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#############################################################################
##
#W grpffmat.gi GAP Library Frank Celler
#W Frank Lübeck
#W Stefan Kohl
##
##
#Y Copyright (C) 1996, 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 the operations for matrix groups over finite field.
##
#############################################################################
##
#M FieldOfMatrixGroup( <ffe-mat-grp> )
##
InstallMethod( FieldOfMatrixGroup,
true,
[ IsFFEMatrixGroup ],
0,
function( grp )
local gens;
gens := GeneratorsOfGroup(grp);
if Length(gens)=0 then
return FieldOfMatrixList([One(grp)]);
else
return FieldOfMatrixList(gens);
fi;
end );
#############################################################################
##
#M FieldOfMatrixList
##
InstallMethod(FieldOfMatrixList,"finite field matrices",true,
[IsListOrCollection and IsFFECollCollColl],0,
function(l)
local deg, i, j, char;
if Length(l)=0 then Error("list must be nonempty");fi;
deg := 1;
for i in l do
for j in i do
deg := LcmInt( deg, DegreeFFE(j) );
od;
od;
char := Characteristic(l[1][1]);
return GF(char^deg);
end);
#############################################################################
##
#M DefaultScalarDomainOfMatrixList
##
InstallMethod(DefaultScalarDomainOfMatrixList,"finite field matrices",true,
[IsListOrCollection and IsFFECollCollColl],0,
function(l)
local deg, i, j, char,m;
if Length(l)=0 then Error("list must be nonempty");fi;
deg := 1;
for i in l do
# treat compact matrices quickly
if IsGF2MatrixRep(i) then
deg:=deg; # always in
elif Is8BitMatrixRep(i) then
j:=Q_VEC8BIT(i![2]);
deg:=LcmInt( deg, Length(Factors(j)));
else
for j in i do
deg := LcmInt( deg, DegreeFFE(j) );
od;
fi;
od;
char := Characteristic(l[1][1]);
return GF(char^deg);
end);
BindGlobal("NonemptyGeneratorsOfGroup",function(grp)
local l;
l:=GeneratorsOfGroup(grp);
if Length(l)=0 then l:=[One(grp)]; fi;
return l;
end);
#############################################################################
##
#M IsNaturalGL( <ffe-mat-grp> )
##
InstallMethod( IsNaturalGL,
"size comparison",
true,
[ IsFFEMatrixGroup and IsFinite ],
0,
function( grp )
return MTX.IsAbsolutelyIrreducible(
GModuleByMats(NonemptyGeneratorsOfGroup(grp),DefaultFieldOfMatrixGroup(grp))) and
Size( grp ) = Size( GL( DimensionOfMatrixGroup( grp ),
Size( FieldOfMatrixGroup( grp ) ) ) );
end );
InstallMethod( IsNaturalSL,
"size comparison",
true,
[ IsFFEMatrixGroup and IsFinite ],
0,
function( grp )
local gen, d, f;
f := FieldOfMatrixGroup( grp );
d := DimensionOfMatrixGroup( grp );
gen := GeneratorsOfGroup( grp );
return MTX.IsAbsolutelyIrreducible(
GModuleByMats(NonemptyGeneratorsOfGroup(grp),DefaultFieldOfMatrixGroup(grp))) and
ForAll(gen, x-> DeterminantMat(x) = One(f))
and Size(grp) = Size(SL(d, Size(f)));
end );
#############################################################################
##
#M NiceMonomorphism( <ffe-mat-grp> )
##
MakeThreadLocal("FULLGLNICOCACHE"); # avoid recreating same homom. repeatedly
FULLGLNICOCACHE:=[];
InstallGlobalFunction( NicomorphismFFMatGroupOnFullSpace, function( grp )
local field, dim, V, xset, nice;
field := FieldOfMatrixGroup( grp );
dim := DimensionOfMatrixGroup( grp );
#check cache
V:=Size(field);
nice:=First(FULLGLNICOCACHE,x->x[1]=V and x[2]=dim);
if nice<>fail then return nice[3];fi;
if not (HasIsNaturalGL(grp) and IsNaturalGL(grp)) then
grp:=GL(dim,field); # enforce map on full GL
fi;
V := field ^ dim;
xset := ExternalSet( grp, V );
# STILL: reverse the base to get point sorting compatible with lexicographic
# vector arrangement
SetBaseOfGroup( xset, One( grp ));
nice := ActionHomomorphism( xset,"surjective" );
if not HasNiceMonomorphism(grp) then
SetNiceMonomorphism(grp,nice);
fi;
SetIsInjective( nice, true );
SetFilterObj(nice,IsNiceMonomorphism);
# because we act on the full space we are canonical.
SetIsCanonicalNiceMonomorphism(nice,true);
if Size(V)>10^5 then
# store only one big one and have it get thrown out quickly
FULLGLNICOCACHE[1]:=[Size(field),dim,nice];
else
if Length(FULLGLNICOCACHE)>4 then
FULLGLNICOCACHE:=FULLGLNICOCACHE{[2..5]};
fi;
Add(FULLGLNICOCACHE,[Size(field),dim,nice]);
fi;
return nice;
end );
InstallMethod( NiceMonomorphism, "falling back on GL", true,
[ IsFFEMatrixGroup and IsFinite ], 0,
function( grp )
local tt;
# is it GL?
if (HasIsNaturalGL( grp ) and IsNaturalGL( grp ))
or (HasIsNaturalSL( grp ) and IsNaturalSL( grp )) then
return NicomorphismFFMatGroupOnFullSpace(grp);
fi;
# is the GL domain small enough in comparison to the group to simply use it?
tt:=2000;
if HasSize(grp) and Size(grp)<tt then
tt:=Size(grp);
fi;
if IsTrivial(grp)
or Size(FieldOfMatrixGroup(Parent(grp)))^DimensionOfMatrixGroup(grp)>tt
then
# if the permutation image would be too large, compute the orbit.
TryNextMethod();
fi;
return NicomorphismFFMatGroupOnFullSpace( GL( DimensionOfMatrixGroup( grp ),
Size( FieldOfMatrixGroup( Parent(grp) ) ) ) );
end );
#############################################################################
##
#M ProjectiveActionOnFullSpace(<G>,<f>,<n>)
##
InstallGlobalFunction(ProjectiveActionOnFullSpace,function(g,f,n)
local o;
# as the groups are large, we can take all normed vectors
o:=NormedRowVectors(f^n);
o:=Set(o, r -> ImmutableVector(f, r));
return Action(g,o,OnLines);
end);
#############################################################################
##
#M Size( <general-linear-group> )
##
InstallMethod( Size,
"general linear group",
true,
[ IsFFEMatrixGroup and IsFinite and IsNaturalGL ],
0,
function( G )
local n, q, size, qi, i;
n := DimensionOfMatrixGroup(G);
q := Size( FieldOfMatrixGroup(G) );
size := q-1;
qi := q;
for i in [ 2 .. n ] do
qi := qi * q;
size := size * (qi-1);
od;
return q^(n*(n-1)/2) * size;
end );
InstallMethod(Size,"natural SL",true,
[IsFFEMatrixGroup and IsNaturalSL and IsFinite],0,
function(G)
local q,n,size,i,qi;
n:=DimensionOfMatrixGroup(G);
q:=Size(FieldOfMatrixGroup(G));
size := 1;
qi := q;
for i in [ 2 .. n ] do
qi := qi * q;
size := size * (qi-1);
od;
return q^(n*(n-1)/2) * size;
end);
InstallMethod( \in, "general linear group", IsElmsColls,
[ IsMatrix, IsFFEMatrixGroup and IsFinite and IsNaturalGL ], 0,
function( mat, G )
return Length( mat ) = Length( mat[ 1 ] )
and Length( mat ) = DimensionOfMatrixGroup( G )
and ForAll( mat, row -> IsSubset( FieldOfMatrixGroup( G ), row ) )
and Length( mat ) = RankMat( mat );
end );
InstallMethod( \in, "special linear group", IsElmsColls,
[ IsMatrix, IsFFEMatrixGroup and IsFinite and IsNaturalSL ], 0,
function( mat, G )
return Length( mat ) = Length( mat[ 1 ] )
and Length( mat ) = DimensionOfMatrixGroup( G )
and ForAll( mat, row -> IsSubset( FieldOfMatrixGroup( G ), row ) )
and Length( mat ) = RankMat( mat )
and DeterminantMat(mat)=One(FieldOfMatrixGroup( G ));
end );
#############################################################################
##
#F SizePolynomialUnipotentClassGL( <la> ) . . . . . . centralizer order of
#F unipotent elements in GL_n( q )
##
## <la> must be a partition of, say, n.
##
## This function returns a pair [coefficient list, valuation] defining a
## polynomial over the integers, having the following property: The order
## of the centralizer of a unipotent element in GL_n(q), q a prime power,
## with Jordan block sizes given by <la>, is the value of this polynomial
## at q.
##
SizePolynomialUnipotentClassGL := function(la)
local lad, n, nla, ri, tmp, phila, i, a;
lad := AssociatedPartition(la);
n := Sum(la);
nla := Sum(lad, i->i*(i-1)/2);
ri := List([1..Maximum(la)], i-> Number(la, x-> x=i));
## the following should be
## T := Indeterminate(Rationals);
## phila := Product(Concatenation(List(ri,
## r-> List([1..r], j-> (1-T^j)))));
## return T^(n+2*nla)*Value(phila,1/T);
## but for now (or ever?) we avoid polynomials
tmp := Concatenation(List(ri, r-> [1..r]));
phila := [1];
for i in tmp do
a := 0*[1..i+1];
a[1] := 1;
a[i+1] := -1;
phila := ProductCoeffs(phila, a);
od;
return [Reversed(phila), n+2*nla-Length(phila)+1];
end;
#############################################################################
##
#M ConjugacyClassesOfNaturalGroup( <G> )
##
InstallGlobalFunction( ConjugacyClassesOfNaturalGroup,
function( G, flag )
local mycartesian, fill, myval, pols, nrpols, pairs,
types, a, a2, pos, i, tup, arr, mat, cen, b,
c, cl, powerpol, one, n, q, cls, new, o, m, rep, gcd;
# to handle single argument
mycartesian := function(arg)
if Length(arg[1]) = 1 then
return List(arg[1][1], x-> [x]);
else
return Cartesian(arg[1]);
fi;
end;
# small helper
fill := function(l, pos, val)
l := ShallowCopy(l);
l{pos} := val;
return l;
end;
# since polynomials are just lists of coefficients
powerpol := function(c, r)
local pr, i;
if r=1 then
return c;
else
pr := c;
for i in [2..r] do
pr := ProductCoeffs(pr, c);
od;
return pr;
fi;
end;
# Value for polynomials as [coeffs,val]
myval := function(p, x)
local r, c;
r := 0*x;
for c in Reversed(p[1]) do
r := x*r+c;
od;
return r*x^p[2];
end;
# set up
n := DimensionOfMatrixGroup( G );
q := Size( FieldOfMatrixGroup( G ) );
o := Size( G );
cls := [];
# irreducible polynomials up to degree n
pols := List([1..n],
i-> AllIrreducibleMonicPolynomialCoeffsOfDegree(i, q));
# remove minimal polynomial of 0
pols[1] := Difference(pols[1],[[0,1]*Z(q)^0]);
nrpols := List(pols, Length);
# parameters for semisimple class types
# typ in types is of form [[m1,n1],...,[mr,nr]] standing for a centralizer
# of type GL_m1(q^n1) x ... x GL_mr(q^nr)
pairs := List([1..n], i->List(DivisorsInt(i), j-> [j,i/j]));
types := Concatenation(List(Partitions(n), a-> mycartesian(pairs{a})));
for a in types do Sort(a); od;
# 'Reversed' to get central elements first
types := Reversed(Set(types));
for a in types do
a2 := List(a,x->x[2]);
pos := [];
for i in Set(a2) do
pos[i] := [];
od;
for i in [1..Length(a2)] do
Add(pos[a2[i]], i);
od;
# find representatives of semisimple classes corresponding to type
tup := [[]];
for i in Set(a2) do
arr := Arrangements([1..nrpols[i]], Length(pos[i]));
tup := Concatenation(List(tup, b-> List(arr, c-> fill(b, pos[i], c))));
od;
# merge with 'a' to remove duplicates
tup := List(tup, b-> List([1..Length(a)],
i-> Concatenation(a[i],[b[i]])));
tup := Set(List(tup,Set));
# now append partitions for distinguishing the unipotent parts
tup := Concatenation(List(tup, a->
Cartesian(List(a,x->List(Partitions(x[1]),b->
Concatenation(x,[b]))))));
Append(cls, tup);
od;
# in the sl-case
if flag then
rep := List([1..Gcd(q-1, n)-1], i-> IdentityMat(n, GF(q)));
for i in [1..Gcd(q-1, n)-1] do
rep[i][n][n] := Z(q)^i;
od;
fi;
# now convert into actual matrices and compute centralizer order
cl := [];
one := One(GF(q));
for a in cls do
mat := [];
cen := 1;
for b in a do
for c in b[4] do
Add(mat, powerpol(pols[b[2]][b[3]], c));
od;
cen := cen * myval(SizePolynomialUnipotentClassGL(b[4]), q^b[2]);
od;
mat := one * DirectSumMat(List(mat, CompanionMat));
# in the sl-case we have to split this class
if flag then
if DeterminantMat(mat)=Z(q)^0 then
gcd := Gcd(Concatenation(List(a, b-> b[4])));
gcd := Gcd(gcd, q-1);
mat := [mat];
for i in [1..gcd-1] do
Add(mat, mat[1]^rep[i]);
od;
for m in mat do
new := ConjugacyClass( G, m );
SetSize( new, (o*(q-1))/(cen*gcd) );
Add( cl, new );
od;
fi;
else
new := ConjugacyClass( G, mat );
SetSize( new, o/cen );
Add(cl, new );
fi;
od;
# obey general rule in GAP to put class of identity first
i := First([1..Length(cl)], c-> Representative(cl[c]) = One(G));
if i <> 1 then
a := cl[i];
cl[i] := cl[1];
cl[1] := a;
fi;
return cl;
end );
#############################################################################
##
#M ConjugacyClasses( <G> ) . . . . . . . . . . . . . . . . . for natural GL
##
InstallMethod( ConjugacyClasses, "for natural gl", true,
[IsFFEMatrixGroup and IsFinite and IsNaturalGL],
0,
G -> ConjugacyClassesOfNaturalGroup( G, false ) );
#############################################################################
##
#M ConjugacyClasses( <G> ) . . . . . . . . . . . . . . . . . for natural SL
##
InstallMethod( ConjugacyClasses, "for natural sl", true,
[IsFFEMatrixGroup and IsFinite and IsNaturalSL],
0,
G -> ConjugacyClassesOfNaturalGroup( G, true ) );
#############################################################################
##
#M ConjugacyClasses
##
InstallMethod(ConjugacyClasses,"matrix groups: test naturality",true,
[IsFFEMatrixGroup and IsFinite and IsHandledByNiceMonomorphism],0,
function(g)
local mon,cl,clg,c,i;
if (((not HasIsNaturalGL(g)) and IsNaturalGL(g))
or ((not HasIsNaturalSL(g)) and IsNaturalSL(g))) then
# redispatch as we found something out
return ConjugacyClasses(g);
fi;
TryNextMethod();
end);
#############################################################################
##
#M Random( <G> ) . . . . . . . . . . . . . . . . . . . . . . for natural GL
##
InstallMethodWithRandomSource( Random,
"for a random source and natural GL",
[ IsRandomSource, IsFFEMatrixGroup and IsFinite and IsNaturalGL ],
function(rs, G)
local m;
m := RandomInvertibleMat( rs, DimensionOfMatrixGroup( G ),
FieldOfMatrixGroup( G ) );
return ImmutableMatrix(FieldOfMatrixGroup(G), m, true);
end);
#############################################################################
##
#M Random( <G> ) . . . . . . . . . . . . . . . . . . . . . . for natural SL
##
## We use that the matrices obtained from the identity matrix by setting the
## entry in the upper left corner to arbitrary nonzero values in the field
## $F$ form a set of coset representatives of $SL(n,F)$ in $GL(n,F)$.
##
InstallMethodWithRandomSource( Random,
"for a random source and natural SL",
[ IsRandomSource, IsFFEMatrixGroup and IsFinite and IsNaturalSL ],
function(rs, G)
local m;
m:= RandomInvertibleMat( rs, DimensionOfMatrixGroup( G ),
FieldOfMatrixGroup( G ) );
MultRowVector(m[1], DeterminantMat(m)^-1);
return ImmutableMatrix(FieldOfMatrixGroup(G), m, true);
end);
#############################################################################
##
#F Phi2( <n> ) . . . . . . . . . . . . Modification of Euler's Phi function
##
## This is needed for the computation of the class numbers of SL(n,q),
## PSL(n,q), SU(n,q) and PSU(n,q)
##
InstallGlobalFunction(Phi2,
n -> n^2 * Product(Set(Filtered(Factors(Integers,n), m -> m <> 1)),
p -> (1 - 1/p^2)));
#############################################################################
##
#F NrConjugacyClassesGL( <n>, <q> ) . . . . . . . . Class number for GL(n,q)
##
## This is also needed for the computation of the class numbers of PGL(n,q),
## SL(n,q) and PSL(n,q)
##
InstallGlobalFunction(NrConjugacyClassesGL,
function(n,q)
return Sum(Partitions(n),
v -> Product(List(Set(v), i -> Number(v, j -> j = i)),
n_i -> q^n_i - q^(n_i - 1)));
end);
#############################################################################
##
#F NrConjugacyClassesSLIsogeneous( <n>, <q>, <f> )
##
## Class number for group isogeneous to SL(n,q)
##
InstallGlobalFunction(NrConjugacyClassesSLIsogeneous,
function(n,q,f)
return Sum(Cartesian(DivisorsInt(Gcd( f,q - 1)),
DivisorsInt(Gcd(n/f,q - 1))),
d -> Phi(d[1]) * Phi2(d[2])
* NrConjugacyClassesGL(n/Product(d),q))/(q - 1);
end);
#############################################################################
##
#F NrConjugacyClassesSL( <n>, <q> ) . . . . . . . Class number for SL(n,q)
##
InstallGlobalFunction(NrConjugacyClassesSL,
function(n,q)
return NrConjugacyClassesSLIsogeneous(n,q,1);
end);
#############################################################################
##
#F NrConjugacyClassesPGL( <n>, <q> ) . . . . . . . Class number for PGL(n,q)
##
InstallGlobalFunction(NrConjugacyClassesPGL,
function(n,q)
return NrConjugacyClassesSLIsogeneous(n,q,n);
end);
#############################################################################
##
#F NrConjugacyClassesPSL( <n>, <q> ) . . . . . . . Class number for PSL(n,q)
##
InstallGlobalFunction(NrConjugacyClassesPSL,
function(n,q)
return Sum(Filtered(Cartesian(DivisorsInt(q - 1),DivisorsInt(q - 1)),
d -> n mod Product(d) = 0),
d -> Phi(d[1]) * Phi2(d[2])
* NrConjugacyClassesGL(n/Product(d),q)/(q - 1))/Gcd(n,q - 1);
end);
#############################################################################
##
#F NrConjugacyClassesGU( <n>, <q> ) . . . . . . . . Class number for GU(n,q)
##
## This is also needed for the computation of the class numbers of PGU(n,q),
## SU(n,q) and PSU(n,q)
##
InstallGlobalFunction(NrConjugacyClassesGU,
function(n,q)
return Sum(Partitions(n),
v -> Product(List(Set(v), i -> Number(v, j -> j = i)),
n_i -> q^n_i + q^(n_i - 1)));
end);
#############################################################################
##
#F NrConjugacyClassesSUIsogeneous( <n>, <q>, <f> )
##
## Class number for group isogeneous to SU(n,q)
##
InstallGlobalFunction(NrConjugacyClassesSUIsogeneous,
function(n,q,f)
return Sum(Cartesian(DivisorsInt(Gcd( f,q + 1)),
DivisorsInt(Gcd(n/f,q + 1))),
d -> Phi(d[1]) * Phi2(d[2])
* NrConjugacyClassesGU(n/Product(d),q))/(q + 1);
end);
#############################################################################
##
#F NrConjugacyClassesSU( <n>, <q> ) . . . . . . . Class number for SU(n,q)
##
InstallGlobalFunction(NrConjugacyClassesSU,
function(n,q)
return NrConjugacyClassesSUIsogeneous(n,q,1);
end);
#############################################################################
##
#F NrConjugacyClassesPGU( <n>, <q> ) . . . . . . . Class number for PGU(n,q)
##
InstallGlobalFunction(NrConjugacyClassesPGU,
function(n,q)
return NrConjugacyClassesSUIsogeneous(n,q,n);
end);
#############################################################################
##
#F NrConjugacyClassesPSU( <n>, <q> ) . . . . . . . Class number for PSU(n,q)
##
InstallGlobalFunction(NrConjugacyClassesPSU,
function(n,q)
return Sum(Filtered(Cartesian(DivisorsInt(q + 1),DivisorsInt(q + 1)),
d -> n mod Product(d) = 0),
d -> Phi(d[1]) * Phi2(d[2])
* NrConjugacyClassesGU(n/Product(d),q)/(q + 1))/Gcd(n,q + 1);
end);
#############################################################################
##
#M NrConjugacyClasses( <G> ) . . . . . . . . . . Method for natural GL(n,q)
##
InstallMethod( NrConjugacyClasses,
"for natural GL",
true,
[ IsFFEMatrixGroup and IsFinite and IsNaturalGL ],
0,
function ( G )
local n,q;
n := DimensionOfMatrixGroup(G);
q := Size(FieldOfMatrixGroup(G));
return NrConjugacyClassesGL(n,q);
end );
#############################################################################
##
#M NrConjugacyClasses( <G> ) . . . . . . . . . . Method for natural SL(n,q)
##
InstallMethod( NrConjugacyClasses,
"for natural SL",
true,
[ IsFFEMatrixGroup and IsFinite and IsNaturalSL ],
0,
function ( G )
local n,q;
n := DimensionOfMatrixGroup(G);
q := Size(FieldOfMatrixGroup(G));
return NrConjugacyClassesSL(n,q);
end );
#############################################################################
##
#M NrConjugacyClasses( <G> ) . . . . . . . . . . . . . . Method for GU(n,q)
##
InstallMethod( NrConjugacyClasses,
"for GU(n,q)",
true,
[ IsFFEMatrixGroup and IsFinite
and IsFullSubgroupGLorSLRespectingSesquilinearForm ],
0,
function ( G )
local n,q;
if IsSubgroupSL(G) then TryNextMethod(); fi;
n := DimensionOfMatrixGroup(G);
q := RootInt(Size(FieldOfMatrixGroup(G)));
return NrConjugacyClassesGU(n,q);
end );
#############################################################################
##
#M NrConjugacyClasses( <G> ) . . . . . . . . . . Method for natural SU(n,q)
##
InstallMethod( NrConjugacyClasses,
"for natural SU",
true,
[ IsFFEMatrixGroup and IsFinite
and IsFullSubgroupGLorSLRespectingSesquilinearForm
and IsSubgroupSL ],
0,
function ( G )
local n,q;
n := DimensionOfMatrixGroup(G);
q := RootInt(Size(FieldOfMatrixGroup(G)));
return NrConjugacyClassesSU(n,q);
end );
InstallGlobalFunction(ClassesProjectiveImage,function(act)
local G,PG,cl,c,i,r,s,sel,p,z,a,x,prop,fus,f,reps,repi,repo,zel,fcl,
real,goal,good,e;
G:=Source(act);
# elementary divisors for GL-class identification
x:=X(DefaultFieldOfMatrixGroup(G),1);
prop:=y->Set(Filtered(ElementaryDivisorsMat(y-x*y^0),
y->DegreeOfUnivariateLaurentPolynomial(y)>0));
# compute real fusion
real:=function(set)
local new,i,a,b;
new:=[];
for i in set do
if i in set then # might have been removed by now
b:=ConjugacyClass(PG,repi[i]);
a:=Filtered(set,x->x<>i and repi[x] in b);
a:=Union(a,[i]);
fcl[a[1]]:=b;
Add(new,a);
set:=Difference(set,a);
fi;
od;
return new;
end;
#dom:=NormedRowVectors(DefaultFieldOfMatrixGroup(G)^Length(One(G)));
#act:=ActionHomomorphism(G,dom,OnLines,"surjective");
PG:=Image(act); # this will be PSL etc.
StabChainMutable(PG);; # needed anyhow and will speed up images under act
z:=Size(Centre(G));
zel:=Filtered(AsSSortedList(Centre(G)),x->Order(x)>1);
cl:=ConjugacyClasses(G);
if IsNaturalGL(G) then
goal:=NrConjugacyClassesPGL(Length(One(G)),
Size(DefaultFieldOfMatrixGroup(G)));
elif IsNaturalSL(G) then
goal:=NrConjugacyClassesPSL(Length(One(G)),
Size(DefaultFieldOfMatrixGroup(G)));
else
goal:=Length(cl); # this is too loose, but upper limit
fi;
s:=[]; # count how much of pre-images we still need to account for
sel:=[];
reps:=List(cl,Representative);
repi:=List(reps,x->ImagesRepresentative(act,x));
repo:=List(repi,Order);
e:=List(reps,prop);
sel:=[1..Length(cl)];
fcl:=[]; # cached factor group classes
if z=1 then
fus:=List(sel,x->[x]);
else
# fuse maximally under centre multiplication
fus:=[];
while Length(sel)>0 do
a:=sel[1]; sel:=sel{[2..Length(sel)]};
p:=Union(e{[a]},List(zel,x->prop(reps[a]*x)));
f:=Filtered(sel,x->e[x] in p and repo[a]=repo[x]);
sel:=Difference(sel,f);
AddSet(f,a);
Add(fus,f);
od;
# separate those that clearly cannot fuse fully
good:=[];
for i in Filtered(fus,x->Length(x)>z or z mod Length(x)<>0) do
a:=real(i);
fus:=Union(Filtered(fus,x->x<>i),a);
good:=Union(good,a); # record that we properly tested
od;
# now go through and test properly and fuse, unless we reached the
# proper class number
for i in fus do
if not i in good and Length(fus)<goal then
# fusion could split up -- test
a:=real(i);
fus:=Union(Filtered(fus,x->x<>i),a);
fi;
od;
fi;
# now fusion is good -- form classes
c:=[];
for i in fus do
if IsBound(fcl[i[1]]) then
a:=fcl[i[1]];
else
a:=ConjugacyClass(PG,repi[i[1]]);
fi;
Add(c,a);
f:=Sum(cl{i},Size)/z;
SetSize(a,f);
od;
SetConjugacyClasses(PG,c);
return [act,PG,c];
end);
#############################################################################
##
#E