|
#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Heiko Theißen, Alexander Hulpke, Martin Schönert.
##
## Copyright of GAP belongs to its developers, whose names are too numerous
## to list here. Please refer to the COPYRIGHT file for details.
##
## SPDX-License-Identifier: GPL-2.0-or-later
##
## This file contains the methods for symmetric and alternating groups
##
#############################################################################
##
#M <perm> in <nat-alt-grp>
##
InstallMethod( \in,
"alternating",
true,
[ IsPerm,
IsNaturalAlternatingGroup ],
0,
function( g, S )
local m, l;
if SignPerm(g)=-1 then
return false;
fi;
m := MovedPoints(S);
l := NrMovedPoints(S);
if g = One( g ) then
return true;
elif l = 0 then
return false;
elif IsRange(m) and ( l = 1 or m[2] - m[1] = 1 ) then
return SmallestMovedPoint(g) >= m[1]
and LargestMovedPoint(g) <= m[l];
else
return IsSubset( m, MovedPoints( [g] ) );
fi;
end );
#############################################################################
##
#M RepresentativeAction( <G>, <d>, <e>, <opr> ). . for alternating groups
##
## This method may fail if d and e are the same integer. RepresentativeAction
## catches this case first, so this may be acceptable.
##
##
InstallOtherMethod( RepresentativeActionOp, "natural alternating group",
true, [ IsNaturalAlternatingGroup, IsObject, IsObject, IsFunction ],
# the objects might be group elements: rank up
{} -> 2*RankFilter(IsMultiplicativeElementWithInverse),
function ( G, d, e, opr )
local dom,dom2,sortfun,cd,ce,rep,dp,ep;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
dom:=Set(MovedPoints(G));
if opr=OnPoints then
if IsInt(d) and IsInt(e) then
if d in dom and e in dom and Length(dom)>2 then
return (d,e,First(dom,i->i<>d and i<>e));
else
return fail;
fi;
elif IsPerm(d) and IsPerm(e) and d in G and e in G then
sortfun:=function(a,b) return Length(a)<Length(b);end;
if Order(d)=1 then #LargestMovedPoint does not work for ().
if Order(e)=1 then
return ();
else
return fail;
fi;
fi;
if CycleStructurePerm(d)<>CycleStructurePerm(e) then
return fail;
fi;
dp:=d;
ep:=e;
cd:=ShallowCopy(Cycles(d,dom));
ce:=ShallowCopy(Cycles(e,dom));
Sort(cd,sortfun);
Sort(ce,sortfun);
rep:=MappingPermListList(Concatenation(cd),Concatenation(ce));
if SignPerm(rep)=-1 then
dom2:=Difference(dom,Union(Concatenation(cd),Concatenation(ce)));
if Length(dom2)>1 then
rep:=rep*(dom2[1],dom2[2]);
else
#this is more complicated
TryNextMethod();
# temporarily disabled, Situation is more complicated
cd:=Filtered(cd,i->IsSubset(dom,i));
d:=CycleStructurePerm(d);
e:=PositionProperty([1..Length(d)],i->IsBound(d[i]) and
# cycle structure is shifted, so this is even length
# we need either to swap a pair of even cycles or to 3-cycle
# odd cycles
((IsInt((i+1)/2) and d[i]>1) or
(IsInt(i/2) and d[i]>2)));
if e=fail then
rep:=fail;
elif IsInt((e+1)/2) then
cd:=Filtered(cd,i->Length(i)=e+1);
cd:=cd{[1,2]};
rep:=MappingPermListList(Concatenation(cd),
Concatenation([cd[2],cd[1]]))*rep;
else
cd:=Filtered(cd,i->Length(i)=e+1);
cd:=cd{[1,2,3]};
rep:=MappingPermListList(Concatenation(cd),
Concatenation([cd[2],cd[3],cd[1]]))*rep;
fi;
fi;
fi;
if rep<>fail then
Assert(1,dp^rep=ep);
fi;
return rep;
fi;
elif (opr=OnSets or opr=OnTuples) and (IsDuplicateFreeList(d) and
IsDuplicateFreeList(e)) then
if Length(d)<>Length(e) then
return fail;
fi;
if IsSubset(dom,Set(d)) and IsSubset(dom,Set(e)) then
rep:=MappingPermListList(d,e);
if SignPerm(rep)=-1 then
cd:=Difference(dom,e);
if Length(cd)>1 then
rep:=rep*(cd[1],cd[2]);
elif opr=OnSets then
if Length(d)>1 then
rep:=(d[1],d[2])*rep;
else
rep:=fail; # set Length <2, maximal 1 further point in dom,imposs.
fi;
else # opr=OnTuples, not enough points left
rep:=fail;
fi;
fi;
return rep;
fi;
fi;
TryNextMethod();
end);
InstallMethod( SylowSubgroupOp,
"alternating",
true,
[ IsNaturalAlternatingGroup, IsPosInt ], 0,
function ( G, p )
local S, # <p>-Sylow subgroup of <G>, result
sgs, # strong generating set of <G>
q, # power of <p>
i, # loop variable
trf,
mov,
deg;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
mov:=MovedPoints(G);
deg:=Length(mov);
# make the strong generating set
sgs := [];
for i in [3..deg] do
q := p;
if p = 2 and i mod 2 = 0 then
Add( sgs, (mov[1],mov[2])(mov[i-1],mov[i]) );
q := q * p;
fi;
trf:=MappingPermListList([1..deg],mov); # translating perm
while i mod q = 0 do
Add( sgs, PermList( Concatenation(
[1..i-q], [i-q+1+q/p..i], [i-q+1..i-q+q/p] ) )^trf );
q := q * p;
od;
od;
# make the Sylow subgroup
S := SubgroupNC( G, sgs );
SetSize(S,p^Length(sgs));
# add the stabilizer chain
#MakeStabChainStrongGenerators( S, Reversed([1..G.degree]), sgs );
if Size( S ) > 1 then
SetIsPGroup( S, true );
SetPrimePGroup( S, p );
SetHallSubgroup(G, [p], S);
fi;
# return the Sylow subgroup
return S;
end);
InstallMethod( ConjugacyClasses,
"alternating",
[ IsNaturalAlternatingGroup ], OverrideNice,
function ( G )
local classes, # conjugacy classes of <G>, result
prt, # partition of <G>
sum, # partial sum of the entries in <prt>
rep, # representative of a conjugacy class of <G>
mov,deg,trf, # degree, moved points, transfer
i; # loop variable
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
mov:=MovedPoints(G);
deg:=Length(mov);
if deg=0 then
TryNextMethod();
fi;
trf:=MappingPermListList([1..deg],mov);
# loop over the partitions
classes := [];
for prt in Partitions( deg ) do
# only take those partitions that lie in the alternating group
if Number( prt, i -> i mod 2 = 0 ) mod 2 = 0 then
# compute the representative of the conjugacy class
rep := [2..deg];
sum := 1;
for i in prt do
rep[sum+i-1] := sum;
sum := sum + i;
od;
rep := PermList( rep )^trf;
# add the new class to the list of classes
Add( classes, ConjugacyClass( G, rep ) );
# some classes split in the alternating group
if ForAll( prt, i -> i mod 2 = 1 )
and Length( prt ) = Length( Set( prt ) )
then
Add( classes, ConjugacyClass(G,rep^(mov[deg-1],mov[deg])) );
fi;
fi;
od;
# return the classes
return classes;
end);
BindGlobal("CheapIsomSymAlt",function(a,b)
local hom;
if IsPermGroup(a) then
hom:=SmallerDegreePermutationRepresentation(a:cheap);
if Image(hom)=b then return hom;
elif NrMovedPoints(Range(hom))<NrMovedPoints(a) then
return hom*IsomorphismGroups(Image(hom),b);
fi;
fi;
return IsomorphismGroups(a,b);
end);
InstallMethod( IsomorphismFpGroup, "alternating group, supply name", true,
[ IsAlternatingGroup ],
40, # override `IsSimple...' method
function(G)
if Size(G)=1 then TryNextMethod();fi;
return IsomorphismFpGroup(G,
Concatenation("A_",String(AlternatingDegree(G)),".") );
end);
InstallOtherMethod( IsomorphismFpGroup, "alternating perm group,name",
true,
[ IsAlternatingGroup and IsPermGroup, IsString ],
35, # override `IsSimpleGroup' method
function ( G,str )
local F, # free group
imgs,
premap, # map to apply first
hom, # bijection
mov,deg,# moved pts, degree
m, #[n/2]
relators,
r,s, # generators
d, # subset of pts
p, # permutation
j; # loop variables
# test for internal rep
if (HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep)) or Size(G)=1 then
TryNextMethod();
fi;
#are we natural?
if not IsNaturalAlternatingGroup(G) then
F:=G;
G:=AlternatingGroup(AlternatingDegree(G));
premap:=CheapIsomSymAlt(F,G);
else
premap:=fail;
fi;
mov:=MovedPoints(G);
deg:=Length(mov);
#special case for degree 3, cyclic
if deg=3 then
F := FreeGroup( 1, str);
imgs:=[(mov[1],mov[2],mov[3])];
relators:=[F.1^3];
else
# create the finitely presented group with <G>.degree-1 generators
F := FreeGroup( 2, str);
# add the relations according to the presentation by Coxeter
# (see Coxeter/Moser)
r:=F.1;
s:=F.2;
if IsOddInt(deg) then
m:=(deg-1)/2;
relators:=[r^deg/s^deg,r^deg/(r*s)^m];
for j in [2..m] do
Add(relators,(r^-j*s^j)^2);
od;
#(1,2,3,..deg) and (1,3,2,4,5,..deg)
p:=MappingPermListList(mov,Concatenation(mov{[2..deg]},[mov[1]]));
imgs:=[p,p^(mov[2],mov[3])];
else
m:=deg/2;
relators:=[r^(deg-1)/s^(deg-1),r^(deg-1)/(r*s)^m];
for j in [1..m-1] do
Add(relators,(r^-j*s^-1*r*s^j)^2);
od;
# (1,2,3,4..,deg-2,deg),(1,2,3,4,deg-3,deg-1,deg);
d:=Concatenation(mov{[1..deg-2]},[mov[deg]]);
p:=MappingPermListList(d,Concatenation(d{[2..deg-1]},[d[1]]));
imgs:=[p];
d:=Concatenation(mov{[1..deg-3]},mov{[deg-1,deg]});
p:=MappingPermListList(d,Concatenation(d{[2..deg-1]},[d[1]]));
Add(imgs,p);
fi;
fi;
F:=F/relators;
SetSize(F,Size(G));
UseIsomorphismRelation( G, F );
# return the isomorphism to the finitely presented group
hom:= GroupHomomorphismByImagesNC(G,F,imgs,GeneratorsOfGroup(F));
if premap<>fail then hom:=premap*hom;fi;
SetIsBijective( hom, true );
ProcessEpimorphismToNewFpGroup(hom);
return hom;
end);
# Presentation that gives nicer rewriting (from Derek Holt)
# A_n = < x_1, ..., x_{n-2} | x_1^3, x_i^2 (i > 1),
# (x_i x_{i+1})^3 (1 <= i <= n-3)
# (x_i x_j)^2 (1 <= i, i+1 < j, j <= n-2) >.
InstallMethod(IsomorphismFpGroupForRewriting,"alternating",
[IsNaturalAlternatingGroup],0,
function ( G )
local F, # free group
gens, #generators of F
imgs,
hom, # bijection
mov,deg,
relators,
i, j; # loop variables
if Size(G)=1 then TryNextMethod();fi;
mov:=MovedPoints(G);
deg:=Length(mov);
# create the finitely presented group with <G>.degree-1 generators
F := FreeGroup( deg-2, Concatenation("A_",String(deg),".") );
gens:=GeneratorsOfGroup(F);
relators := [];
Add(relators,gens[1]^3);
for i in [2..deg-2] do
Add(relators,gens[i]^2);
od;
for i in [1..deg-3] do
Add(relators,(gens[i]*gens[i+1])^3);
for j in [i+2..deg-2] do
Add(relators,(gens[i]*gens[j])^2);
od;
od;
F:=F/relators;
SetSize(F,Size(G));
UseIsomorphismRelation( G, F );
# compute the bijection
imgs:=[];
Add(imgs,(mov[1],mov[2],mov[3]));
for i in [3..deg-1] do
Add(imgs,(mov[1],mov[2])(mov[i],mov[i+1]));
od;
# return the isomorphism to the finitely presented group
hom:= GroupHomomorphismByImagesNC(G,F,imgs,GeneratorsOfGroup(F));
SetIsBijective( hom, true );
return hom;
end);
################################################
# h has socle A_n^{l1}, acting on l1-tuples of l2-sets
#output is a subset of the permutation domain, consisting
#of tuples which agree in l1-1 coordinates and intersect in l2-1 points
#in the last coordinate
BindGlobal( "PermNatAnTestDetect", function(h,n,l1,l2)
local schreiertree, cosetrepresentative, flag, schtree, stab, k, p, j,
cosetrep, orbits, neworb, int, set, count, flag2, neworb2, o, i,dom,pt1;
#permutation group h, on m points, Schreier tree with root k
schreiertree:=function(h,m,k)
local mark, gens, inv, schtree, i, j, list;
mark:=BlistList([1..m],[k]);
gens:=GeneratorsOfGroup(h);
inv:=List(gens,x->x^(-1));
list:=[k];
schtree:=[ ];
schtree[k]:=();
for i in list do
for j in [1..Length(gens)] do
if mark[i^(inv[j])]=false then
Add(list,i^(inv[j]));
mark[i^(inv[j])]:=true;
schtree[i^(inv[j])]:=gens[j];
fi;
od;
od;
return schtree;
end;
cosetrepresentative:=function(schtree,k,j)
local cosetrep;
cosetrep:=();
repeat
cosetrep:=cosetrep*schtree[j];
j:=j^schtree[j];
until j=k;
return cosetrep;
end;
flag:=true;
# create a domain of moved points, at least (n choose l2)^l1 long
dom:=Set(MovedPoints(h));
k:=Binomial(n,l2)^l1;
while Length(dom)<k do
AddSet(dom,Last(dom)+1);
od;
pt1:=dom[1];
schtree:= schreiertree(h,Last(dom),pt1);
# group generated by ten random elements of the stabilizer of k in h
stab:=[];
k:=pt1;
for i in [1..10] do
p:=PseudoRandom(h);
j:=k^p;
cosetrep:=cosetrepresentative(schtree,k,j);
Add(stab,p*cosetrep);
od;
stab:=Group(stab);
orbits:=Orbits(stab,dom{[1..Binomial(n,l2)^l1]},OnPoints);
k:=Position(List(orbits, Length),l1*l2*(n-l2));
if k = fail then
flag:= false;
else
j:=orbits[k][1];
cosetrep:=cosetrepresentative(schtree,1,j);
cosetrep:=cosetrep^(-1);
neworb:=List(orbits[k],x->x^cosetrep);
int:=Intersection(orbits[k],neworb);
Add(int,orbits[k][1]);
Add(int,1);
if Length(int) <> n then
flag:=false;
else
# int contains l2-1 extra l2-sets
if l2=1 then
set:=Set(int);
else
count:=1;
flag2:=false;
repeat
j:=int[count];
cosetrep:=cosetrepresentative(schtree,pt1,j);
cosetrep:=cosetrep^(-1);
neworb2:=List(orbits[k],x->x^cosetrep);
if Length(Intersection(int,neworb2))=n-l2 then
set:= Union(Intersection(int,neworb2),[int[count]]);
flag2:=true;
else
count:=count+1;
fi;
until flag2 or count>l2;
if not flag2 then
flag:=false;
fi;
fi;
fi;
fi;
if flag=true then
o:=Orbit(h,set,OnSets);
if Length(o) <> l1*Binomial(n,l2)^(l1-1)*Binomial(n,l2-1) then
flag:=false;
fi;
fi;
if flag=false then
return fail;
else
return set;
fi;
end );
# see Ákos Seress, Permutation group algorithms. Cambridge Tracts in
# Mathematics, 152. Section 10.2 for the background of this function.
BindGlobal("DoSnAnGiantTest",function(g,dom,kind)
local bound, n, i, p, l, pnt;
pnt := dom[1];
n:=Length(dom);
# From the above reference we see that with these bounds this function
# will fail on a symmetric group with probability < 10^-10.
if kind=1 then
bound:=10*LogInt(n,2);
else
bound:=50*LogInt(n,2);
fi;
i:=0;
# We are looking for an element with a cycle of prime length > n/2
# and < n-2. Instead of computing the complete cycle structure we just
# look at the cycle length of one moved point (if there is a cycle as
# desired, it will contain this point with probability > 1/2).
repeat
i:=i+1;
p:=PseudoRandom(g);
l:=CYCLE_LENGTH_PERM_INT(p,pnt);
until (i>bound) or (l> n/2 and l<n-2 and IsPrime(l));
if i>bound then
return fail;
else
return true;
fi;
end);
BindGlobal("PermgpContainsAn",function(g)
local dom, n, mine, root, d, k, b, m, l,lh;
dom:=MovedPoints(g);
n:=Length(dom);
if not IsTransitive(g,dom) then
return false;
fi;
if DoSnAnGiantTest(g,dom,1)=true then
# we've found elements that prove the group must contain A_n.
return true;
fi;
# otherwise, the group is likely (but not proven) to be different
if not IsPrimitive(g,dom) then
return false;
fi;
# so the group is primitive
if n>10 then # otherwise the size is immediate
# test whether its socle could be A_l^m, acting on k-tuples in product
# action.
mine:=Minimum(List(Collected(Factors(n)),i->i[2]));
for m in [1..mine] do
root:=RootInt(n,m);
if root^m=n then
# case k=1 -> A_root on points
if m>1 then # k=1, m=1: then it's A_n
d:=PermNatAnTestDetect(g,root,m,1);
if d<>fail then
Info(InfoGroup,3,"Detected ",root,",",m,",",1,"\n");
return d;
fi;
fi;
for l in [2..RootInt(2*root,2)+1] do
lh:=Int(l/2)+1;
k:=2;
b:=Binomial(l,k);
while b<root and k<lh do
k:=k+1;
b:=Binomial(l,k);
od;
if b=root then
d:=PermNatAnTestDetect(g,l,m,k);
if d<>fail then
Info(InfoGroup,3,"Detected ",l,",",m,",",k,"\n");
return d;
fi;
fi;
od;
fi;
od;
fi;
if DoSnAnGiantTest(g,dom,2)=true then
# we've found elements that prove the group must contain A_n.
return true;
fi;
# now the socle is not a power of A_l or n is small. So the group is small
# base enforce a stabilizer chain calculation
return Size(g)>=Factorial(n)/2;
end);
#############################################################################
##
#M IsNaturalAlternatingGroup( <sym> )
##
InstallMethod(IsNaturalAlternatingGroup,"knows size",true,[IsPermGroup
and HasSize],0,
function( g )
local s, i, n;
s := Size(g);
n := NrMovedPoints(g);
# avoid computing Factorial(n)
for i in [3..n] do
if s mod i <> 0 then
return false;
else
s := s/i;
fi;
od;
return s = 1;
end );
InstallMethod(IsNaturalAlternatingGroup,"comprehensive",true,[IsPermGroup],0,
G -> IsTrivial(G) or
ForAll(GeneratorsOfGroup(G),i->SignPerm(i)=1) and PermgpContainsAn(G)=true);
#############################################################################
##
#M IsNaturalSymmetricGroup( <sym> )
##
InstallMethod(IsNaturalSymmetricGroup,"knows size",true,[IsPermGroup
and HasSize],0,
function( g )
local s, i, n;
s := Size(g);
n := NrMovedPoints(g);
# avoid computing Factorial(n)
for i in [2..n] do
if s mod i <> 0 then
return false;
else
s := s/i;
fi;
od;
return true;
end );
InstallMethod(IsNaturalSymmetricGroup,"comprehensive",true,[IsPermGroup],0,
G -> IsTrivial(G) or
ForAny(GeneratorsOfGroup(G),i->SignPerm(i)=-1) and PermgpContainsAn(G)=true);
#############################################################################
##
#M <perm> in <nat-sym-grp>
##
InstallMethod( \in,"perm in natsymmetric group",
true,
[ IsPerm,
IsNaturalSymmetricGroup ],
0,
function( g, S )
local m, l;
m := MovedPoints(S);
l := NrMovedPoints(S);
if g = One( g ) then
return true;
elif l = 0 then
return false;
elif IsRange(m) and ( l = 1 or m[2] - m[1] = 1 ) then
return SmallestMovedPoint(g) >= m[1]
and LargestMovedPoint(g) <= m[l];
else
return IsSubset( m, MovedPoints( [g] ) );
fi;
end );
#############################################################################
##
#M IsSubset(<nat-sym-grp>,<permgrp>
##
InstallMethod( IsSubset,"permgrp of natsymmetric group", true,
[ IsNaturalSymmetricGroup,IsPermGroup ],
# we need to override a method that computes the size.
SUM_FLAGS,
function( S,G )
return IsSubset(MovedPoints(S),MovedPoints(G));
end );
#############################################################################
##
#M Socle( <nat-sym/alt-grp> )
##
InstallMethod( Socle,
true, [ IsNaturalSymmetricGroup ], 0,
function(sym)
if NrMovedPoints(sym)<=4 then
TryNextMethod();
else
return AlternatingGroup(MovedPoints(sym));
fi;
end);
InstallMethod( Socle, true, [ IsNaturalAlternatingGroup ], 0,
function(alt)
if NrMovedPoints(alt)<=4 then
TryNextMethod();
else
return alt;
fi;
end);
#############################################################################
##
#M Size( <nat-sym-grp> )
##
InstallMethod( Size,
true,
[ IsNaturalSymmetricGroup ], 0,
sym -> Factorial( NrMovedPoints(sym) ) );
BindGlobal("FLOYDS_ALGORITHM", function(rs, deg, even)
local rnd, sgn, i, k, tmp;
rnd := [1..deg];
sgn := 1;
for i in [1..deg-1] do
k := Random( rs, i, deg );
if k <> i then
tmp := rnd[i];
rnd[i] := rnd[k];
rnd[k] := tmp;
sgn := -sgn;
fi;
od;
if even and sgn = -1 then
tmp := rnd[deg];
rnd[deg] := rnd[deg-1];
rnd[deg-1] := tmp;
fi;
return rnd;
end);
InstallMethodWithRandomSource( Random,
"for a random source and a natural symmetric group: floyd's algorithm",
true,
[ IsRandomSource, IsNaturalSymmetricGroup ],
10, # override perm group method
function ( rs, G )
local rnd, # random permutation, result
deg,
mov;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
# use Floyd\'s algorithm
mov:=MovedPoints(G);
deg:=Length(mov);
rnd:=FLOYDS_ALGORITHM(rs,deg,false);
# return the permutation
return PermList( rnd )^MappingPermListList([1..deg],mov);
end);
InstallMethodWithRandomSource( Random,
"for a random source and a natural alternating group: floyd's algorithm",
true,
[ IsRandomSource, IsNaturalAlternatingGroup ],
10, # override perm group method
function ( rs, G )
local rnd, # random permutation, result
deg,
mov;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
# use Floyd\'s algorithm
mov:=MovedPoints(G);
deg:=Length(mov);
rnd:=FLOYDS_ALGORITHM(rs,deg,true);
# return the permutation
return PermList( rnd )^MappingPermListList([1..deg],mov);
end);
#############################################################################
##
#M Size( <nat-alt-grp> )
##
InstallMethod( Size,
true,
[ IsNaturalAlternatingGroup ], 0,
function(a)
local n;
n := NrMovedPoints(a);
if n <= 2 then
return 1;
fi;
return Factorial( n )/2;
end);
#############################################################################
##
#M Intersection2
InstallMethod( Intersection2, [IsNaturalSymmetricGroup, IsNaturalSymmetricGroup],
function(s1,s2)
return SymmetricGroup(Intersection(MovedPoints(s1),MovedPoints(s2)));
end);
InstallMethod( Intersection2, [IsNaturalSymmetricGroup, IsNaturalAlternatingGroup],
function(s1,s2)
return AlternatingGroup(Intersection(MovedPoints(s1),MovedPoints(s2)));
end);
InstallMethod( Intersection2, [IsNaturalAlternatingGroup, IsNaturalSymmetricGroup],
function(s1,s2)
return AlternatingGroup(Intersection(MovedPoints(s1),MovedPoints(s2)));
end);
InstallMethod( Intersection2, [IsNaturalAlternatingGroup, IsNaturalAlternatingGroup],
function(s1,s2)
return AlternatingGroup(Intersection(MovedPoints(s1),MovedPoints(s2)));
end);
InstallMethod( Intersection2, [IsNaturalSymmetricGroup, IsPermGroup],
function(s,g)
return Stabilizer(g,Difference(MovedPoints(g),MovedPoints(s)), OnTuples);
end);
InstallMethod( Intersection2, [IsPermGroup, IsNaturalSymmetricGroup],
function(g,s)
return Stabilizer(g,Difference(MovedPoints(g),MovedPoints(s)), OnTuples);
end);
InstallMethod( Intersection2, [IsNaturalAlternatingGroup, IsPermGroup],
function(s,g)
return AlternatingSubgroup(Stabilizer(g,Difference(MovedPoints(g),MovedPoints(s)), OnTuples));
end);
InstallMethod( Intersection2, [IsPermGroup, IsNaturalAlternatingGroup],
function(g,s)
return AlternatingSubgroup(Stabilizer(g,Difference(MovedPoints(g),MovedPoints(s)), OnTuples));
end);
#############################################################################
##
#M DerivedSubgroup
##
InstallMethod( DerivedSubgroup, [IsNaturalSymmetricGroup],
function(g)
local d;
d := AlternatingGroup(MovedPoints(g));
d := AsSubgroup(g,d);
SetIsNaturalAlternatingGroup(d, true);
return d;
end);
#############################################################################
##
#M StabilizerOp( <nat-sym-grp>, ...... )
##
BindGlobal( "SYMGP_STABILIZER", function(sym, arg...)
local k, act, pt, mov, stab, nat, diff, int, bls, mov1, parts,
part, bl, i, gens, size, alt;
k := Length(arg);
act := arg[k];
pt := arg[k-3];
if arg[k-1] <> arg[k-2] then
TryNextMethod();
fi;
alt := IsNaturalAlternatingGroup(sym);
if alt then
sym := SymmetricParentGroup(sym);
fi;
mov := MovedPoints(sym);
if act = OnPoints and IsPosInt(pt) then
if pt in mov then
stab := SymmetricGroup(Difference(mov,[pt]));
else
stab := sym;
fi;
nat := true;
elif (act = OnTuples or act = OnPairs) and IsList(pt) and ForAll(pt, IsPosInt) then
stab := SymmetricGroup(Difference(mov, Set(pt)));
nat := true;
elif act = OnSets and IsSet(pt) and ForAll(pt, IsPosInt) then
diff := Difference(mov, pt);
int := Intersection(mov,pt);
if Length(diff) = 0 or Length(int) = 0 then
stab := sym;
nat := true;
else
stab := Group(Concatenation(GeneratorsOfGroup(SymmetricGroup(diff)),
GeneratorsOfGroup(SymmetricGroup(int))),());
SetSize(stab, Factorial(Length(diff))*Factorial(Length(int)));
nat := Length(diff) = 1 or Length(int) = 1;
fi;
elif act = OnTuplesTuples and IsList(pt) and ForAll(pt, x->IsList(x) and ForAll(x,IsPosInt)) then
stab := SymmetricGroup(Difference(mov,Set(Flat(pt))));
nat := true;
elif act = OnTuplesSets and IsList(pt) and ForAll(pt, x-> IsSet(x) and ForAll(x,IsPosInt)) then
bls := List(mov, x-> List(pt, y-> x in y));
mov1 := ShallowCopy(mov);
SortParallel(bls, mov1);
parts := [];
part := [mov1[1]];
bl := bls[1];
for i in [2..Length(mov)] do
if bls[i] <> bl then
if Length(part) > 1 then
Add(parts, part);
fi;
bl := bls[i];
part := [mov1[i]];
else
Add(part, mov1[i]);
fi;
od;
if Length(part) > 1 then
Add(parts, part);
fi;
gens := [];
size := 1;
for part in parts do
Append(gens, GeneratorsOfGroup(SymmetricGroup(part)));
size := size*Factorial(Length(part));
od;
stab := Group(gens,());
SetSize(stab,size);
nat := Length(parts) <= 1;
else
TryNextMethod();
fi;
stab := AsSubgroup(sym, stab);
if nat then
SetIsNaturalSymmetricGroup(stab,true);
fi;
if alt then
stab := AlternatingSubgroup(stab);
fi;
return stab;
end );
InstallOtherMethod( StabilizerOp,"symmetric group", true,
[ IsNaturalSymmetricGroup, IsObject, IsList, IsList, IsFunction ],
# the objects might be a group element: rank up
{} -> RankFilter(IsMultiplicativeElementWithInverse) +
RankFilter(IsSolvableGroup),
SYMGP_STABILIZER);
InstallOtherMethod( StabilizerOp,"symmetric group", true,
[ IsNaturalSymmetricGroup, IsDomain, IsObject, IsList, IsList, IsFunction ],
# the objects might be a group element: rank up
{} -> RankFilter(IsMultiplicativeElementWithInverse) +
RankFilter(IsSolvableGroup),
SYMGP_STABILIZER);
InstallOtherMethod( StabilizerOp,"alternating group", true,
[ IsNaturalAlternatingGroup, IsObject, IsList, IsList, IsFunction ],
# the objects might be a group element: rank up
{} -> RankFilter(IsMultiplicativeElementWithInverse) +
RankFilter(IsSolvableGroup),
SYMGP_STABILIZER);
InstallOtherMethod( StabilizerOp,"alternating group", true,
[ IsNaturalAlternatingGroup, IsDomain, IsObject, IsList, IsList, IsFunction ],
# the objects might be a group element: rank up
{} -> RankFilter(IsMultiplicativeElementWithInverse) +
RankFilter(IsSolvableGroup),
SYMGP_STABILIZER);
InstallMethod( CentralizerOp,
"element in natural alternating group",
IsCollsElms,
[ IsNaturalAlternatingGroup, IsPerm ], 0,
function ( G, g )
return AlternatingSubgroup(Centralizer(SymmetricParentGroup(G),g));
end);
InstallMethod( CentralizerOp,
"element in natural symmetric group",
IsCollsElms,
[ IsNaturalSymmetricGroup, IsPerm ], 0,
function ( G, g )
local C, # centralizer of <g> in <G>, result
sgs, # strong generating set of <C>
gen, # one generator in <sgs>
cycles, # cycles of <g>
cycle, # one cycle from <cycles>
lasts, # '<lasts>[<l>]' is the last cycle of length <l>
last, # one cycle from <lasts>
counts, # number of cycles of each length
mov, # moved points of group
siz, # size of centraliser
l;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
if not g in G then
TryNextMethod();
fi;
# handle special case
mov:=MovedPoints(G);
# start with the empty strong generating system
sgs := [];
# compute the cycles and find for each length the last one
# and the count
cycles := Cycles( g, mov );
lasts := [];
counts := [];
for cycle in cycles do
l := Length(cycle);
lasts[l] := cycle;
if not IsBound(counts[l]) then
counts[l] := 1;
else
counts[l] := counts[l]+1;
fi;
od;
# loop over the cycles
for cycle in cycles do
l := Length(cycle);
# add that cycle itself to the strong generators
if l <> 1 then
gen := MappingPermListList(cycle,
Concatenation(cycle{[2..l]},[cycle[1]]));
Add( sgs, gen );
fi;
# and this cycle can be mapped to the last cycle of this length
if cycle <> lasts[ Length(cycle) ] then
last := lasts[ Length(cycle) ];
gen := MappingPermListList(Concatenation(cycle,last),
Concatenation(last,cycle));
Add( sgs, gen );
fi;
od;
siz := 1;
for l in [1..Length(counts)] do
if IsBound(counts[l]) then
siz := siz*l^counts[l]*Factorial(counts[l]);
fi;
od;
# make the centralizer
C := SubgroupNC( G , sgs );
SetSize(C,siz);
# return the centralizer
return C;
end);
BindGlobal("AllNormalizerfixedBlockSystem",function(G,dom)
local b, bl,prop;
# what properties can we find easily
prop:=function(s)
local p;
s:=Set(dom{s});
p:=[Length(s)];
if ValueOption(NO_PRECOMPUTED_DATA_OPTION)<>true then
Info(InfoPerformance,2,"Using Transitive Groups Library");
# type of action on blocks
if TransitiveGroupsAvailable(Length(dom)/Length(s)) then
Add(p,TransitiveIdentification(Action(G,Orbit(G,s,OnSets),OnSets)));
fi;
# type of action on blocks
if TransitiveGroupsAvailable(Length(s)) then
Add(p,TransitiveIdentification(Action(Stabilizer(G,s,OnSets),s)));
fi;
fi;
if Length(p)=1 then
return p[1];
else
return p;
fi;
end;
if IsPrimeInt(Length(dom)) then
# no need trying
return fail;
fi;
b:=AllBlocks(Action(G,dom));
b:=ShallowCopy(b);
#Print(List(b,Length),"\n");
bl:=Collected(List(b,prop));
bl:=Filtered(bl,i->i[2]=1);
if Length(bl)=0 then
Info(InfoGroup,3,"No normalizerfixed block found");
return fail;
fi;
b:=Filtered(b,i->prop(i)=bl[1][1]);
Info(InfoGroup,3,"Normalizerfixed block system blocksize ",List(b,Length));
return List(b,x->Set(Orbit(G,Set(dom{x}),OnSets)));
end);
# Calculate subgroup of Sn/An that must contain the normalizer. Then a
# subsequent backtrack search is in a smaller group and thus much faster.
# Parameters: Overgroup (must be symmetric or alternating, otherwise just
# returns this overgroup), subgroup.
InstallGlobalFunction(NormalizerParentSA,function(s,u)
local dom, issym, o, b, beta, alpha, emb, nb, na, w, perm, pg, l, is, ie, ll,
syll, act, typ, sel, bas, wdom, comp, lperm, other, away, i, j,b0,opg,bp;
dom:=Set(MovedPoints(s));
issym:=IsNaturalSymmetricGroup(s);
if not IsSubset(dom,MovedPoints(u)) or
(not issym and (not IsNaturalAlternatingGroup(s) or
ForAny(GeneratorsOfGroup(u),x->SignPerm(x)=-1))) then
return s; # cannot get parent, as not contained
fi;
# get orbits
o:=ShallowCopy(Orbits(u,dom));
Info(InfoGroup,1,"SymmAlt normalizer: orbits ",List(o,Length));
if Length(o)=1 and IsAbelian(u) then
b:=List(PrimeDivisors(Size(u)),p->Omega(SylowSubgroup(u,p),p,1));
if Length(b)=1 and IsTransitive(b[1],dom) then
# elementary abelian, regular -- construct the correct AGL
b:=b[1];
bas:=Pcgs(b);
pg:=Centralizer(s,b);
for i in GeneratorsOfGroup(GL(Length(bas),RelativeOrders(bas)[1])) do
bp:=dom[1];
w:=GroupHomomorphismByImagesNC(b,b,bas,
List([1..Length(bas)],x->PcElementByExponents(bas,i[x])));
w:=List(dom,x->bp^Image(w,First(AsSSortedList(b),a->bp^a=x)));
w:=MappingPermListList(dom,w);
pg:=ClosureGroup(pg,w);
od;
return Intersection(s,pg);
else
SortBy(b,Size);
b:=Reversed(b); # larger ones should give most reduction.
pg:=NormalizerParentSA(s,b[1]);
for i in [2..Length(b)] do
pg:=Normalizer(pg,b[i]);
od;
return Intersection(s,pg);
fi;
elif Length(o)=1 and IsPrimitive(u,dom) then
# natural symmetric/alternating
if IsNormal(s,u) then
return s;
fi;
# can there be more in the normalizer -- primitive groups
b:=Socle(u);
if IsElementaryAbelian(b) then
return NormalizerParentSA(s,b);
fi;
# nonabelian socle
if PrimitiveGroupsAvailable(Length(dom))
and ValueOption(NO_PRECOMPUTED_DATA_OPTION)<>true then
Info(InfoPerformance,2,"Using Primitive Groups Library");
# use library
beta:=Factorial(Length(dom))/2;
w:=CallFuncList(ValueGlobal("AllPrimitiveGroups"),
[NrMovedPoints,Length(dom),IsSolvableGroup,false,
x->Size(x)>Size(u) and Size(x) mod Size(u)=0 and
Size(x)<beta,true]);
if Length(w)=0 then
return u; # must be self-normalizing
fi;
fi;
# find right automorphisms (socle cannot have centralizer)
w:=AutomorphismGroup(b);
opg:=NaturalHomomorphismByNormalSubgroupNC(w,
InnerAutomorphismsAutomorphismGroup(w));
ll:=List(AsSSortedList(Image(opg)),x->PreImagesRepresentative(opg,x));
ll:=Filtered(ll,IsConjugatorAutomorphism);
ll:=List(ll,ConjugatorInnerAutomorphism);
pg:=b;
for i in ll do
pg:=ClosureGroup(pg,i);
od;
return Intersection(s,pg);
elif Length(o)=1 then
b0:=AllNormalizerfixedBlockSystem(u,o[1]);
if b0=fail then
# none -- no improvement
return s;
fi;
# the normalizer must fix these block system
opg:=fail;
for b in b0 do
beta:=ActionHomomorphism(u,b,OnSets,"surjective");
alpha:=ActionHomomorphism(Stabilizer(u,b[1],OnSets),b[1],"surjective");
emb:=KuKGenerators(u,beta,alpha);
nb:=Normalizer(SymmetricGroup(Length(b)),Image(beta));
na:=Normalizer(SymmetricGroup(Length(b[1])),Image(alpha));
w:=WreathProduct(na,nb);
if issym then
perm:=s;
else
perm:=SymmetricGroup(MovedPoints(s));
fi;
perm:=RepresentativeAction(perm,emb,GeneratorsOfGroup(u),OnTuples);
if perm<>fail then
pg:=w^perm;
else
#Print("Embedding Problem!\n");
w:=WreathProduct(SymmetricGroup(Length(b[1])),SymmetricGroup(Length(b)));
perm:=MappingPermListList([1..Length(o[1])],Concatenation(b));
pg:=w^perm;
fi;
if opg<>fail then
pg:=Intersection(pg,opg);
fi;
opg:=pg;
od;
if Length(GeneratorsOfGroup(pg))>5 then
opg:=Group(SmallGeneratingSet(pg));
SetSize(opg,Size(pg));
pg:=opg;
fi;
else
# first sort by Length
SortBy(o, Length);
l:=Length(o);
pg:=[]; # parent generators
is:=1;
ie:=1;
while is<=l do
ll:=Length(o[is]);
while ie<=l and Length(o[ie])=ll do
ie:=ie+1;
od;
# now length block is from is to ie-1
syll:=SymmetricGroup(ll);
# if the degrees are small enough, even get local types
if ll>1 and TransitiveGroupsAvailable(ll)
and ValueOption(NO_PRECOMPUTED_DATA_OPTION)<>true then
Info(InfoPerformance,2,"Using Transitive Groups Library");
Info(InfoGroup,1,"Length ",ll," sort by types");
act:=[];
typ:=[];
for i in [is..ie-1] do
act[i]:=Action(u,o[i]);
typ[i]:=TransitiveIdentification(act[i]);
od;
# rearrange
for i in Set(typ) do
sel:=Filtered([is..ie-1],j->typ[j]=i);
bas:=NormalizerParentSA(syll,act[sel[1]]);
bas:=Normalizer(bas,act[sel[1]]);
w:=WreathProduct(bas,SymmetricGroup(Length(sel)));
wdom:=[1..ll*Length(sel)];
comp:=WreathProductInfo(w).components;
# now the suitable permutation
perm:=();
# first permutation on each component
for j in [1..Length(sel)] do
if j=1 then
lperm:=();
else
lperm:=RepresentativeAction(syll,act[sel[1]],act[sel[j]]);
fi;
other:=Difference(wdom,comp[j]);
away:=[1..Length(other)]+Length(wdom);
perm:=perm*MappingPermListList(Concatenation(comp[j],other),
Concatenation([1..ll],away)) # j-th component
*lperm # standard form
*MappingPermListList(Concatenation([1..ll],away),
Concatenation(comp[j],other)); # to j orbit
od;
# and then of components
perm:=perm*MappingPermListList(wdom,Concatenation(o{sel}));
for i in SmallGeneratingSet(w) do
Add(pg,i^perm);
od;
od;
else
bas:=syll;
w:=WreathProduct(bas,SymmetricGroup(ie-is));
perm:=MappingPermListList([1..ll*(ie-is)],Concatenation(o{[is..ie-1]}));
for i in SmallGeneratingSet(w) do
Add(pg,i^perm);
od;
fi;
is:=ie;
od;
pg:=Group(pg,());
fi;
if not issym then
pg:=AlternatingSubgroup(pg);
fi;
if (Size(pg)/Size(u))>10000 and IsSolvableGroup(pg) then
perm:=IsomorphismPcGroup(pg);
pg:=PreImage(perm,Normalizer(Image(perm,pg),Image(perm,u)));
fi;
return pg;
end);
BindGlobal("DoNormalizerSA",function ( G, U )
local P;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
P:=NormalizerParentSA(G,U);
if Size(P)<Size(G) then
Info(InfoGroup,1,"Normalizer parent deg ",NrMovedPoints(G),
" reduces by ",Index(G,P));
return AsSubgroup(G,Normalizer(P,U));
else
Info(InfoGroup,2,"No improvement by symm/alt normalizer");
TryNextMethod(); # go way of permutations
fi;
end);
InstallMethod( NormalizerOp, "subgp of natural symmetric group",
IsIdenticalObj, [ IsNaturalSymmetricGroup, IsPermGroup ], 0,
DoNormalizerSA);
InstallMethod( NormalizerOp, "subgp of natural alternating group",
IsIdenticalObj, [ IsNaturalAlternatingGroup, IsPermGroup ], 0,
DoNormalizerSA);
# conjugate subgroups of symmetric group.
# false indicates the method does not work
BindGlobal("SubgpConjSymmgp",function(s,g,h)
local og,oh,cb,cc,cac,perm1,perm2,
dom,n,a,c,b,b2,w,p1,p2,perm,t,ac,ac2,no,no2,i;
p1:=Set(MovedPoints(g));
p2:=Set(MovedPoints(h));
dom:=Set(MovedPoints(s));
og:=Orbits(g,p1);
oh:=Orbits(h,p2);
# different orbits lengths -- cannot be conjugate
if Collected(List(og,Length))<>Collected(List(oh,Length)) then
return fail;
fi;
if Length(og)>1 or p1<>p2 or p1<>dom then
# intransitive
if not (IsSubset(dom,p1) and IsSubset(dom,p2)) then
return false;
fi;
if Collected(List(og,Length))<>Collected(List(oh,Length)) then
return fail;
fi;
og:=Set(og,Set);
oh:=Set(oh,Set);
ac:=[];
a:=1;
perm:=();
perm1:=[];
perm2:=[];
if p1<>p2 then
Add(perm1,Difference(dom,p1));
Add(perm2,Difference(dom,p2));
fi;
for i in (Set(og,Length)) do
c:=Filtered(og,x->Length(x)=i);
#Append(p1,c);
ac2:=Filtered(oh,x->Length(x)=i);
#Append(p2,ac2);
w:=WreathProduct(SymmetricGroup(i),SymmetricGroup(Length(ac2)));
b:=Blocks(w,MovedPoints(w),[1..i]);
cc:=Concatenation(c);
cb:=Concatenation(b);
cac:=Concatenation(ac2);
Add(perm1,cc);
Add(perm2,cac);
c:=MappingPermListList(cc,cb);
b:=MappingPermListList(cb,cac);
# make projections the same
p1:=List(GeneratorsOfGroup(g),x->RestrictedPerm(x,cc)^c);
p1:=SubgroupNC(w,p1);
p2:=List(GeneratorsOfGroup(h),x->b*RestrictedPerm(x,cac)/b);
p2:=SubgroupNC(w,p2);
no:=Normalizer(w,p2);
#Print(i," ",Length(ac2)," ",Size(w)," ",Index(w,no),"\n");
t:=RepresentativeAction(w,p1,p2);
if t=fail then return fail;fi; # can't map projection OK
#Print(List(Filtered(og,x->Length(x)=i),x->Position(oh,OnSets(x,c*b))),"\n");
Append(ac,List(GeneratorsOfGroup(no),x->x^b));
perm:=perm*t^b;
a:=a*Size(no);
od;
perm1:=MappingPermListList(Concatenation(perm1),Concatenation(perm2));
perm:=perm1*perm;
ac:=SubgroupNC(s,ac);
SetSize(ac,a);
a:=RepresentativeAction(ac,g^perm,h);
if a=fail then
return fail;
else
return perm*a;
fi;
fi;
n:=NrMovedPoints(s);
a:=AllBlocks(g);
c:=Collected(List(a,Length));
c:=Filtered(c,i->i[2]=1);
if Length(c)=0 then
return false;
else
c:=c[1][1];
a:=First(a,i->Length(i)=c);
b:=Blocks(g,MovedPoints(g),a);
ac:=Action(g,b,OnSets);
a:=AllBlocks(h);
a:=Filtered(a,i->Length(i)=c);
if Length(a)<>1 then
# different blocks
return fail;
fi;
b2:=Blocks(h,MovedPoints(h),a[1]);
ac2:=Action(h,b2,OnSets);
t:=SymmetricGroup(n/c);
perm:=RepresentativeAction(t,ac,ac2);
if perm=fail then
return fail;
else
b:=Permuted(b,perm);
Assert(1,Action(g,b,OnSets)=ac2);
fi;
p1:=MappingPermListList(Concatenation(b),[1..n]);
p2:=MappingPermListList(Concatenation(b2),[1..n]);
no:=Normalizer(t,ac2);
#Print(" using blocks ",c," factorgp size ",Size(no),"\n");
g:=g^p1;
h:=h^p2;
b:=List(b,i->OnSets(Set(i),p1));
ac:=Action(Stabilizer(g,b[1],OnSets),b[1]);
t:=SymmetricGroup(c);
for i in [1..Length(b)] do
ac2:=Action(Stabilizer(g,b[i],OnSets),b[i]);
perm:=RepresentativeAction(t,ac2,ac);
if perm=fail then
# b cannot be conjugated -- inconsistent
Error("inconsistence");
fi;
perm:=perm^MappingPermListList([1..c],b[i]);
g:=g^perm;
p1:=p1*perm;
ac2:=Action(Stabilizer(h,b[i],OnSets),b[i]);
perm:=RepresentativeAction(t,ac2,ac);
if perm=fail then
# cannot map onto -- wrong
return fail;
fi;
perm:=perm^MappingPermListList([1..c],b[i]);
h:=h^perm;
p2:=p2*perm;
od;
no2:=Normalizer(t,ac);
w:=WreathProduct(no2,no);
perm:=RepresentativeAction(w,g,h);
if perm<>fail then
Assert(1,ForAll(GeneratorsOfGroup(g),i->i^perm in h));
perm:=p1*perm/p2;
fi;
return perm;
fi;
end);
InstallMethod( IsConjugate, "for natural symmetric group",
true, [ IsNaturalSymmetricGroup, IsGroup, IsGroup ],
function (s, g, h)
local res;
res := SubgpConjSymmgp(s, g, h);
if IsPerm(res) then
return true;
elif res = fail then
return false;
else
TryNextMethod();
fi;
end);
#############################################################################
##
#M RepresentativeAction( <G>, <d>, <e>, <opr> ) . . for symmetric groups
##
InstallOtherMethod( RepresentativeActionOp, "for natural symmetric group",
true, [ IsNaturalSymmetricGroup, IsObject, IsObject, IsFunction ],
# the objects might be group elements: rank up
{} -> 2*RankFilter(IsMultiplicativeElementWithInverse),
function ( G, d, e, opr )
local dom,n,sortfun,cd,ce,p1,p2;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
dom:=Set(MovedPoints(G));
n:=Length(dom);
if opr=OnPoints then
if IsInt(d) and IsInt(e) then
if d in dom and e in dom then
return (d,e);
else
return fail;
fi;
elif IsPerm(d) and IsPerm(e) and d in G and e in G then
sortfun:=function(a,b) return Length(a)<Length(b);end;
if Order(d)=1 then #LargestMovedPoint does not work for ().
if Order(e)=1 then
return ();
else
return fail;
fi;
fi;
if CycleStructurePerm(d)<>CycleStructurePerm(e) then
return fail;
fi;
cd:=ShallowCopy(Cycles(d,dom));
ce:=ShallowCopy(Cycles(e,dom));
Sort(cd,sortfun);
Sort(ce,sortfun);
return MappingPermListList(Concatenation(cd),Concatenation(ce));
elif IsPermGroup(d) and IsPermGroup(e)
#and IsTransitive(d,dom) and IsTransitive(e,dom)
and IsSubset(G,d) and IsSubset(G,e) then
if dom<>[1..n] then
# translate
p1:=MappingPermListList(dom,[1..n]);
p2:=SubgpConjSymmgp(G^p1,d^p1,e^p1);
if p2=false then
TryNextMethod();
elif p2<>fail then
p2:=p2^Inverse(p1);
fi;
return p2;
else
p2:=SubgpConjSymmgp(G,d,e);
if p2=false then
TryNextMethod();
fi;
return p2;
fi;
fi;
elif (opr=OnSets or opr=OnTuples) and (IsDuplicateFreeList(d) and
IsDuplicateFreeList(e)) then
if Length(d)<>Length(e) then
return fail;
fi;
if IsSubset(dom,Set(d)) and IsSubset(dom,Set(e)) then
if dom <> [1..n] then
p1 := MappingPermListList(dom,[1..n]);
return p1*MappingPermListList(OnTuples(d,p1),OnTuples(e,p1))/p1;
fi;
return MappingPermListList(d,e);
fi;
fi;
TryNextMethod();
end);
InstallMethod( SylowSubgroupOp,
"symmetric",
true,
[ IsNaturalSymmetricGroup, IsPosInt ], 0,
function ( G, p )
local S, # <p>-Sylow subgroup of <G>, result
sgs, # strong generating set of <G>
q, # power of <p>
mov,deg,trf, # degree, moved points, transfer
i; # loop variable
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
mov:=MovedPoints(G);
deg:=Length(mov);
trf:=MappingPermListList([1..deg],mov);
# make the strong generating set
sgs := [];
for i in [1..deg] do
q := p;
while i mod q = 0 do
Add( sgs, PermList( Concatenation(
[1..i-q], [i-q+1+q/p..i], [i-q+1..i-q+q/p] ) )^trf );
q := q * p;
od;
od;
# make the Sylow subgroup
S := SubgroupNC( G , sgs );
SetSize(S,p^Length(sgs));
if Size( S ) > 1 then
SetIsPGroup( S, true );
SetPrimePGroup( S, p );
SetHallSubgroup(G, [p], S);
fi;
# return the Sylow subgroup
return S;
end);
InstallMethod( ConjugacyClasses,
"symmetric",
[ IsNaturalSymmetricGroup ], OverrideNice,
function ( G )
local classes, # conjugacy classes of <G>, result
prt, # partition of <G>
sum, # partial sum of the entries in <prt>
rep, # representative of a conjugacy class of <G>
mov,deg,trf, # degree, moved points, transfer
i; # loop variable
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
mov:=MovedPoints(G);
deg:=Length(mov);
trf:=MappingPermListList([1..deg],mov);
# loop over the partitions
classes := [];
for prt in Partitions( deg ) do
# compute the representative of the conjugacy class
rep := [2..deg];
sum := 1;
for i in prt do
rep[sum+i-1] := sum;
sum := sum + i;
od;
rep := PermList( rep )^trf;
# add the new class to the list of classes
Add( classes, ConjugacyClass( G, rep ) );
od;
# return the classes
return classes;
end);
InstallMethod( IsomorphismFpGroup, "symmetric group, supply name", true,
[ IsSymmetricGroup ],
30, # override `IsNatural...' method
function(G)
return IsomorphismFpGroup(G,
Concatenation("S_",String(SymmetricDegree(G)),".") );
end);
InstallOtherMethod( IsomorphismFpGroup, "symmetric perm group,name", true,
[ IsSymmetricGroup and IsPermGroup,IsString ], 0,
function ( G,nam )
local F, # free group
gens, #generators of F
imgs,
premap, # map to apply first
hom, # bijection
mov,deg,
relators,
i, k; # loop variables
if IsTrivial(G) then
return GroupHomomorphismByFunction(G, TRIVIAL_FP_GROUP,
x->One(TRIVIAL_FP_GROUP),
x->One(G):noassert);
fi;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
#are we natural?
if not IsNaturalSymmetricGroup(G) then
F:=G;
G:=SymmetricGroup(SymmetricDegree(G));
premap:=CheapIsomSymAlt(F,G);
else
premap:=fail;
fi;
mov:=MovedPoints(G);
deg:=Length(mov);
# create the finitely presented group with <G>.degree-1 generators
F := FreeGroup( deg-1, nam );
gens:=GeneratorsOfGroup(F);
# add the relations according to the Coxeter presentation $a-b-c-...-d$
relators := [];
for i in [1..deg-1] do
Add( relators, gens[i]^2 );
od;
for i in [1..deg-2] do
Add( relators, (gens[i] * gens[i+1])^3 );
for k in [i+2..deg-1] do
Add( relators, (gens[i] * gens[k])^2 );
od;
od;
F:=F/relators;
SetSize(F,Size(G));
UseIsomorphismRelation( G, F );
# compute the bijection
imgs:=[];
for i in [2..deg] do
Add(imgs,(mov[i-1],mov[i]));
od;
# return the isomorphism to the finitely presented group
hom:= GroupHomomorphismByImagesNC(G,F,imgs,GeneratorsOfGroup(F));
if premap<>fail then hom:=premap*hom;fi;
SetIsBijective( hom, true );
ProcessEpimorphismToNewFpGroup(hom);
return hom;
end);
# use this presentation also for rewriting
InstallMethod(IsomorphismFpGroupForRewriting,"symmetric",
[IsNaturalSymmetricGroup],0,
IsomorphismFpGroup);
#############################################################################
##
#M ViewObj( <nat-sym-grp> )
##
InstallMethod( ViewString,
"for natural alternating group",
true,
[ IsNaturalAlternatingGroup ], 0,
function(alt)
alt:=MovedPoints(alt);
if Length(alt)=0 then TryNextMethod();fi;
IsRange(alt);
return Concatenation( "Alt( ", String(alt), " )" );
end );
InstallMethod( ViewString,
"for natural symmetric group",
true,
[ IsNaturalSymmetricGroup ], 0,
function(sym)
sym:=MovedPoints(sym);
if Length(sym)=0 then TryNextMethod();fi;
IsRange(sym);
return Concatenation( "Sym( ",String(sym), " )" );
end );
InstallMethod( ViewObj,
"for natural alternating group",
true,
[ IsNaturalAlternatingGroup ], 0,
function(alt)
Print(ViewString(alt));
end );
InstallMethod( ViewObj,
"for natural symmetric group",
true,
[ IsNaturalSymmetricGroup ], 0,
function(sym)
Print(ViewString(sym));
end );
#############################################################################
##
#M PrintObj( <nat-sym-grp> )
##
InstallMethod( String,
"for natural symmetric group",
true,
[ IsNaturalSymmetricGroup ], 0,
function(sym)
sym:=MovedPoints(sym);
if Length(sym)=0 then TryNextMethod();fi;
IsRange(sym);
return Concatenation( "SymmetricGroup( ",String(sym), " )" );
end );
InstallMethod( String,
"for natural alternating group",
true,
[ IsNaturalAlternatingGroup ], 0,
function(alt)
alt:=MovedPoints(alt);
if Length(alt)=0 then TryNextMethod();fi;
IsRange(alt);
return Concatenation( "AlternatingGroup( ",String(alt), " )" );
end );
InstallMethod( PrintObj,
"for natural alternating group",
true,
[ IsNaturalAlternatingGroup ], 0,
function(alt)
Print(String(alt));
end );
InstallMethod( PrintObj,
"for natural symmetric group",
true,
[ IsNaturalSymmetricGroup ], 0,
function(sym)
Print(String(sym));
end );
#############################################################################
##
#M SymmetricParentGroup( <grp> )
##
InstallMethod( SymmetricParentGroup,
"symm(moved pts)",
true,
[ IsPermGroup ], 0,
G -> SymmetricGroup( MovedPoints( G ) ) );
InstallMethod( SymmetricParentGroup,
"natural symmetric group",
true,
[ IsNaturalSymmetricGroup ], 0,
IdFunc );
#############################################################################
##
#M OrbitStabilizingParentGroup( <grp> )
##
InstallMethod( OrbitStabilizingParentGroup, "direct product of S_n's",
true, [ IsPermGroup ], 0,
function(G)
local o,d,i,j,l,s;
o:=ShallowCopy(OrbitsDomain(G,MovedPoints(G)));
SortBy(o, Length);
d:=false;
i:=1;
while i<=Length(o) do
l:=Length(o[i]);
j:=i+1;
while j<=Length(o) and Length(o[j])=l do
j:=j+1;
od;
s:=SymmetricGroup(l);
if j-1>i then
s:=WreathProduct(s,SymmetricGroup(j-i));
fi;
if d=false then
d:=s;
else
d:=DirectProduct(d,s);
fi;
Assert(1,HasSize(d));
i:=j;
od;
d:=ConjugateGroup(d,MappingPermListList(Set(MovedPoints(d)),
Concatenation(o)));
Assert(1,IsSubset(d,G));
return d;
end);
InstallOtherMethod( StabChainOp, "symmetric group", true,
[ IsNaturalSymmetricGroup,IsRecord ], 0,
function(G,r)
local dom, l, sgs, nondupbase;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
if IsBound(r.reduced) and r.reduced=false then
TryNextMethod();
fi;
dom:=Set(MovedPoints(G));
l:=Length(dom);
if IsBound(r.base) then
nondupbase:=DuplicateFreeList(r.base);
dom:=Concatenation(Filtered(nondupbase,i->i in dom),Difference(dom,nondupbase));
fi;
sgs:=List([1..l-1],i->(dom[i],dom[l]));
return StabChainBaseStrongGenerators(dom{[1..Length(dom)-1]},sgs,());
end);
InstallOtherMethod( StabChainOp, "alternating group", true,
[ IsNaturalAlternatingGroup,IsRecord ], 0,
function(G,r)
local dom, l, sgs, nondupbase;
# test for internal rep
if HasGeneratorsOfGroup(G) and
not ForAll(GeneratorsOfGroup(G),IsInternalRep) then
TryNextMethod();
fi;
if IsBound(r.reduced) and r.reduced=false then
TryNextMethod();
fi;
dom:=Set(MovedPoints(G));
l:=Length(dom);
if IsBound(r.base) then
nondupbase:=DuplicateFreeList(r.base);
dom:=Concatenation(Filtered(nondupbase,i->i in dom),Difference(dom,nondupbase));
fi;
sgs:=List([1..l-2],i->(dom[i],dom[l-1],dom[l]));
return StabChainBaseStrongGenerators(dom{[1..Length(dom)-2]},sgs,());
end);
#############################################################################
##
#M AlternatingSubgroup( <grp> )
##
InstallMethod(AlternatingSubgroup,"for perm groups",true,[IsPermGroup],0,
function(G)
local a,b,x,i;
if SignPermGroup(G)=1 then
return G;
fi;
# otherwise construct
# this is faster than intersecting with A_n, because no stabchain for A_n
# needs to be built
a:=[];
b:=[];
for i in GeneratorsOfGroup(G) do
if SignPerm(i)=1 then
Add(a,i);
else
Add(b,i);
if Order(i)>2 then
Add(a,i^2);
fi;
if Length(b)>1 then
Add(a,b[1]/i);
fi;
fi;
od;
a:=SubgroupNC(G,a);
StabChainOptions(a).limit:=Size(G)/2;
while Size(a)<Size(G)/2 do
repeat
#Print("close\n");
x:=Random(GeneratorsOfGroup(a))^Random(b);
until not x in a;
a:=ClosureSubgroupNC(a,x);
od;
return a;
end);
# maximal subgroups routine.
# precomputed data up to degree 50 (so it will be quick is most cases).
# (As there is no independent check for the primitive groups of degree >50,
# we rather do not refer to them, but only use them in a calculation.)
BindGlobal("SNMAXPRIMS", MakeImmutable([[],[],[],[],[],[2],[],[5],[],[7],[],[4],
[],[2],[],[],[],[2],[],[2],[1,3,7],[2],[],[3],[],[5],[],[12],[],[2],[],[5],[],
[],[],[12],[],[2],[],[4,6],[],[2],[],[2],[5],[],[],[2],[],[7],[],[],[],[2],[6],
[7,5],[],[],[],[7],[],[2],[2],[],[2],[],[],[3,5],[],[],[],[2],[],[2],[],[],[],
[2,4],[],[2],[],[8],[],[2,4],[4],[],[],[],[],[2],[8],[],[],[],[],[],[],[2],[],
[4,2],[],[3],[],[2],[7,9],[],[],[2],[],[2],[],[],[],[2],[],[],[],[],[],[10],[],
[5],[],[],[],[2,17,11],[],[5],[],[5],[],[2],[],[],[],[12],[],[2],[],[2],[],[],
[],[],[],[],[],[],[],[2],[],[2],[],[],[],[7],[],[2],[],[],[],[5],[],[2],[2],
[],[],[7],[],[5],[4,2],[],[],[2],[2],[],[],[],[],[2],[],[2],[],[],[],[],[],[],
[],[],[],[2],[],[2],[],[],[],[2],[],[2],[],[],[],[],[],[],[],[],[],[4],[],[2],
[],[],[],[],[],[],[],[3],[],[],[],[2],[],[],[],[2],[],[2],[],[],[],[4],[],[],
[],[],[],[2],[],[2],[],[4],[],[],[],[],[],[],[],[2],[7,2],[],[],[],[],[2],[],
[],[],[],[],[2],[],[],[],[],[],[2],[],[2],[],[],[],[],[],[2],[],[2,20,22],[],
[2],[],[2],[],[2],[],[],[],[5],[],[],[],[2],[],[],[2],[],[],[9,5,7],[],[],[],[],
[],[],[],[2],[],[],[],[2],[],[2],[3],[],[],[2],[],[],[],[],[],[],[5],[],[],[],
[],[],[],[2],[],[],[],[],[],[2],[],[],[2],[],[],[6],[],[],[],[2],[],[2],
[9,4,6],[],[],[2],[],[],[5],[],[],[18],[],[5],[],[9,4],[],[],[],[2],[3],[],[],
[],[],[2],[],[],[],[],[],[2],[],[],[],[2],[],[],[],[],[],[2],[],[],[],[],[],
[],[],[2],[],[6],[],[2],[],[],[],[4,2],[],[],[],[2],[],[],[],[],[],[],[],[],
[],[2],[],[2],[],[],[1],[],[],[],[],[],[],[2],[],[2],[],[],[],[],[],[2],[],[],
[],[2],[],[],[],[],[],[2],[],[],[],[],[],[],[],[2],[],[],[],[6],[],[2],[5,2,3],
[],[],[2],[],[],[],[],[],[],[],[],[],[],[],[2],[],[],[],[],[],[],[],[2],[],
[],[],[2],[],[],[],[],[],[],[],[2],[],[],[],[6],[],[],[],[],[],[2],[],[],[],
[],[],[],[],[],[],[7],[],[2],[],[2],[6],[],[],[7],[],[5],[],[],[],[],[],[],[],
[],[],[8],[],[2],[],[],[],[],[],[2],[],[],[],[],[],[],[],[],[],[2],[],[4],[],
[],[],[2],[],[],[],[],[],[2],[],[2],[],[],[],[],[],[2],[],[],[],[],[],[],[],
[],[],[2],[],[],[],[],[],[2],[2],[],[],[],[],[2],[],[2],[],[],[],[],[],[2],[],
[],[],[],[],[2],[],[],[],[2],[],[3],[],[],[],[],[],[8],[],[],[],[],[],[2],[],
[],[],[],[],[],[],[],[],[2],[],[2],[],[],[],[2],[],[],[],[],[],[2],[],[],[],
[],[],[7],[],[2],[],[],[],[4,2],[],[],[],[],[],[],[],[2],[],[],[],[2],[],[2],
[],[],[],[2],[],[],[],[],[],[],[],[2],[4],[],[],[],[],[],[],[],[],[2],[],[],
[],[],[],[],[],[2],[],[],[],[],[2],[],[],[],[],[2],[],[],[],[],[],[],[],[2],
[],[13,3],[],[],[],[2],[],[],[],[],[],[2],[2],[],[],[2],[],[],[],[],[],[1],[],
[2],[],[],[],[],[],[2],[],[],[],[2],[],[],[2],[],[],[],[],[2],[],[],[],[2],[1],
[],[],[],[],[],[],[],[],[],[],[],[],[2],[],[],[],[],[],[],[],[],[],[2],[],
[],[],[],[],[],[],[8],[],[],[],[2],[],[2],[],[],[],[],[],[],[4],[11,2,19],[],
[2],[],[2],[],[],[],[2],[],[2],[],[],[],[],[],[],[],[],[],[6],[],[5],[],[],[],
[],[],[],[],[],[],[],[],[2],[],[],[],[2],[],[2],[],[],[],[2],[],[],[],[],[],
[],[],[],[],[],[],[],[],[2],[],[],[],[2],[],[2],[],[],[],[2],[],[],[],[],[],
[],[],[],[],[],[],[],[],[],[4,2],[],[],[],[],[2],[],[3],[],[2],[],[],[],[],[],
[],[],[2],[],[],[],[],[],[],[],[],[],[2],[],[],[],[],[],[],[],[2],[],[],[],
[2],[],[],[],[],[],[2],[],[],[],[],[],[2],[],[],[],[],[],[],[],[5],[],[],[],
[],[],[2],[],[],[],[2],[],[],[],[],[],[2],[],[],[],[],[],[2],[],[],[],[],[],
[2],[],[2],[],[],[],[],[],[2],[]]));
BindGlobal("ANMAXPRIMS", MakeImmutable([[],[],[],[],[],[1],[5],[],[9],[6],[6],
[2],[7],[1],[4],[],[8],[1],[],[1],[2,6],[1],[5],[1],[],[3],[13],[6,11],[],[1],
[9,10],[4],[2],[],[2],[10,11],[],[1],[],[3,5],[],[1],[],[1],[4,7],[],[],[1],[],
[2,6],[],[1],[],[1],[5],[4,6],[1,3],[],[],
[6],[],[1],[1,4,6],[],[1,5,7,11],[5],[],
[2,4],[],[],[],[1],[14],[1],[],[],[2],[1,3],[],[1],[],[7],[],[1,3],[3],[],[],
[],[],[1],[6,7],[],[],[],[],[],[],[1],[],[1,3],[],[1,2],[],[1],[6,8],[],[],[1],
[],[1],[],[8],[],[1],[],[],[1,3],[],[2],
[5,9,14,15,17,21],[49],[4],[],[],[],[1,6,8,16],
[13],[4],[2],[3],[],[1],[1],[],[3],[6,11],
[],[1],[],[1],[],[],[],[2,4,5],[],[],[],[],[],[1],[],[1],[4],[],[1],[6],[],[1],
[],[],[],[2],[],[1],[1,5],[],[],[6],[],
[3],[1,3],[],[],[1],[1,4],[2,4],[],[],[],[1],[],[1],[2],[],[],[1],[],[],[],[4],
[],[1],[],[1],[],[],[],[1],[],[1],[],[],[1],[],[],[],[],[3],[],[2,3],[],[1],[],
[],[],[],[],[],[],[2],[],[],[],[1],[],[],[],[1],[],[1],[4],[],[],[2,3],[],[],
[],[],[],[1],[],[1],[],[3],[],[],[],[1],[],[],[],[1],[1,3,6],[],[2],[],[4],[1],
[],[],[],[],[],[1],[],[1],[],[],[],[1],[],[1],[6],[],[2],[3,6],[],[1],[],
[1,17,21],[],[1],[],[1],[1],[1],[],[],[],
[3],[],[],[],[1],[],[],[1],[],[],[2,6,8],
[],[],[],[],[],[],[1],[1],[],[],[],[1],[],[1],[1,2],[],[],[1],[],[],[],[],[],
[],[2,4,12],[],[],[],[],[2,4],[],[1],[],
[],[],[6,7],[],[1],[],[],[1],[],[],[2,5],
[],[],[],[1],[],[1],[3,5,8],[],[],[1],[],
[],[4],[],[],[17],[],[4],[],[3,7,8],[],
[],[],[1],[2],[],[],[],[],[1],[],[],[],[6,9],[],[1],[2],[],[],[1],[],[],[],[],
[],[1],[],[],[],[],[],[2],[],[1],[],[5],
[],[1],[],[],[],[1,3],[],[],[],[1],[],[],[],[],[],[4],[],[],[],[1],[],[1],[],
[],[],[],[],[],[],[],[],[1],[],[1],[4],
[],[],[],[],[1],[],[],[],[1],[],[],[],[],[],[1],[],[],[],[],[2],[2],[],[1],[],
[],[],[2,5],[],[1],[1,4],[],[],[1],[],[],[],[],[],[],[],[],[],[],[],[1],[],[],
[],[],[],[],[],[1],[],[],[],[1],[],[],[2],[3,7,10],[],[],[],[1],[],[],[],[5],
[],[1],[],[],[],[1],[1],[],[9,12],[],[],
[],[],[],[],[4,5],[],[1],[],[1],[4,5],[],[2],[3,6],[],[4],[],[],[],[],[],[],[],
[],[],[7],[],[1],[],[],[],[],[],[1],[],[],[],[],[1],[],[],[],[],[1],[],[2,3],
[2],[],[],[1],[],[],[5],[],[],[1],[],[1],[],[],[],[],[],[1],[],[],[],[],[],
[],[4],[],[],[1],[],[],[],[],[],[1],[1],[],[],[],[],[1],[],[1],[],[],[],[],[],
[1],[],[],[],[],[],[1],[],[2],[],[1],[],[1,2],[],[],[],[],[],[6],[],[],[],[2],
[],[1],[],[],[],[],[],[],[],[],[],[1],[],[1],[],[],[],[1],[],[],[1,5],[],[],
[1],[],[],[2],[],[],[6],[],[1],[],[],[],[1,3],[],[],[],[],[2],[4],[],[1],[],[],
[],[1],[],[1],[],[],[],[1],[],[],[],[],[],[],[],[1],[3],[],[],[],[],[],[],[],
[],[1],[4],[],[],[],[],[],[],[1],[],[],[],[],[1],[],[],[],[],[1],[],[],[],[],
[],[],[],[1],[],[2,12],[],[],[],[1],[],[],[],[],[],[1],[1],[],[],[1],[],[],[],
[],[],[],[],[1],[],[],[],[5],[2],[1],[1],[],[],[1],[],[],[1],[],[],[],[],[1],
[],[],[],[1],[],[],[],[],[],[2],[1],[],[],[],[],[],[],[1],[],[],[],[2],[],[],
[],[],[],[1],[],[],[],[],[],[],[],[2],[],[],[],[1],[],[1],[],[],[],[2],[],[],
[3,6],[1,10,16],[],[1],[],[1],[],[],[],[1],[],[1],[],[],[],[],[],[],[],[],[],
[2,4,5],[],[3],[],[],[],[],[],[],[],[],[],[],[],[1],[],[],[],[1],[],[1],[4],[],
[],[1],[],[],[],[],[],[],[1],[],[],[],[],[],[],[1],[],[1],[],[1],[],[1],[],[],
[],[1],[],[],[4],[],[],[],[],[],[],[],[],[],[],[],[1,3],[],[],[],[],[1],[],
[1],[],[1],[],[],[],[],[],[],[],[1],[],[],[],[],[],[],[],[],[],[1],[],[],[],
--> --------------------
--> maximum size reached
--> --------------------
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