=runtime();
best:=TrivialSubgroup(g);
cnt:=0;
while runtime()-start<time or cnt<100 do
cnt:=cnt+1;
u:=TrivialSubgroup(g);
repeat
v:=u;
u:=ClosureGroup(u,Random(g));
until IsSubset(u,n);
if IndexNC(g,v)<IndexNC(g,best) then best:=v;fi;
od;
return best;
end);
InstallGlobalFunction(FpGroupCocycle,function(arg)
local r,z,ogens,n,gens,str,dim,i,j,f,rels,new,quot,g,p,collect,m,e,fp,sim,
it,hom,trysy,prime,mindeg,fps,ei,mgens,mwrd,nn,newfree,mfpi,mmats,sub,
tab,tab0,evalprod,gensmrep,invsmrep,zerob,step,simi,simiq,wasbold,
mon,ord,mn,melmvec,killgens,frew,fffam,ofgens,rws,formalinverse;
# function to evaluate product (as integer list) in gens (and their
# inverses invs) with corresponding action mats
evalprod:=function(w,gens,invs,mats)
local new,i;
new:=[[],zerob];
for i in w do
if i>0 then
collect:=r.tailforword(Concatenation(new[1],gens[i][1]),z);
new:=[collect[1],collect[2]+new[2]*mats[i]+gens[i][2]];
else
collect:=r.tailforword(Concatenation(new[1],invs[-i][1]),z);
new:=[collect[1],collect[2]+new[2]/mats[-i]+invs[-i][2]];
fi;
od;
return new;
end;
melmvec:=function(v)
local i,a,w;
w:=One(f);
for i in [1..Length(v)] do
a:=Int(v[i]);
if prime=2 or a*2<prime then
w:=w*gens[2*i-1+mn]^a;
else
a:=prime-a;
w:=w*gens[2*i+mn]^a;
fi;
od;
return w;
end;
# formal inverse of module element
formalinverse:=function(w)
local l,i;
l:=[];
for i in Reversed(LetterRepAssocWord(w)) do
if IsEvenInt(i) then Add(l,i-1);
else Add(l,i+1);fi;
od;
return AssocWordByLetterRep(FamilyObj(w),l);
end;
r:=arg[1];
z:=arg[2];
ogens:=GeneratorsOfGroup(r.presentation.group);
dim:=r.module.dimension;
prime:=Size(r.module.field);
if ValueOption("normalform")<>true then
mon:=fail;
else
# Construct the monoid and rewriting system, as this is cheap to do but
# will allow for reduced multiplication. Do so before the group, so there is no
# issue about overwriting variables that might be needed later
mon:=Image(r.monhom);
ofgens:=FreeGeneratorsOfFpMonoid(mon);
fffam:=FamilyObj(One(FreeMonoidOfFpMonoid(mon)));
frew:=ReducedConfluentRewritingSystem(mon);
# generators that get killed. Do not use in RHS
killgens:=Filtered(GeneratorsOfMonoid(mon),x->UnderlyingElement(x)<>
ReducedForm(frew,UnderlyingElement(x)));
killgens:=Union(List(killgens,x->LetterRepAssocWord(UnderlyingElement(x))));
str:=List(GeneratorsOfMonoid(mon),String);
mn:=Length(str);
for i in [1..dim] do
Add(str,Concatenation("m",String(i)));
Add(str,Concatenation("m",String(i),"^-1"));
od;
f:=FreeMonoid(str);
gens:=GeneratorsOfMonoid(f);
rels:=[];
# inverse rules
for i in [1,3..Length(gens)-1] do
Add(rels,[gens[i]*gens[i+1],One(f)]);
Add(rels,[gens[i+1]*gens[i],One(f)]);
od;
# module is elementary abelian
for i in [mn+1,mn+3..Length(str)-1] do
if prime=2 then
Add(rels,[gens[i]^2,One(f)]);
Add(rels,[gens[i+1],gens[i]]);
else
j:=QuoInt(prime+1,2); # power that changes the exponent sign
Add(rels,[gens[i]^j,gens[i+1]^(j-1)]);
Add(rels,[gens[i+1]^j,gens[i]^(j-1)]);
fi;
for j in [i+2..Length(str)] do
Add(rels,[gens[j]*gens[i],gens[i]*gens[j]]);
Add(rels,[gens[j]*gens[i+1],gens[i+1]*gens[j]]);
od;
od;
# module rules
for i in [1..mn] do
if not i in killgens then
for j in [mn+1..Length(str)] do
new:=r.module.generators[QuoInt(i+1,2)];
if IsEvenInt(i) then new:=new^-1; fi;
new:=new[QuoInt(j-mn+1,2)];
if IsEvenInt(j) then new:=-new;fi;
Add(rels,[gens[j]*gens[i],gens[i]*melmvec(new)]);
od;
fi;
od;
# rules with tails
for i in [1..Length(r.presentation.monrulpos)] do
new:=RelationsOfFpMonoid(mon)[r.presentation.monrulpos[i]];
new:=List(new,x->MappedWord(x,ofgens,gens{[1..mn]}));
new[2]:=new[2]*melmvec(z{[(i-1)*dim+1..i*dim]});
Add(rels,new);
od;
# Any killed generators -- use just the same word expression
if r.presentation.killrelators<>[] then
for i in Filtered(killgens,IsOddInt) do
new:=AssocWordByLetterRep(fffam,[i]);
new:=[new,ReducedForm(frew,new)];
new:=List(new,x->MappedWord(x,
ofgens,gens{[1..mn]}));
Add(rels,new);
RemoveSet(killgens,i);
od;
fi;
if HasReducedConfluentRewritingSystem(mon) then
ord:=OrderingOfRewritingSystem(ReducedConfluentRewritingSystem(mon));
if HasLevelsOfGenerators(ord) then
ord:=WreathProductOrdering(f,
Concatenation(LevelsOfGenerators(ord)+1,
ListWithIdenticalEntries(2*dim,1)));
else
ord:=WreathProductOrdering(f,
Concatenation(ListWithIdenticalEntries(mn,2),
ListWithIdenticalEntries(2*dim,1)));
fi;
else
ord:=WreathProductOrdering(f,
Concatenation(ListWithIdenticalEntries(mn,2),
ListWithIdenticalEntries(2*dim,1)));
fi;
# if there is any [x^2,1] relation, this means the inverse needs to be
# mapped to the generator. (This will have been skipped in the
# monrulpos.)
for i in rels do
if Length(i[2])=0 and Length(i[1])=2
and Length(Set(LetterRepAssocWord(i[1])))=1 then
new:=LetterRepAssocWord(i[1])[1];
if IsOddInt(new) then
Add(rels,[gens[new+1],gens[new]]);
fi;
fi;
od;
# temporary build to be able to reduce
mon:=FactorFreeMonoidByRelations(f,rels);
rws:=KnuthBendixRewritingSystem(FamilyObj(One(mon)),ord);
# handle inverses that get killed
for i in killgens do
new:=AssocWordByLetterRep(fffam,[i]);
new:=ReducedForm(frew,new); # what is it in the factor?
new:=MappedWord(new,ofgens,gens{[1..mn]});
# now left multiply with non-inverse generator
j:=ReducedForm(rws,gens[i-1]*new); #must be a tail.
j:=ReducedForm(rws,formalinverse(j)); # invert
Add(rels,[gens[i],new*j]);
od;
mon:=FactorFreeMonoidByRelations(f,rels);
rws:=KnuthBendixRewritingSystem(FamilyObj(One(mon)),ord:isconfluent);
ReduceRules(rws);
MakeConfluent(rws); # will add rules to kill inverses, if needed
SetReducedConfluentRewritingSystem(mon,rws);
fi;
# now construct the group
str:=List(ogens,String);
zerob:=ImmutableVector(r.module.field,
ListWithIdenticalEntries(dim,Zero(r.module.field)));
n:=Length(ogens);
gensmrep:=List(GeneratorsOfGroup(r.presentation.group),
x->[LetterRepAssocWord(UnderlyingElement(ImagesRepresentative(r.monhom,
ElementOfFpGroup(FamilyObj(One(Range(r.fphom))),x)))),zerob]);
# module generators
Append(gensmrep,List(IdentityMat(r.module.dimension,r.module.field),
x->[[],x]));
invsmrep:=List(gensmrep,x->r.myinvers(x[1],z));
for i in [1..dim] do
Add(str,Concatenation("m",String(i)));
od;
f:=FreeGroup(str);
gens:=GeneratorsOfGroup(f);
rels:=[];
for i in [1..Length(r.presentation.relators)] do
new:=MappedWord(r.presentation.relators[i],ogens,gens{[1..n]});
for j in [1..dim] do
new:=new*gens[n+j]^Int(z[(i-1)*dim+j]);
od;
Add(rels,new);
od;
for i in [1..Length(r.presentation.killrelators)] do
new:=MappedWord(r.presentation.killrelators[i],ogens,gens{[1..n]});
Add(rels,new);
od;
for i in [n+1..Length(gens)] do
Add(rels,gens[i]^prime);
for j in [i+1..Length(gens)] do
Add(rels,Comm(gens[i],gens[j]));
od;
od;
for i in [1..n] do
for j in [1..dim] do
Add(rels,gens[n+j]^gens[i]/
LinearCombinationPcgs(gens{[n+1..Length(gens)]},r.module.generators[i][j]));
od;
od;
fp:=f/rels;
SetSize(fp,Size(r.group)*prime^r.module.dimension);
simi:=fail;
wasbold:=false;
if mon<>fail then
rels:=MakeFpGroupToMonoidHomType1(fp,mon);
SetReducedMultiplication(fp);
fi;
if Length(arg)>2 and arg[3]=true then
if IsZero(z) and MTX.IsIrreducible(r.module) then
# make SDP directly
m:=PermrepSemidirectModule(r.group,r.module:cheap);
p:=m.group;
# test is cheap here, thus no NC
new:=GroupHomomorphismByImages(fp,p,GeneratorsOfGroup(fp),
Concatenation(m.ggens,m.basis));
else
#sim:=IsomorphismSimplifiedFpGroup(fp);
sim:=IdentityMapping(fp);
fps:=Image(sim,fp);
g:=r.group;
quot:=InverseGeneralMapping(sim)
*GroupHomomorphismByImages(fp,g,GeneratorsOfGroup(fp),
Concatenation(GeneratorsOfGroup(g),
ListWithIdenticalEntries(r.module.dimension,One(g))));
hom:=GroupHomomorphismByImages(r.group,Group(r.module.generators),
GeneratorsOfGroup(r.group),r.module.generators);
p:=Image(quot);
trysy:=Maximum(1000,50*IndexNC(p,SylowSubgroup(p,prime)));
# allow to enforce test coverage
if ValueOption("forcetest")=true then trysy:=2;fi;
mindeg:=2;
if IsPermGroup(p) then
mindeg:=Minimum(List(Orbits(p,MovedPoints(p)),Length));
fi;
it:=fail;
while Size(p)<Size(fp) do
# we allow do go immediately to normal subgroup of index up to 4.
# This reduces search space
repeat
if it=fail then
# re-use the first quotient, helps with repeated subgroups iterator
if p=r.group then
m:=r.group;
else
m:=p;
fi;
# avoid working hard for outer automorphisms
e:=Filtered(DerivedSeriesOfGroup(m),
# roughly 5 for 1000, 30 for 10^6, 170 for 10^9
x->IndexNC(m,x)^4<=Size(m));
m:=Last(e);
it:=DescSubgroupIterator(m:skip:=LogInt(Size(p),2));
fi;
wasbold:=false;
m:=NextIterator(it);
# catch case of large permdegree, try naive first
if Index(p,m)=1 and IsPermGroup(p)
and NrMovedPoints(p)^2>Size(p) then
# take set stabilizer of orbit points.
e:=Set(List(Orbits(p,MovedPoints(p)),x->x[1]));
m:=Stabilizer(p,e,OnSets);
if IndexNC(p,m)>10*NrMovedPoints(p) then
m:=Intersection(MaximalSubgroupClassReps(p));
fi;
if IndexNC(p,m)>10*NrMovedPoints(p) then
m:=p; # after all..
wasbold:=false;
else
wasbold:=true;
fi;
fi;
Info(InfoExtReps,3,"Found index ",Index(p,m));
e:=fail;
if Index(p,m)>=mindeg and (hom=false or Size(m)=1 or
false<>MatricesStabilizerOneDim(r.module.field,
List(GeneratorsOfGroup(m),
x->TransposedMat(ImagesRepresentative(hom,x))^-1))) then
Info(InfoExtReps,2,"Attempt index ",Index(p,m));
if hom=false then Info(InfoExtReps,2,"hom is false");fi;
if hom<>false and Index(p,m)>
# up to index
50
# the rewriting seems to be sufficiently spiffy that we don't
# need to worry about this more involved process.
# this might fail if generators are killed by rewriting
and Length(r.presentation.killrelators)=0
then
# Rewriting produces a bad presentation. Rather rebuild a new
# fp group using rewriting rules, finds its abelianization,
# and then lift that quotient
sub:=PreImage(quot,m);
tab0:=AugmentedCosetTableInWholeGroup(sub);
# primary generators
mwrd:=List(tab0!.primaryGeneratorWords,
x->ElementOfFpGroup(FamilyObj(One(fps)),x));
Info(InfoExtReps,4,"mwrd=",mwrd);
mgens:=List(mwrd,x->ImagesRepresentative(quot,x));
nn:=Length(mgens);
if IsPermGroup(m) then
e:=SmallerDegreePermutationRepresentation(m);
mfpi:=IsomorphismFpGroupByGenerators(Image(e,m),
List(mgens,x->ImagesRepresentative(e,x)):cheap);
mfpi:=e*mfpi;
else
mfpi:=IsomorphismFpGroupByGenerators(m,mgens:cheap);
fi;
mmats:=List(mgens,x->ImagesRepresentative(hom,x));
i:=Concatenation(r.module.generators,
ListWithIdenticalEntries(r.module.dimension,
IdentityMat(r.module.dimension,r.module.field)));
e:=List(mwrd,
x->evalprod(LetterRepAssocWord(UnderlyingElement(x)),
gensmrep,invsmrep,i));
ei:=List(e,function(x)
local i;
i:=r.myinvers(x[1],z);
return [i[1],i[2]-x[2]];
end);
newfree:=FreeGroup(nn+dim);
gens:=GeneratorsOfGroup(newfree);
rels:=[];
# module relations
for i in [1..dim] do
Add(rels,gens[nn+i]^prime);
for j in [1..i-1] do
Add(rels,Comm(gens[nn+i],gens[nn+j]));
od;
for j in [1..Length(mgens)] do
Add(rels,gens[nn+i]^gens[j]/
LinearCombinationPcgs(gens{[nn+1..nn+dim]},mmats[j][i]));
od;
od;
#extended presentation
for i in RelatorsOfFpGroup(Range(mfpi)) do
i:=LetterRepAssocWord(i);
str:=evalprod(i,e,ei,mmats);
if Length(str[1])>0 then Error("inconsistent");fi;
Add(rels,AssocWordByLetterRep(FamilyObj(One(newfree)),i)
/LinearCombinationPcgs(gens{[nn+1..nn+dim]},str[2]));
od;
newfree:=newfree/rels; # new extension
Assert(2,
AbelianInvariants(newfree)=AbelianInvariants(PreImage(quot,m)));
mfpi:=GroupHomomorphismByImagesNC(newfree,m,
GeneratorsOfGroup(newfree),Concatenation(mgens,
ListWithIdenticalEntries(dim,One(m))));
# first just try a bit, and see whether this gets all (e.g. if
# module is irreducible).
e:=LargerQuotientBySubgroupAbelianization(mfpi,m:cheap);
if e<>fail then
step:=0;
while step<=1 do
# Now write down the combined representation in wreath
e:=DefiningQuotientHomomorphism(e);
# define map on subgroup.
tab:=CopiedAugmentedCosetTable(tab0);
tab.primaryImages:=Immutable(
List(GeneratorsOfGroup(newfree){[1..nn]},
x->ImagesRepresentative(e,x)));
TrySecondaryImages(tab);
i:=GroupHomomorphismByImagesNC(sub,Range(e),mwrd,
List(GeneratorsOfGroup(newfree){[1..nn]},
x->ImagesRepresentative(e,x)):noassert);
SetCosetTableFpHom(i,tab);
e:=PreImage(i,TrivialSubgroup(Range(e)));
e:=Intersection(e,
KernelOfMultiplicativeGeneralMapping(quot));
# this check is very cheap, in comparison. So just be safe
Assert(0,ForAll(RelatorsOfFpGroup(fps),
x->IsOne(MappedWord(x,FreeGeneratorsOfFpGroup(fps),
GeneratorsOfGroup(e!.quot)))));
if step=0 then
i:=GroupHomomorphismByImagesNC(e!.quot,p,
GeneratorsOfGroup(e!.quot),
GeneratorsOfGroup(p));
j:=PreImage(i,m);
if AbelianInvariants(j)=AbelianInvariants(newfree) then
step:=2;
Info(InfoExtReps,2,"Small bit did good");
else
Info(InfoExtReps,2,"Need expensive version");
e:=LargerQuotientBySubgroupAbelianization(mfpi,m:
cheap:=false);
e:=Intersection(e,
KernelOfMultiplicativeGeneralMapping(quot));
fi;
fi;
step:=step+1;
od;
fi;
else
if simi=fail then
simi:=IsomorphismSimplifiedFpGroup(Source(quot));
simiq:=InverseGeneralMapping(simi)*quot;
fi;
e:=LargerQuotientBySubgroupAbelianization(simiq,m);
if e<>fail then
i:=simi*DefiningQuotientHomomorphism(e);
j:=Image(DefiningQuotientHomomorphism(e));;
j:=List(Orbits(j,MovedPoints(j)),x->Stabilizer(j,x[1]));
j:=List(j,x->PreImage(i,x));
e:=Intersection(j);
e:=Intersection(e,KernelOfMultiplicativeGeneralMapping(quot));
fi;
fi;
if e<>fail then
# can we do better degree -- greedy block reduction?
nn:=e!.quot;
if IsTransitive(nn,MovedPoints(nn)) then
repeat
ei:=ShallowCopy(RepresentativesMinimalBlocks(nn,
MovedPoints(nn)));
SortBy(ei,x->-Length(x)); # long ones first
j:=false;
i:=1;
while i<=Length(ei) and j=false do
str:=Stabilizer(nn,ei[i],OnSets);
if Size(Core(nn,str))=1 then
j:=ei[i];
fi;
i:=i+1;
od;
if j<>false then
Info(InfoExtReps,4,"deg. improved by blocks ",Size(j));
# action on blocks
i:=ActionHomomorphism(nn,Orbit(nn,j,OnSets),OnSets);
j:=Size(nn);
# make new group to not cache anything about old.
nn:=Group(List(GeneratorsOfGroup(nn),
x->ImagesRepresentative(i,x)),());
SetSize(nn,j);
fi;
until j=false;
fi;
if not IsIdenticalObj(nn,e!.quot) then
Info(InfoExtReps,2,"Degree improved by factor ",
NrMovedPoints(e!.quot)/NrMovedPoints(nn));
e:=SubgroupOfWholeGroupByQuotientSubgroup(FamilyObj(fps),nn,
Stabilizer(nn,1));
fi;
elif e=fail and IndexNC(p,m)>trysy then
trysy:=Size(fp); # never try again
# can the Sylow subgroup get us something?
m:=SylowSubgroup(p,prime);
e:=PreImage(quot,m);
Info(InfoExtReps,2,"Sylow test for index ",IndexNC(p,m));
i:=IsomorphismFpGroup(e:cheap); # only one TzGo
e:=EpimorphismPGroup(Range(i),prime,PClassPGroup(m)+1);
e:=i*e; # map onto pgroup
j:=KernelOfMultiplicativeGeneralMapping(
InverseGeneralMapping(e)*quot);
i:=RandomSubgroupNotIncluding(Range(e),j,20000); # 20 seconds
Info(InfoExtReps,2,"Sylow found ",IndexNC(p,m)," * ",
IndexNC(Range(e),i));
if IndexNC(Range(e),i)*IndexNC(p,m)<
# consider permdegree up to
100000
# as manageable
then
e:=PreImage(e,i);
#e:=KernelOfMultiplicativeGeneralMapping(
# DefiningQuotientHomomorphism(i));
else
e:=fail; # not good
fi;
fi;
else Info(InfoExtReps,4,"Don't index ",Index(p,m));
fi;
until e<>fail;
i:=p;
quot:=DefiningQuotientHomomorphism(Intersection(e,
KernelOfMultiplicativeGeneralMapping(quot)));
p:=Image(quot);
Info(InfoExtReps,1,"index ",Index(i,m)," increases factor by ",
Size(p)/Size(i)," at degree ",NrMovedPoints(p));
hom:=false; # we don't have hom cheaply any longer as group changed.
# this is not an issue if module is irreducible
it:=fail; simi:=fail; # cleanout info for first factor
od;
quot:=sim*quot;
new:=GroupHomomorphismByImages(fp,p,GeneratorsOfGroup(fp),
List(GeneratorsOfGroup(fp),x->ImagesRepresentative(quot,x)));
fi;
# if we used factor perm rep, be bolder
if IsPermGroup(p) then
new:=new*SmallerDegreePermutationRepresentation(p:cheap:=wasbold<>true);
SetIsomorphismPermGroup(fp,new);
elif IsPcGroup(p) then
SetIsomorphismPcGroup(fp,new);
fi;
fi;
if HasIsSolvableGroup(r.group) then
SetIsSolvableGroup(fp,IsSolvableGroup(r.group));
fi;
return fp;
end);
InstallGlobalFunction("CompatiblePairOrbitRepsGeneric",function(cp,coh)
local bas,ran,mats,o;
if Length(coh.cohomology)=0 then return [coh.zero];fi;
if Length(coh.cohomology)=1 and Size(coh.module.field)=2 then
# 1-dim space over GF(2)
return [coh.zero,coh.cohomology[1]];
fi;
# make basis
bas:=Concatenation(coh.coboundaries,coh.cohomology);
ran:=[Length(coh.coboundaries)+1..Length(bas)];
mats:=List(GeneratorsOfGroup(cp),g->List(coh.cohomology,
v->SolutionMat(bas,coh.pairact(v,g)){ran}));
o:=Orbits(Group(mats),coh.module.field^Length(coh.cohomology));
o:=List(o,x->x[1]*coh.cohomology);
return o;
end);
#############################################################################
##
#M Extensions( G, M )
##
InstallOtherMethod(Extensions,"generic method for finite groups",
true,[IsGroup and IsFinite,IsObject],
-RankFilter(IsGroup and IsFinite),
function(G,M)
local coh;
coh:=TwoCohomologyGeneric(G,M);
return List(Elements(VectorSpace(coh.module.field,coh.cohomology,coh.zero)),
x->FpGroupCocycle(coh,x,true:normalform));
end);
InstallOtherMethod(ExtensionRepresentatives,"generic method for finite groups",
true,[IsGroup and IsFinite,IsObject,IsGroup],
-RankFilter(IsGroup and IsFinite),
function(G,M,P)
local coh;
coh:=TwoCohomologyGeneric(G,M);
return List(CompatiblePairOrbitRepsGeneric(P,coh),
x->FpGroupCocycle(coh,x,true:normalform));
end);
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