Quelle grpperm.gi Sprache: unbekannt

#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Heiko Theißen.
##
## 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
##

#############################################################################
##
#M CanEasilyTestMembership( <permgroup> )
##
InstallTrueMethod(CanEasilyTestMembership,IsPermGroup);

#############################################################################
##
#M CanEasilyComputePcgs( <permgroup> )
##
## solvable permgroups can compute a pcgs. (The group may have been told it
## is solvable without actually doing a solvability test, so we need the
## implication.)
InstallTrueMethod(CanEasilyComputePcgs,IsPermGroup and IsSolvableGroup);

#M CanComputeSizeAnySubgroup
InstallTrueMethod(CanComputeSizeAnySubgroup,IsPermGroup);

#############################################################################
##
#M AsSubgroup( <G>, ) . . . . . . . . . . . . with stab chain transfer
##
InstallMethod( AsSubgroup,"perm groups",
 IsIdenticalObj, [ IsPermGroup, IsPermGroup ], 0,
 function( G, U )
 local S;
 # test if the parent is already alright
 if HasParent(U) and IsIdenticalObj(Parent(U),G) then
 return U;
 fi;

 if not IsSubset( G, U ) then
 return fail;
 fi;
 S:= SubgroupNC( G, GeneratorsOfGroup( U ) );
 UseIsomorphismRelation( U, S );
 UseSubsetRelation( U, S );
 if HasStabChainMutable( U ) then
 SetStabChainMutable( S, StabChainMutable( U ) );
 fi;
 return S;
end );

#############################################################################
##
#M CanEasilyComputeWithIndependentGensAbelianGroup( <permgroup> )
##
InstallTrueMethod(CanEasilyComputeWithIndependentGensAbelianGroup,
 IsPermGroup and IsAbelian);

#############################################################################
##
#F IndependentGeneratorsAbelianPPermGroup( , ) . . . nice generators
##
InstallGlobalFunction( IndependentGeneratorsAbelianPPermGroup,
 function ( P, p )
 local inds, # independent generators, result
 pows, # their powers
 base, # the base of the vectorspace
 size, # the size of the group generated by <inds>
 orbs, # orbits
 trns, # transversal
 gens, # remaining generators
 one, # identity of `P'
 gens2, # remaining generators for next round
 exp, # exponent of 
 g, # one generator from <gens>
 h, # its power
 b, # basepoint
 c, # other point in orbit
 i, j, k; # loop variables

 # initialize the list of independent generators
 inds := [];
 pows := [];
 base := [];
 size := 1;
 orbs := [];
 trns := [];

 # gens are the generators for the remaining group
 gens := GeneratorsOfGroup( P );
 one:= One( P );

 # loop over the exponents
 exp := Maximum( List( gens, g -> LogInt( Order( g ), p ) ) );
 for i in [exp,exp-1..1] do

 # loop over the remaining generators
 gens2 := [];
 for j in [1..Length(gens)] do
 g := gens[j];
 h := g ^ (p^(i-1));

 # reduce <g> and <h>
 while not IsOne(h)
 and IsBound(trns[SmallestMovedPoint(h)^h])
 do
 g := g / pows[ trns[SmallestMovedPoint(h)^h] ];
 h := h / base[ trns[SmallestMovedPoint(h)^h] ];
 od;

 # if this is linear independent, add it to the generators
 if not IsOne(h) then
 Add( inds, g );
 Add( pows, g );
 Add( base, h );
 size := size * p^i;
 b := SmallestMovedPoint(h);
 if not IsBound( orbs[b] ) then
 orbs[b] := [ b ];
 trns[b] := [ one ];
 fi;
 for c in ShallowCopy(orbs[b]) do
 for k in [1..p-1] do
 Add( orbs[b], c ^ (h^k) );
 trns[c^(h^k)] := Length(base);
 od;
 od;

 # otherwise reduce and add to gens2
 else
 Add( gens2, g );

 fi;

 od;

 # prepare for the next round
 gens := gens2;
 pows := List( pows, i->i^p );

 od;

 # return the independent generators
 return inds;
end );

#############################################################################
##
#M IndependentGeneratorsOfAbelianGroup( <G> ) . . . . . . . nice generators
##
InstallMethod( IndependentGeneratorsOfAbelianGroup, "for perm group",
 [ IsPermGroup and IsAbelian ],
 function ( G )
 local inds, # independent generators, result
 one, # identity of `G'
 p, # prime factor of group size
 gens, # generators of -Sylowsubgroup
 g, # one generator
 o; # its order

 # loop over all primes
 inds := [];
 one:= One( G );
 for p in Union( List( GeneratorsOfGroup( G ),
 g -> Factors(Order(g)) ) ) do
 if p <> 1 then

 # compute the generators for the Sylow subgroup
 gens := [];
 for g in GeneratorsOfGroup( G ) do
 o := Order(g);
 while o mod p = 0 do o := o / p; od;
 g := g^o;
 if not IsOne(g) then Add( gens, g ); fi;
 od;

 # append the independent generators for the Sylow subgroup
 Append( inds,
 IndependentGeneratorsAbelianPPermGroup(
 GroupByGenerators( gens, one ), p ) );

 fi;
 od;

 # Sort the independent generators by increasing order
 SortBy( inds, Order );

 # return the independent generators
 return inds;
end );

#############################################################################
##
#F OrbitPerms( <gens>, <d> ) . . . . . . . . . . . . . orbit of permutations
##
InstallGlobalFunction( OrbitPerms, function( gens, d )
 local max, orb, new, pnt, img, gen;

 # get the largest point <max> moved by the group <G>
 max := LargestMovedPoint( gens );

 # handle fixpoints
 if not d in [1..max] then
 return [ d ];
 fi;

 # start with the singleton orbit
 orb := [ d ];
 new := BlistList( [1..max], [1..max] );
 new[d] := false;

 # loop over all points found
 for pnt in orb do

 # apply all generators <gen>
 for gen in gens do
 img := pnt ^ gen;

 # add the image <img> to the orbit if it is new
 if new[img] then
 Add( orb, img );
 new[img] := false;
 fi;

 od;

 od;
 return orb;
end );

#############################################################################
##
#F OrbitsPerms( <gens>, <D> ) . . . . . . . . . . . orbits of permutations
##
InstallGlobalFunction( OrbitsPerms, function( gens, D )
 local max, dom, new, orbs, fst, orb, pnt, gen, img;

 # get the largest point <max> moved by the group <G>
 max := LargestMovedPoint( gens );
 dom := BlistList( [1..max], D );
 new := BlistList( [1..max], [1..max] );
 orbs := [];

 # repeat until the domain is exhausted
 fst := Position( dom, true );
 while fst <> fail do

 # start with the singleton orbit
 orb := [ fst ];
 new[fst] := false;
 dom[fst] := false;

 # loop over all points found
 for pnt in orb do

 # apply all generators <gen>
 for gen in gens do
 img := pnt ^ gen;

 # add the image <img> to the orbit if it is new
 if new[img] then
 Add( orb, img );
 new[img] := false;
 dom[img] := false;
 fi;

 od;

 od;

 # add the orbit to the list of orbits and take next point
 Add( orbs, orb );
 fst := Position( dom, true, fst );

 od;

 # add the remaining points of <D>, they are fixed
 for pnt in D do
 if pnt>max then
 Add( orbs, [ pnt ] );
 fi;
 od;

 return orbs;
end );

#############################################################################
##
#M SmallestMovedPoint( <C> ) . . . . . . . for a collection of permutations
##
BindGlobal( "SmallestMovedPointPerms", function( C )
 local min, m, gen;

 min := infinity;
 for gen in C do
 if not IsOne( gen ) then
 m := SmallestMovedPointPerm( gen );
 if m < min then
 min := m;
 fi;
 fi;
 od;
 return min;
end );

InstallMethod( SmallestMovedPoint,
 "for a collection of permutations",
 true,
 [ IsPermCollection ], 0,
 SmallestMovedPointPerms );

InstallMethod( SmallestMovedPoint,
 "for an empty list",
 true,
 [ IsList and IsEmpty ], 0,
 SmallestMovedPointPerms );

#############################################################################
##
#M LargestMovedPoint( <C> ) . . . . . . . for a collection of permutations
##
BindGlobal( "LargestMovedPointPerms", function( C )
 local max, m, gen;

 max := 0;
 for gen in C do
 if not IsOne( gen ) then
 m := LargestMovedPointPerm( gen );
 if max < m then
 max := m;
 fi;
 fi;
 od;
 return max;
end );

InstallMethod( LargestMovedPoint,
 "for a collection of permutations",
 true,
 [ IsPermCollection ], 0,
 LargestMovedPointPerms );

InstallMethod( LargestMovedPoint,
 "for an empty list",
 true,
 [ IsList and IsEmpty ], 0,
 LargestMovedPointPerms );

#############################################################################
##
#F MovedPoints( <C> ) . . . . . . . . . . for a collection of permutations
##
InstallGlobalFunction(MovedPointsPerms,function( C )
 local mov, gen, pnt;

 mov := [ ];
 for gen in C do
 if gen <> One( gen ) then
 for pnt in [ SmallestMovedPoint( gen ) ..
 LargestMovedPoint( gen ) ] do
 if pnt ^ gen <> pnt then
 mov[ pnt ] := pnt;
 fi;
 od;
 fi;
 od;
 return Set( mov );
end);

InstallMethod( MovedPoints, "for a collection of permutations", true,
 [ IsPermCollection ], 0,
 MovedPointsPerms );

InstallMethod( MovedPoints, "for an empty list", true,
 [ IsList and IsEmpty ], 0,
 MovedPointsPerms );

InstallMethod( MovedPoints, "for a permutation", true, [ IsPerm ], 0,
function(p)
 return MovedPointsPerms([p]);
end);

#############################################################################
##
#M NrMovedPoints( <C> ) . . . . . . . . . for a collection of permutations
##
BindGlobal( "NrMovedPointsPerms", function( C )
 local mov, sma, pnt, gen;

 mov := 0;
 sma := SmallestMovedPoint( C );
 if sma = infinity then
 return 0;
 fi;
 for pnt in [ sma .. LargestMovedPoint( C ) ] do
 for gen in C do
 if pnt ^ gen <> pnt then
 mov:= mov + 1;
 break;
 fi;
 od;
 od;
 return mov;
end );

InstallMethod( NrMovedPoints,
 "for a collection of permutations",
 [ IsPermCollection ],
 NrMovedPointsPerms );

InstallMethod( NrMovedPoints,
 "for an empty list",
 [ IsList and IsEmpty ],
 NrMovedPointsPerms );

#############################################################################
##
#M LargestMovedPoint( <G> ) . . . . . . . . . . . . for a permutation group
##
InstallMethod( LargestMovedPoint,
 "for a permutation group",
 true,
 [ IsPermGroup ], 0,
 G -> LargestMovedPoint( GeneratorsOfGroup( G ) ) );

#############################################################################
##
#M SmallestMovedPoint( <G> ) . . . . . . . . . . . . for a permutation group
##
InstallMethod( SmallestMovedPoint,
 "for a permutation group",
 true,
 [ IsPermGroup ], 0,
 G -> SmallestMovedPoint( GeneratorsOfGroup( G ) ) );

#############################################################################
##
#M MovedPoints( <G> ) . . . . . . . . . . . . . . . for a permutation group
##
InstallMethod( MovedPoints,
 "for a permutation group",
 true,
 [ IsPermGroup ], 0,
 G -> MovedPoints( GeneratorsOfGroup( G ) ) );

#############################################################################
##
#M NrMovedPoints( <G> ) . . . . . . . . . . . . . . for a permutation group
##
InstallMethod( NrMovedPoints,
 "for a permutation group",
 true,
 [ IsPermGroup ], 0,
 G -> NrMovedPoints( GeneratorsOfGroup( G ) ) );

#############################################################################
##
#M OrbitsMovedPoints( <G> )
##
InstallMethod( OrbitsMovedPoints, "for a permutation group", [IsPermGroup],
function(G)
local o,i;
 o:=Orbits(G,MovedPoints(G));
 o:=List(o,Set);
 List(o,IsRange); # save memory if long orbits
 Sort(o);
 MakeImmutable(o);
 for i in o do
 SetIsSSortedList(i,true);
 od;
 SetIsSSortedList(o,true);
 return o;
end);

#############################################################################
##
#M \=( <G>, <H> ) . . . . . . . . . . . test if two perm groups are equal
##
InstallMethod( \=, "perm groups", IsIdenticalObj, [ IsPermGroup, IsPermGroup ],
function ( G, H )
 # avoid new stabilizer chains if they are not computed yet
 if HasSize(G) and HasSize(H) and Size(G)<>Size(H) then return false;fi;

 if not (HasGeneratorsOfGroup(G) and HasGeneratorsOfGroup(H)) then
 TryNextMethod();
 fi;

 if LargestMovedPoint(G)<>LargestMovedPoint(H)
 or OrbitsMovedPoints(G)<>OrbitsMovedPoints(H) then
 return false;
 fi;

 if IsEqualSet( GeneratorsOfGroup( G ), GeneratorsOfGroup( H ) ) then
 return true;
 fi;

 # try to use one existing stab chain
 if HasStabChainMutable(H) or HasStabChainImmutable(H) then
 return Size(G)=Size(H) and ForAll(GeneratorsOfGroup(G),gen->gen in H);
 fi;
 return Size(G)=Size(H) and ForAll(GeneratorsOfGroup(H),gen->gen in G);
end);

#############################################################################
##
#M BaseOfGroup( <G> )
##
InstallMethod( BaseOfGroup,
 "for a permutation group",
 true,
 [ IsPermGroup ], 0,
 G -> BaseStabChain( StabChainMutable( G ) ) );

#############################################################################
##
#M Size( <G> ) . . . . . . . . . . . . . . . . . . size of permutation group
##
InstallMethod( Size,
 "for a permutation group",
 true,
 [ IsPermGroup ], 0,
 G -> SizeStabChain( StabChainMutable( G ) ) );

#############################################################################
##
#M Enumerator( <G> ) . . . . . . . . . . . . enumerator of permutation group
##
BindGlobal( "ElementNumber_PermGroup",
 function( enum, pos )
 local elm, S, len, n;

 S := enum!.stabChain;
 elm := S.identity;
 n:= pos;
 pos := pos - 1;
 while Length( S.genlabels ) <> 0 do
 len := Length( S.orbit );
 elm := LeftQuotient( InverseRepresentative
 ( S, S.orbit[ pos mod len + 1 ] ), elm );
 pos := QuoInt( pos, len );
 S := S.stabilizer;
 od;

 if pos <> 0 then
 Error( "<enum>[", n, "] must have an assigned value" );
 fi;
 return elm;
end );

BindGlobal( "NumberElement_PermGroup",
 function( enum, elm )
 local pos, val, S, img;

 if not IsPerm( elm ) then
 return fail;
 fi;

 pos := 1;
 val := 1;
 S := enum!.stabChain;
 while Length( S.genlabels ) <> 0 do
 img := S.orbit[ 1 ] ^ elm;
 #pos := pos + val * ( Position( S.orbit, img ) - 1 );
 if IsBound(S.orbitpos[img]) then
 pos := pos + val * S.orbitpos[img];
 #val := val * Length( S.orbit );
 val := val * S.ol;
 elm := elm * InverseRepresentative( S, img );
 S := S.stabilizer;
 else
 return fail;
 fi;
 od;
 if elm <> S.identity then
 return fail;
 fi;
 return pos;
end );

InstallMethod( Enumerator,
 "for a permutation group",
 [ IsPermGroup ],
function(G)
local e,S,i,p;
 # get a good base
 S:=G;
 p:=[];
 while Size(S)>1 do
 e:=Orbits(S,MovedPoints(S));
 i:=List(e,Length);
 i:=Position(i,Maximum(i));
 Add(p,e[i][1]);
 S:=Stabilizer(S,e[i][1]);
 od;
 Info(InfoGroup,1,"choose base ",p);

 S:=StabChainOp(G,rec(base:=p));
 e:=EnumeratorByFunctions( G, rec(
 ElementNumber := ElementNumber_PermGroup,
 NumberElement := NumberElement_PermGroup,
 Length := enum -> SizeStabChain( enum!.stabChain ),
 PrintObj := function( G )
 Print( "<enumerator of perm group>" );
 end,

 stabChain := S ) );
 while Length(S.genlabels)<>0 do
 S.ol:=Length(S.orbit);
 S.orbitpos:=[];
 for i in [1..Maximum(S.orbit)] do
 p:=Position(S.orbit,i);
 if p<>fail then
 S.orbitpos[i]:=p-1;
 fi;
 od;
 S:=S.stabilizer;
 od;
 return e;
end);

#############################################################################
##
#M Iterator( <G> ) . . . . . . . . . . . . . . iterator of permutation group
##
InstallMethod( Iterator,
 "for a trivial permutation group",
 [ IsPermGroup and IsTrivial ],
function(G)
 return IteratorList([()]);
end);

InstallMethod( Iterator,
 "for a permutation group",
 [ IsPermGroup ],
function(G)
 return IteratorStabChain(StabChain(G));
end);

#############################################################################
##
#M Random( <G> ) . . . . . . . . . . . . . . . . . . . . . . random element
##
InstallMethodWithRandomSource( Random,
 "for a random source and a permutation group",
 [ IsRandomSource, IsPermGroup ], 10,
 function( rs, G )
 local S, rnd;

 # go down the stabchain and multiply random representatives
 S := StabChainMutable( G );
 rnd := S.identity;
 while Length( S.genlabels ) <> 0 do
 rnd := LeftQuotient( InverseRepresentative( S,
 Random( rs, S.orbit ) ), rnd );
 S := S.stabilizer;
 od;

 # return the random element
 return rnd;
end );

#############################################################################
##
#M <g> in <G> . . . . . . . . . . . . . . . . . . . . . . . membership test
##
InstallMethod( \in,
 "for a permutation, and a permutation group",
 true,
 [ IsPerm, IsPermGroup and HasGeneratorsOfGroup], 0,
 function( g, G )
 if g = One( G ) or g in GeneratorsOfGroup( G ) then
 return true;
 else
 G := StabChainMutable( G );
 return SiftedPermutation( G, g ) = G.identity;
 fi;
end );

#############################################################################
##
#M ClosureGroup( <G>, <gens>, <options> ) . . . . . . closure with options
##
## The following function implements the method and may be called even with
## incomplete (temp) stabilizer chains.
BindGlobal("DoClosurePrmGp",function( G, gens, options )
 local C, # closure of < <G>, <obj> >, result
 P, inpar, # parent of the closure
 g, # an element of gens
 o, # order
 newgens, # new generator list
 chain; # the stabilizer chain created

 options:=ShallowCopy(options); # options will be overwritten
 # if all generators are in <G>, <G> is the closure
 gens := Filtered( gens, gen -> not gen in G );
 if IsEmpty( gens ) then
 return G;
 fi;

 # is the closure normalizing?
 if Length(gens)=1 and
 ForAll(gens,i->ForAll(GeneratorsOfGroup(G),j->j^i in G)) then
 g:=gens[1];
 if Size(G)>1 then
 o:=First(Difference(DivisorsInt(Order(g)),[1]),j->g^j in G);
 options.limit:=Size(G)*o;
 else
 o:=First(Difference(DivisorsInt(Order(g)),[1]),j->IsOne(g^j));
 options.limit:=o;
 fi;
 options.size:=options.limit;
 #Print(options.limit,"<<\n");
 fi;

 # otherwise decide between random and deterministic methods
 P := Parent( G );
 inpar := IsSubset( P, gens );
 while not inpar and not IsIdenticalObj( P, Parent( P ) ) do
 P := Parent( P );
 inpar := IsSubset( P, gens );
 od;
 if inpar then
 CopyOptionsDefaults( P, options );
 elif not IsBound( options.random ) then
 options.random := DefaultStabChainOptions.random;
 fi;

 # perhaps <G> is normal in <C> with solvable factor group

#AH 5-feb-96: Disabled (see gap-dev discussion).
# if DefaultStabChainOptions.tryPcgs
# and ForAll( gens, gen -> ForAll( GeneratorsOfGroup( G ),
# g -> g ^ gen in G ) ) then
# if inpar then
# C := SubgroupNC( P,
# Concatenation( GeneratorsOfGroup( G ), gens ) );
# else
# C := GroupByGenerators
# ( Concatenation( GeneratorsOfGroup( G ), gens ) );
# fi;
# pcgs:=TryPcgsPermGroup( [ C, G ], false, false, false );
# if IsPcgs( pcgs ) then
# chain:=pcgs!.stabChain;
# if inpar then C := GroupStabChain( P, chain, true );
# else C := GroupStabChain( chain ); fi;
# SetStabChainOptions( C, rec( random := options.random ) );
#
# UseSubsetRelation( C, G );
# return C;
# fi;
# fi;

 # make the base of G compatible with options.base
 chain := StructuralCopy( StabChainMutable( G ) );
 if IsBound( options.base ) then
 ChangeStabChain( chain, options.base,
 IsBound( options.reduced ) and options.reduced );
 fi;

 if LargestMovedPoint( Concatenation( GeneratorsOfGroup( G ),
 gens ) ) <= 100 then
 options := ShallowCopy( options );
 options.base := BaseStabChain( chain );
 for g in gens do
 if SiftedPermutation( chain, g ) <> chain.identity then
 StabChainStrong( chain, [ g ], options );
 fi;
 od;
 else
 o:=ValueOption("knownClosureSize");
 if IsInt(o) then options.limit:=o;fi;
 chain := ClosureRandomPermGroup( chain, gens, options );
 fi;

 newgens:=Concatenation(GeneratorsOfGroup(G),gens);
 if Length(chain.generators)<=Length(newgens) then
 if inpar then C := GroupStabChain( P, chain, true );
 else C := GroupStabChain( chain ); fi;
 else
 if inpar then C := SubgroupNC(P,newgens);
 else C := Group( newgens,One(G) ); fi;
 SetStabChainMutable(C,chain);
 fi;
 SetStabChainOptions( C, rec( random := options.random ) );

 UseSubsetRelation( C, G );
 return C;
end );

InstallOtherMethod( ClosureGroup,"permgroup, elements, options",
 true,
 [ IsPermGroup, IsList and IsPermCollection, IsRecord ], 0,
 DoClosurePrmGp);

InstallOtherMethod( ClosureGroup, "empty list",true,
 [ IsPermGroup, IsList and IsEmpty ], 0,
ReturnFirst);

InstallMethod( ClosureGroup, "permgroup, element",true,
 [ IsPermGroup, IsPerm ], 0,
function( G, g )
 return DoClosurePrmGp( G, [ g ], rec() );
end );

InstallMethod( ClosureGroup, "permgroup, elements",true,
 [ IsPermGroup, IsList and IsPermCollection ], 0,
function( G, gens )
 return DoClosurePrmGp( G, gens, rec() );
end );

InstallOtherMethod( ClosureGroup, "permgroup, element, options",true,
 [ IsPermGroup, IsPerm, IsRecord ], 0,
function( G, g, options )
 return DoClosurePrmGp( G, [ g ], options );
end );

InstallOtherMethod( ClosureGroup, "permgroup, permgroup, options", true,
 [ IsPermGroup, IsPermGroup, IsRecord ], 0,
function( G, H, options )
 return DoClosurePrmGp( G, GeneratorsOfGroup( H ), options );
end );

InstallOtherMethod( ClosureGroup, "empty list and options",true,
 [ IsPermGroup, IsList and IsEmpty, IsRecord ], 0,
ReturnFirst);

#############################################################################
##
#M NormalClosureOp( <G>, ) . . . . . . . . . . . . . . . . in perm group
##
BindGlobal("DoNormalClosurePermGroup",function ( G, U )
 local N, # normal closure of in <G>, result
 chain, # stabilizer chain for the result
 rchain, # restored version of <chain>, for `VerifySGS'
 options, # options record for stabilizer chain construction
 gensG, # generators of the group <G>
 genG, # one generator of the group <G>
 gensN, # generators of the group <N>
 genN, # one generator of the group <N>
 cnj, # conjugated of a generator of 
 random, k, # values measuring randomness of <chain>
 param, missing, correct, result, i, one;

 # get a set of monoid generators of <G>
 gensG := GeneratorsOfGroup( G );
 one:= One( G );

 # make a copy of the group to be closed
 N := SubgroupNC( G, GeneratorsOfGroup(U) );
 UseIsomorphismRelation(U,N);
 UseSubsetRelation(U,N);
 SetStabChainMutable( N, StabChainMutable( U ) );
 options := ShallowCopy( StabChainOptions( U ) );
 if IsBound( options.random ) then random := options.random;
 else random := 1000; fi;
 options.temp := true;

 # make list of conjugates to be added to N
 repeat
 gensN := [ ];
 for i in [ 1 .. 10 ] do
 genG := SCRRandomSubproduct( gensG, One( G ) );
 cnj := SCRRandomSubproduct( Concatenation(
 GeneratorsOfGroup( N ), gensN ), One( G ) ) ^ genG;
 if not cnj in N then
 Add( gensN, cnj );
 fi;
 od;
 if not IsEmpty( gensN ) then
 N := ClosureGroup( N, gensN, options );
 fi;
 until IsEmpty( gensN );

 # Guarantee that all conjugates are in the normal closure: Loop over all
 # generators of N
 gensN := ShallowCopy( GeneratorsOfGroup( N ) );
 for genN in gensN do

 # loop over the generators of G
 for genG in gensG do

 # make sure that the conjugated element is in the closure
 cnj := genN ^ genG;
 if not cnj in N then
 N := ClosureGroup( N, [ cnj ], options );
 Add( gensN, cnj );
 fi;

 od;

 od;

 # Verify the stabilizer chain.
 chain := StabChainMutable( N );
 if not IsBound( chain.orbits ) then
 if IsBound( chain.aux ) then
 chain := SCRRestoredRecord( chain );
 fi;
 result := chain.identity;
 elif random = 1000 then
 missing := chain.missing;
 correct := chain.correct;
 rchain := SCRRestoredRecord( chain );
 result := VerifySGS( rchain, missing, correct );
 if not IsPerm(result) then
 repeat
 result := SCRStrongGenTest2(chain,[0,0,1,10/chain.diam,0,0]);
 until result <> one;
 fi;
 chain := rchain;
 else
 k := First([0..14],x->(3/5)^x <= 1-random/1000);
 if IsBound(options.knownBase) then
 param := [k,4,0,0,0,0];
 else
 param := [QuoInt(k,2),4,QuoInt(k+1,2),4,50,5];
 fi;
 if options.random <= 200 then
 param[2] := 2;
 param[4] := 2;
 fi;
 result := SCRStrongGenTest( chain, param, chain.orbits,
 chain.basesize, chain.base,
 chain.correct, chain.missing );
 if result = chain.identity and not chain.correct then
 result := SCRStrongGenTest2( chain, param );
 fi;
 chain := SCRRestoredRecord( chain );
 fi;
 SetStabChainMutable( N, chain );
 if result <> chain.identity then
 N := ClosureGroup( N, [ result ] );
 fi;

 # give N the proper randomness
 StabChainOptions(N).random:=random;

 # return the normal closure
 UseSubsetRelation( N, U );
 return N;
end );

InstallMethod( NormalClosureOp, "subgroup of perm group",
 IsIdenticalObj, [ IsPermGroup, IsPermGroup ], 0,
function(G,U)
 # test applicability
 if not IsSubset(G,U) then
 TryNextMethod();
 fi;
 return DoNormalClosurePermGroup(G,U);
end);

#############################################################################
##
#M ConjugateGroup( <G>, <g> ) . . . . . . . . . . . . of permutation group
##
InstallMethod( ConjugateGroup, ", <g>", true, [ IsPermGroup, IsPerm ], 0,
function( G, g )
local H, S;

 H := GroupByGenerators( OnTuples( GeneratorsOfGroup( G ), g ), One( G ) );
 if HasStabChainMutable(G) then
 S := EmptyStabChain( [ ], One( H ) );
 ConjugateStabChain( StabChainMutable( G ), S, g, g );
 SetStabChainMutable( H, S );
 elif HasSize(G) then
 SetSize(H,Size(G));
 fi;
 UseIsomorphismRelation( G, H );
 return H;
end );

#############################################################################
##
#M CommutatorSubgroup( , <V> ) . . . . . . . . . . . . . for perm groups
##
InstallMethod( CommutatorSubgroup, "permgroups", IsIdenticalObj,
 [ IsPermGroup, IsPermGroup ], 0,
 function( U, V )
 local C, # the commutator subgroup
 CUV, # closure of U,V
 doneCUV, # boolean; true if CUV is computed
 u, # random subproduct of U.generators
 v, # random subproduct of V.generators
 comm, # commutator of u,v
 list, # list of commutators
 i; # loop variable

 # [ , <V> ] = normal closure of < [ , <v> ] >.
 C := TrivialSubgroup( U );
 doneCUV := false;

 # if there are lot of generators, use random subproducts
 if Length( GeneratorsOfGroup( U ) ) *
 Length( GeneratorsOfGroup( V ) ) > 10 then
 repeat
 list := [];
 for i in [1..10] do
 u := SCRRandomSubproduct( GeneratorsOfGroup( U ), One( U ) );
 v := SCRRandomSubproduct( GeneratorsOfGroup( V ), One( V ) );
 comm := Comm( u, v );
 if not comm in C then
 Add( list, comm ) ;
 fi;
 od;
 if Length(list) > 0 then
 C := ClosureGroup( C, list, rec( random := 0,
 temp := true ) );
 if not doneCUV then
 CUV := ClosureGroup( U, V );
 doneCUV := true;
 fi;
 # force closure test
 StabChainOptions(C).random:=DefaultStabChainOptions.random;
 C := DoNormalClosurePermGroup( CUV, C );
 fi;
 until IsEmpty( list );
 fi;

 # do the deterministic method; it will also check correctness
 list := [];
 for u in GeneratorsOfGroup( U ) do
 for v in GeneratorsOfGroup( V ) do
 comm := Comm( u, v );
 if not comm in C then
 Add( list, comm );
 fi;
 od;
 od;
 if not IsEmpty( list ) then
 C := ClosureGroup( C, list, rec( random := 0,
 temp := true ) );
 if not doneCUV then
 CUV := ClosureGroup( U, V );
 doneCUV := true;
 fi;
 # force closure test
 StabChainOptions(C).random:=DefaultStabChainOptions.random;
 C := DoNormalClosurePermGroup( CUV, C );
 fi;

 return C;
end );

#############################################################################
##
#M DerivedSubgroup( <G> ) . . . . . . . . . . . . . . of permutation group
##
InstallMethod( DerivedSubgroup,"permgrps",true, [ IsPermGroup ], 0,
 function( G )
 local D, # derived subgroup of <G>, result
 g, h, # random subproducts of generators
 comm, # their commutator
 list, # list of commutators
 i,j; # loop variables

 # find the subgroup generated by the commutators of the generators
 D := TrivialSubgroup( G );

 # if there are >4 generators, use random subproducts
 if Length( GeneratorsOfGroup( G ) ) > 4 then
 repeat
 list := [];
 for i in [1..10] do
 g := SCRRandomSubproduct( GeneratorsOfGroup( G ), One( G ) );
 h := SCRRandomSubproduct( GeneratorsOfGroup( G ), One( G ) );
 comm := Comm( g, h );
 if not comm in D then
 Add( list, comm );
 fi;
 od;
 if Length(list) > 0 then
 D := ClosureGroup(D,list,rec( random := 0,
 temp := true ) );
 # force closure test
 if IsBound(StabChainOptions(G).random) then
 StabChainOptions(D).random:=StabChainOptions(G).random;
 else
 StabChainOptions(D).random:=DefaultStabChainOptions.random;
 fi;
 D := DoNormalClosurePermGroup( G, D );
 fi;
 until list = [];
 fi;

 # do the deterministic method; it will also check random result
 list := [];
 for i in [ 2 .. Length( GeneratorsOfGroup( G ) ) ] do
 for j in [ 1 .. i - 1 ] do
 comm := Comm( GeneratorsOfGroup( G )[i],
 GeneratorsOfGroup( G )[j] );
 if not comm in D then
 Add( list, comm );
 fi;
 od;
 od;
 if not IsEmpty( list ) then
 D := ClosureGroup(D,list,rec( random := 0,
 temp := true ) );
 # give D a proper randomness
 if IsBound(StabChainOptions(G).random) then
 StabChainOptions(D).random:=StabChainOptions(G).random;
 else
 StabChainOptions(D).random:=DefaultStabChainOptions.random;
 fi;

 D := DoNormalClosurePermGroup( G, D );
 fi;

 if HasIsNaturalSymmetricGroup(G) and Size(D)<Size(G) then IsNaturalAlternatingGroup(D);fi;

 return D;
end );

#############################################################################
##
#M IsSimpleGroup( <G> ) . . . . . . . test if a permutation group is simple
##
## This is a most interesting function. It tests whether a permutation
## group is simple by testing whether the group is perfect and then only
## looking at the size of the group and the degree of a primitive operation.
## Basically it uses the O'Nan--Scott theorem, which gives a pretty clear
## description of perfect primitive groups. This algorithm is described in
## William M. Kantor,
## Finding Composition Factors of Permutation Groups of Degree $n\leq 10^6$,
## J. Symbolic Computation, 12:517--526, 1991.
##
InstallMethod( IsSimpleGroup,"for permgrp", true, [ IsPermGroup ], 0,
 function ( G )
 local D, # operation domain of <G>
 hom, # transitive constituent or blocks homomorphism
 d, # degree of <G>
 n, m, # $d = n^m$
 simple, # list of orders of simple groups
 transperf, # list of orders of transitive perfect groups
 s, t; # loop variables

 # if <G> is the trivial group, it is not simple
 if IsTrivial( G ) then
 return false;
 fi;

 # first find a transitive representation for <G>
 D := Orbit( G, SmallestMovedPoint( G ) );
 if not IsEqualSet( MovedPoints( G ), D ) then
 hom := ActionHomomorphism( G, D,"surjective" );
 if Size( G ) <> Size( Image( hom ) ) then
 return false;
 fi;
 G := Image( hom );
 fi;

 # next find a primitive representation for <G>
 D := Blocks( G, MovedPoints( G ) );
 while Length( D ) <> 1 do
 hom := ActionHomomorphism( G, D, OnSets,"surjective" );
 if Size( G ) <> Size( Image( hom ) ) then
 return false;
 fi;
 G := Image( hom );
 D := Blocks( G, MovedPoints( G ) );
 od;

 # compute the degree $d$ and express it as $d = n^m$
 D := MovedPoints( G );
 d := Length( D );
 n := SmallestRootInt( d );
 m := LogInt( d, n );
 if 10^6 < d then
 Error("cannot decide whether <G> is simple or not");
 fi;

 # if $G = C_p$, it is simple
 if IsPrimeInt( Size( G ) ) then
 return true;

 # if $G = A_d$, it is simple (unless $d < 5$)
 elif IsNaturalAlternatingGroup(G) then
 return 5 <= d;

 # if $G = S_d$, it is not simple (except $S_2$)
 elif IsNaturalSymmetricGroup(G) then
 return 2 = d;

 # if $G$ is not perfect, it is not simple (unless $G = C_p$, see above)
 elif Size( DerivedSubgroup( G ) ) < Size( G ) then
 return false;

 # if $\|G\| = d^2$, it is not simple (Kantor's Lemma 4)
 elif Size( G ) = d ^ 2 then
 return false;

 # if $d$ is a prime, <G> is simple
 elif IsPrimeInt( d ) then
 return true;

 # if $G = U(4,2)$, it is simple (operation on 27 points)
 elif d = 27 and Size( G ) = 25920 then
 return true;

 # if $G = PSL(n,q)$, it is simple (operations on prime power points)
 elif ( (d = 8 and Size(G) = (7^3-7)/2 ) # PSL(2,7)
 or (d = 9 and Size(G) = (8^3-8) ) # PSL(2,8)
 or (d = 32 and Size(G) = (31^3-31)/2 ) # PSL(2,31)
 or (d = 121 and Size(G) = 237783237120) # PSL(5,3)
 or (d = 128 and Size(G) = (127^3-127)/2 ) # PSL(2,127)
 or (d = 8192 and Size(G) = (8191^3-8191)/2 ) # PSL(2,8191)
 or (d = 131072 and Size(G) = (131071^3-131071)/2) # PSL(2,131071)
 or (d = 524288 and Size(G) = (524287^3-524287)/2)) # PSL(2,524287)
 and IsTransitive( Stabilizer( G, D[1] ), Difference( D, [ D[1] ] ) )
 then
 return true;

 # if $d$ is a prime power, <G> is not simple (except the cases above)
 elif IsPrimePowerInt( d ) then
 return false;

 # if we don't have at least an $A_5$ acting on the top, <G> is simple
 elif m < 5 then
 return true;

 # otherwise we must check for some special cases
 else

 # orders of simple subgroups of $S_n$ with primitive normalizer
 simple := [ ,,,,,
 [60,360],,,, # 5: A(5), A(6)
 [60,360,1814400],, # 10: A(5), A(6), A(10)
 [660,7920,95040,239500800],, # 12: PSL(2,11), M(11), M(12), A(12)
 [1092,43589145600], # 14: PSL(2,13), A(14)
 [360,2520,20160,653837184000] # 15: A(6), A(7), A(8), A(15)
 ];

 # orders of transitive perfect subgroups of $S_m$
 transperf := [ ,,,,
 [60], # 5: A(5)
 [60,360], # 6: A(5), A(6)
 [168,2520], # 7: PSL(3,2), A(7)
 [168,8168,20160] # 8: PSL(3,2), AGL(3,2), A(8)
 ];

 # test the special cases (Kantor's Lemma 3)
 for s in simple[n] do
 for t in transperf[m] do
 if Size( G ) mod (t * s^m) = 0
 and (((t * (2*s)^m) mod Size( G ) = 0 and s <> 360)
 or ((t * (4*s)^m) mod Size( G ) = 0 and s = 360))
 then
 return false;
 fi;
 od;
 od;

 # otherwise <G> is simple
 return true;

 fi;

end );

#############################################################################
##
#M IsSolvableGroup( <G> ) . . . . . . . . . . . . . . . . solvability test
##

# fast test for finding iteratively nontrivial commutators in a permutation group.
# if this goes on too long the group may not be solvable. This can be much faster
# for large degree groups than to use the full pcgs machinery (which uses the same
# idea but builds a pcgs on the way).
# The function returns `true` if the group has been proved to be *non*solvable.
BindGlobal("QuickUnsolvabilityTestPerm",function(G)
local som,elvth,fct,gens,new,l,i,j,a,b,bound;
 # a few moved points
 som:=MovedPoints(G);
 if Length(som) = 0 then SetIsTrivial(G,true); return fail; fi;
 bound:=Int(LogInt(Length(som)^5,3)/2); #Dixon Bound
 if Length(som)>100 then
 som:=som{List([1..100],x->Random(1, Length(som)))};
 fi;

 elvth:=One(G);
 fct:=function(G)
 elvth:=elvth*Product([1..Random(1,5)],x->Random(GeneratorsOfGroup(G))^Random(-3,3));
 return elvth;
 end;

 gens:=GeneratorsOfGroup(G);
 # find one nontrivial commutator
 new:=fail;
 i:=1;
 while i<=Length(gens) and new=fail do
 j:=i+1;
 while j<=Length(gens) and new=fail do
 if ForAny(som,x->(x^gens[i])^gens[j]<>(x^gens[j])^gens[i]) then
 new:=Comm(gens[i],gens[j]);
 fi;
 j:=j+1;
 od;
 i:=i+1;
 od;
 if new=fail then return fail;fi;

 # now take commutators with conjugates to get down
 i:=1;
 repeat
 gens:=new;
 j:=1;
 new:=fail;
 l:=[gens];
 while j<=10 and new=fail do
 a:=Random(l)^fct(G);
 b:=First(l,b->ForAny(som,x->(x^a)^b<>(x^b)^a));
 if b<>fail then
 new:=Comm(a,b);
 else
 Add(l,a);
 fi;
 j:=j+1;
 od;
 if new=fail then
 return fail;
 fi;
 i:=i+1;
 until i>bound;
 return true;
end);

InstallMethod( IsSolvableGroup,"for permgrp", true, [ IsPermGroup ], 0,
function(G)
local pcgs;
 if QuickUnsolvabilityTestPerm(G)=true then return false;fi;

 pcgs:=TryPcgsPermGroup( G, false, false, true );
 if IsPcgs(pcgs) then
 SetIndicesEANormalSteps(pcgs,pcgs!.permpcgsNormalSteps);
 SetIsPcgsElementaryAbelianSeries(pcgs,true);
 if not HasPcgs(G) then
 SetPcgs(G,pcgs);
 fi;
 if not HasPcgsElementaryAbelianSeries(G) then
 SetPcgsElementaryAbelianSeries(G,pcgs);
 fi;
 return true;
 else
 return false;
 fi;
end);

#############################################################################
##
#M IsNilpotentGroup( <G> ) . . . . . . . . . . . . . . . . . nilpotency test
##
InstallMethod( IsNilpotentGroup,"for permgrp", true, [ IsPermGroup ], 0,
 G -> IsPcgs(PcgsCentralSeries(G) ) );

#############################################################################
##
#M PcgsCentralSeries( <G> )
##
InstallOtherMethod( PcgsCentralSeries,"for permgrp", true, [ IsPermGroup ], 0,
function(G)
local pcgs;
 pcgs:=TryPcgsPermGroup( G, true, false, false );
 if IsPcgs(pcgs) then
 SetIndicesCentralNormalSteps(pcgs,pcgs!.permpcgsNormalSteps);
 SetIsPcgsCentralSeries(pcgs,true);
 if not HasPcgs(G) then
 SetPcgs(G,pcgs);
 fi;
 fi;
 return pcgs;
end);

#############################################################################
##
#M PcgsPCentralSeriesPGroup( <G> )
##
InstallOtherMethod( PcgsPCentralSeriesPGroup,"for permgrp", true, [ IsPermGroup ], 0,
function(G)
local pcgs;
 pcgs:=TryPcgsPermGroup( G, true, true, Factors(Size(G))[1] );
 if IsPcgs(pcgs) then
 SetIndicesPCentralNormalStepsPGroup(pcgs,pcgs!.permpcgsNormalSteps);
 SetIsPcgsPCentralSeriesPGroup(pcgs,true);
 fi;
 if IsPcgs(pcgs) and not HasPcgs(G) then
 SetPcgs(G,pcgs);
 fi;
 return pcgs;
end);

#T AH, 3/26/07: This method triggers a bug with indices. It is not clear
#T form the description why TryPcgs... should also give a drived series. Thus
#T disabled.
# #############################################################################
# ##
# #M DerivedSeriesOfGroup( <G> ) . . . . . . . . . . . . . . . derived series
# ##
# InstallMethod( DerivedSeriesOfGroup,"for permgrp", true, [ IsPermGroup ], 0,
# function( G )
# local pcgs, series;
#
# if (not DefaultStabChainOptions.tryPcgs
# or ( HasIsSolvableGroup( G ) and not IsSolvableGroup( G ) ))
# and not (HasIsSolvableGroup(G) and IsSolvableGroup(G)) then
# TryNextMethod();
# fi;
#
# pcgs := TryPcgsPermGroup( G, false, true, false );
# if not IsPcgs( pcgs ) then
# TryNextMethod();
# fi;
# SetIndicesEANormalSteps(pcgs,pcgs!.permpcgsNormalSteps);
# series := EANormalSeriesByPcgs( pcgs );
# if not HasDerivedSubgroup( G ) then
# if Length( series ) > 1 then SetDerivedSubgroup( G, series[ 2 ] );
# else SetDerivedSubgroup( G, G ); fi;
# fi;
# return series;
# end );

#############################################################################
##
#M LowerCentralSeriesOfGroup( <G> ) . . . . . . . . . lower central series
##
InstallMethod( LowerCentralSeriesOfGroup,"for permgrp", true, [ IsPermGroup ], 0,
 function( G )
 local pcgs, series;

 if IsTrivial(G) then
 return [G];
 fi;

 if not DefaultStabChainOptions.tryPcgs
 or HasIsNilpotentGroup( G ) and not IsNilpotentGroup( G )
 and not (HasIsNilpotentGroup(G) and IsNilpotentGroup(G)) then
 TryNextMethod();
 fi;

 pcgs := TryPcgsPermGroup( G, true, true, false );
 if not IsPcgs( pcgs ) then
 TryNextMethod();
 fi;
 SetIndicesCentralNormalSteps(pcgs,pcgs!.permpcgsNormalSteps);
 series := CentralNormalSeriesByPcgs( pcgs );
 if not HasDerivedSubgroup( G ) then
 if Length( series ) > 1 then SetDerivedSubgroup( G, series[ 2 ] );
 else SetDerivedSubgroup( G, G ); fi;
 fi;
 return series;
end );

InstallOtherMethod( ElementaryAbelianSeries, "perm group", true,
[ IsPermGroup and IsFinite],
 # we want this to come *before* the method for Pcgs computable groups
 RankFilter(IsPermGroup and CanEasilyComputePcgs and IsFinite)
 -RankFilter(IsPermGroup and IsFinite),
function(G)
local pcgs;
 pcgs:=PcgsElementaryAbelianSeries(G);
 if not IsPcgs(pcgs) then
 TryNextMethod();
 fi;
 return EANormalSeriesByPcgs(pcgs);
end);

#InstallOtherMethod( ElementaryAbelianSeries, fam -> IsIdenticalObj
# ( fam, CollectionsFamily( CollectionsFamily( PermutationsFamily ) ) ),
# [ IsList ], 0,
# function( list )
# local pcgs;
#
# if IsEmpty( list )
# or not DefaultStabChainOptions.tryPcgs
# or ( HasIsSolvableGroup( list[ 1 ] )
# and not IsSolvableGroup( list[ 1 ] ) ) then
# TryNextMethod();
## fi;
#
## pcgs := TryPcgsPermGroup( list, false, false, true );
# if not IsPcgs( pcgs ) then return fail;
# else return ElementaryAbelianSeries( pcgs ); fi;
#end );

#############################################################################
##
#M PCentralSeries( <G>, ) . . . . . . . . . . . . . . p-central series
##
InstallMethod( PCentralSeriesOp,"for permgrp", true, [ IsPermGroup, IsPosInt ], 0,
function( G, p )
 local pcgs;

 if Length(Factors(Size(G)))>1 then
 TryNextMethod();
 fi;
 pcgs:=PcgsPCentralSeriesPGroup(G);
 if not IsPcgs(pcgs) then
 TryNextMethod();
 fi;
 return PCentralNormalSeriesByPcgsPGroup(pcgs);
end);

#############################################################################
##
#M SylowSubgroup( <G>, ) . . . . . . . . . . . . . . Sylow $p$-subgroup
##
InstallMethod( SylowSubgroupOp,"permutation groups", true,
 [ IsPermGroup, IsPosInt ], 0,
function( G, p )
local S;
 if not HasIsSolvableGroup(G) and IsSolvableGroup(G) then
 # enforce solvable methods if we detected anew that the group is
 # solvable.
 return SylowSubgroupOp(G,p);
 fi;
 S := SylowSubgroupPermGroup( G, p );
 S := GroupStabChain( G, StabChainMutable( S ) );
 SetIsNilpotentGroup( S, true );
 if Size(S) > 1 then
 SetIsPGroup( S, true );
 SetPrimePGroup( S, p );
 SetHallSubgroup(G, [p], S);
 fi;
 return S;
end );

InstallGlobalFunction( SylowSubgroupPermGroup, function( G, p )
 local S, # -Sylow subgroup of <G>, result
 q, # largest power of dividing the size of <G>
 D, # domain of operation of <G>
 O, # one orbit of <G> in this domain
 B, # blocks of the operation of <G> on <D>
 f, # action homomorphism of <G> on <O> or 
 T, # -Sylow subgroup in the image of <f>
 g, g2, # one element of <G>
 C, C2; # centralizer of <g> in <G>

 # get the size of the -Sylow subgroup
 q := 1; while Size( G ) mod (q * p) = 0 do q := q * p; od;

 # handle trivial subgroup
 if q = 1 then
 return TrivialSubgroup( G );
 fi;
 # go down in stabilizers as long as possible
 S := StabChainMutable( G );
 while Length( S.orbit ) mod p <> 0 do
 S := S.stabilizer;
 od;
 G := GroupStabChain( G, S, true );

 # handle full group
 if q = Size( G ) then
 return G;
 fi;

 # handle -Sylow subgroups of size 
 if q = p then
 repeat g := Random( G ); until Order( g ) mod p = 0;
 g := g ^ (Order( g ) / p);
 return SubgroupNC( G, [ g ] );
 fi;

 # if the group is not transitive work with the transitive constituents
 D := MovedPoints( G );
 if not IsTransitive( G, D ) then
 Info(InfoGroup,1,"PermSylow: transitive");
 S := G;
 D := ShallowCopy( D );
 while q < Size( S ) do
 O := Orbit( S, D[1] );
 f := ActionHomomorphism( S, O,"surjective" );
 T := SylowSubgroupPermGroup( Range( f ), p );
 S := PreImagesSet( f, T );
 SubtractSet( D, O );
 od;
 return S;
 fi;

 # if the group is not primitive work in the image first
 B := Blocks( G, D );
 if Length( B ) <> 1 then
 Info(InfoGroup,1,"PermSylow: blocks");
 f := ActionHomomorphism( G, B, OnSets,"surjective" );
 T := SylowSubgroupPermGroup( Range( f ), p );
 if Size( T ) < Size( Range( f ) ) then
 T := PreImagesSet( f, T );
 S := SylowSubgroupPermGroup( T , p) ;
 return S;
 fi;
 fi;

 # find a element whose centralizer contains a full -Sylow subgroup
 Info(InfoGroup,1,"PermSylow: element");
 repeat g := Random( G ); until Order( g ) mod p = 0;
 g := g ^ (Order( g ) / p);
 C := Centralizer( G, g );
 while GcdInt( q, Size( C ) ) < q do
 repeat g2 := Random( C ); until Order( g2 ) mod p = 0;
 g2 := g2 ^ (Order( g2 ) / p);
 C2 := Centralizer( G, g2 );
 if GcdInt( q, Size( C ) ) < GcdInt( q, Size( C2 ) ) then
 C := C2; g := g2;
 fi;
 od;

 # the centralizer acts on the cycles of the element
 B := List( Cycles( g, D ), Set );
 Info(InfoGroup,1,"PermSylow: cycleaction");
 f := ActionHomomorphism( C, B, OnSets,"surjective" );
 T := SylowSubgroupPermGroup( Range( f ), p );
 S := PreImagesSet( f, T );
 return S;

end );

#############################################################################
##
#M Socle( <G> ) . . . . . . . . . . . socle of primitive permutation group
##
InstallMethod( Socle,"test primitive", true, [ IsPermGroup ], 0,
 function( G )
 local Omega, deg, coll, ds, L, z, ord,
 p, i;

 Omega := MovedPoints( G );

 if not IsPrimitive( G, Omega ) then
 TryNextMethod();
 fi;

 # Affine groups first.
 L := Earns( G, Omega );
 if L <> [] then
 return L[1];
 fi;

 deg := Length( Omega );
 if deg >= 6103515625 then
 TryNextMethod();
 # the normal closure actually seems to be faster than derived series, so
 # this is not really a shortcut
 #elif deg < 12960000 then
 # shortcut := true;
 # if deg >= 3125 then
 # coll := Collected( Factors( deg ) );
 # d := Gcd( List( coll, c -> c[ 2 ] ) );
 # if d mod 5 = 0 then
 # m := 1;
 # for c in coll do
 # m := m * c[ 1 ] ^ ( c[ 2 ] / d );
 # od;
 # if m >= 5 then
 # shortcut := false;
 # fi;
 # fi;
 # fi;
 # if shortcut then
 # ds := DerivedSeriesOfGroup( G );
 # return Last(ds);
 # fi;
 fi;

 coll := Collected( Factors(Integers, Size( G ) ) );
 if deg < 78125 then
 p := Last(coll)[ 1 ];
 else
 i := Length( coll );
 while coll[ i ][ 2 ] = 1 do
 i := i - 1;
 od;
 p := coll[ i ][ 1 ];
 fi;
 repeat
 repeat
 z := Random( G );
 ord := Order( z );
 until ord mod p = 0;
 z := z ^ ( ord / p );
 until Index( G, Centralizer( G, z ) ) mod p <> 0;

 # immediately add conjugate as this will be needed anyhow and seems to
 # speed up the closure process
 L := NormalClosure( G, SubgroupNC( G, [ z,z^Random(G) ] ) );
 if deg >= 78125 then
 ds := DerivedSeriesOfGroup( L );
 L := Last(ds);
 fi;
 if IsSemiRegular( L, Omega ) then
 L := ClosureSubgroup( L, Centralizer( G, L ) );
 fi;
 return L;
end );

#############################################################################
##
#M FrattiniSubgroup( <G> ) . . . . . . . . . . . . for permutation p-groups
##
InstallMethod( FrattiniSubgroup,"for permgrp", true, [ IsPermGroup ], 0,
 function( G )
 local p, l, k, i, j;

 if not IsPGroup( G ) then
 TryNextMethod();
 elif IsTrivial( G ) then
 return G;
 fi;
 p := PrimePGroup( G );
 l := GeneratorsOfGroup( G );
 k := [ l[1]^p ];
 for i in [ 2 .. Length(l) ] do
 for j in [ 1 .. i-1 ] do
 Add(k, Comm(l[i], l[j]));
 od;
 Add(k, l[i]^p);
 od;
 return SolvableNormalClosurePermGroup( G, k );
end );

#############################################################################
##
#M ApproximateSuborbitsStabilizerPermGroup(<G>,<pnt>) . . . approximation of
## the orbits of Stab_G(pnt) on all points of the orbit pnt^G. (As not
## all schreier generators are used, the results may be the orbits of a
## subgroup.)
##
InstallGlobalFunction( ApproximateSuborbitsStabilizerPermGroup,
 function(G,punkt)
 local one, orbit, trans, gen, pnt, img, i, changed,
 processStabGen, currPt,currGen, gens,Ggens;

 one:= One( G );
 if HasStabChainMutable( G ) and punkt in StabChainMutable(G).orbit then
# if we already have a stab chain and bother computing the proper
# stabilizer, a trivial orbit algorithm seems best.
 return Concatenation([[punkt]],
 OrbitsDomain(Stabilizer(G,punkt),
 Difference(MovedPoints(G),[punkt])));
# orbit:=Difference(StabChainMutable(G).orbit,[punkt]);
# stab:=Stabilizer(G,punkt));
# # catch trivial case
# if Length(stab)=0 then
# stab:=[ one ];
# fi;
# stgp:=1;
#
# processStabGen:=function()
# stgp:=stgp+1;
# if stgp>Length(stab) then
# StabGenAvailable:=false;
# fi;
# return stab[stgp-1];
# end;

 else
 # compute orbit and transversal
 Ggens:=GeneratorsOfGroup(G);
 orbit := [ punkt ];
 trans := [];
 trans[ punkt ] := one;
 for pnt in orbit do
 for gen in Ggens do
 img:=pnt^gen;
 if not IsBound( trans[ img ] ) then
 Add( orbit, img );
 trans[img ] := gen;
 fi;
 od;
 od;

 fi;

 currPt:=1;
 currGen:=0;

 processStabGen:=function()
 local punkt,img,a,b,gen;
 if Random(1,2)=1 then
 # next one
 currGen:=currGen+1;
 if currGen>Length(Ggens) then
 currGen:=1;
 currPt:=currPt+1;
 if currPt>Length(orbit) then
 return ();
 fi;
 fi;
 a:=currPt;
 b:=currGen;
 else
 a:=Random(1, Length(orbit));
 b:=Random(1, Length(Ggens));
 fi;
 # map a with b
 img:=orbit[a]^Ggens[b];
 if img=orbit[a] then
 return ();
 else
 punkt:=orbit[a];
 gen:=Ggens[b];
 while punkt<>orbit[1] do
 gen:=trans[punkt]*gen;
 punkt:=punkt/trans[punkt];
 od;
 if orbit[1]^gen<>img then Error("EHEH"); fi;
 punkt:=img;
 while punkt<>orbit[1] do
 gen:=gen/trans[punkt];
 punkt:=punkt/trans[punkt];
 od;
 if orbit[1]^gen<>orbit[1] then Error("UHEH"); fi;
 fi;
 return gen;
 end;

 # as the first approximation take a subgroup generated by four nontrivial
 # Schreier generators
 changed:=0;
 gens:=[];
 while changed<=3 and currPt<=Length(orbit) do
 gen:=();
 while IsOne(gen) and currPt<=Length(orbit) do
 gen:=processStabGen();
 od;
 Add(gens,gen);
 changed:=changed+1;
 od;
 if Length(gens)=0 then
 orbit:=List(orbit,i->[i]);
 else
 orbit:=OrbitsPerms(gens,orbit);
 fi;

 # todo: further fuse
 return orbit;
end );

#############################################################################
##
#M AllBlocks . . . Representatives of all block systems
##
InstallMethod( AllBlocks, "generic", [ IsPermGroup ],
function(g)
local gens, one, dom,DoBlocks,pool,subo;

 DoBlocks:=function(b)
 local bl,bld,i,t,n;
 # find representatives of all potential suborbits
 bld:=List(Filtered(subo,i->not ForAny(b,j->j in i)),i->i[1]);
 bl:=[];
 if not IsPrime(Length(dom)/Length(b)) then
 for i in bld do
 t:=Union(b,[i]);
 n:=Blocks(g,dom,t);
 if Length(n)>1 and #ok durch pool:ForAll(Difference(n[1],t),j->j>i) and
 not n[1] in pool then
 t:=n[1];
 Add(pool,t);
 bl:=Concatenation(bl,[t],DoBlocks(t));
 fi;
 od;
 fi;
 return bl;
 end;

 one:= One( g );
 gens:= Filtered( GeneratorsOfGroup(g), i -> i <> one );
 if Length(gens)=0 then return []; fi;
 subo:=ApproximateSuborbitsStabilizerPermGroup(g,
 Minimum(List(gens,SmallestMovedPoint)));
 dom:=Union(subo);
 if not IsTransitive(g,dom) then
 Error("AllBlocks, must be transitive");
 fi;
 pool:=[];
 return DoBlocks(subo[1]);
end);

#############################################################################
##
#F SignPermGroup
##
InstallGlobalFunction( SignPermGroup, function(g)
 if ForAll(GeneratorsOfGroup(g),i->SignPerm(i)=1) then
 return 1;
 else
 return -1;
 fi;
end );

BindGlobal( "CreateAllCycleStructures", function(n)
local i,j,l,m;
 l:=[];
 for i in Partitions(n) do
 m:=[];
 for j in i do
 if j>1 then
 if IsBound(m[j-1]) then
 m[j-1]:=m[j-1]+1;
 else
 m[j-1]:=1;
 fi;
 fi;
 od;
 Add(l,m);
 od;
 return l;
end );

#############################################################################
##
#F CycleStructuresGroup(G) list of all cyclestructures occ. in a perm group
##
InstallGlobalFunction( CycleStructuresGroup, function(g)
local l,m,i, one;
 l:=CreateAllCycleStructures(Length(MovedPoints(g)));
 m:=List([1..Length(l)-1],i->0);
 one:= One( g );
 for i in ConjugacyClasses(g) do
 if Representative(i) <> one then
 m[Position(l,CycleStructurePerm(Representative(i)))-1]:=1;
 fi;
 od;
 return m;
end );

# return e is a^e=b or false otherwise
InstallGlobalFunction("LogPerm",
function(a,b)
local oa,ob,q,aa,m,e,tst,s,c,p,g;
 oa:=Order(a);
 ob:=Order(b);
 if oa mod ob<>0 then Info(InfoGroup,20,"1"); return false;fi;
 q:=oa/ob;
 aa:=a^q;

 m:=1; # order modulo which power is OK so far
 e:=1;
 while m<ob do
 tst:=aa^e/b;
 s:=SmallestMovedPoint(tst); # a point on which there might be a difference
 if s=infinity then # found it
 return q*e mod oa;
 fi;
 c:=Cycle(aa,s);
 g:=Gcd(m,Length(c));
 p:=Position(c,s^b);
 if p=fail or (e mod g<>(p-1) mod g) then return false;fi;
 e:=ChineseRem([m,Length(c)],[e,p-1]);
 m:=Lcm(m,Length(c));
 od;
 if aa^e<>b then return false;fi;
 return q*e mod oa;
end);

#############################################################################
##
#M SmallGeneratingSet(<G>)
##
InstallMethod(SmallGeneratingSet,"random and generators subset, randsims",true,
 [IsPermGroup],0,
function (G)
local i, j, U, gens,a,mintry,orb,orp,isok;

 gens := ShallowCopy(Set(GeneratorsOfGroup(G)));

 # remove obvious redundancies...
 # sort elements by descending order; trivial permutations
 # at the end
 SortBy(gens, g -> -Order(g));

 i := 1;
 while i < Length(gens) do
 j := i + 1;
 while j <= Length(gens) do
 a:=LogPerm(gens[i], gens[j]);
 if a = false then
 j := j + 1;
 else
 Remove(gens, j);
 fi;
 od;
 i := i + 1;
 od;

 if Length(gens)<=Length(AbelianInvariants(G))+2 then
 return gens;
 fi;

 # try pc methods. The solvability test should not exceed cost, nor
 # the number of points.
 if #Length(MovedPoints(G))<50000 and
 #((HasIsSolvableGroup(G) and IsSolvableGroup(G)) or IsAbelian(G))
 IsSolvableGroup(G)
 and Length(gens)>3 then
 return MinimalGeneratingSet(G);
 fi;

 # store orbit data
 orb:=Set(Orbits(G,MovedPoints(G)),Set);
 # longer primitive orbits
 orp:=Filtered([1..Length(orb)],x->Length(orb[x])>100 and IsPrimitive(Action(G,orb[x])));

 isok:=function(U)
 if Length(MovedPoints(G))>Length(MovedPoints(U)) or
 Length(orb)>Length(Orbits(U,MovedPoints(U))) or
 ForAny(orp,x->not IsPrimitive(U,orb[x])) then
 return false;
 fi;

 StabChainOptions(U).random:=100; # randomized size
 return Size(U)=Size(G);
 end;

 mintry:=2;
 if Length(gens)>2 then
 # lower bound on what to accept -- use index of G' instead of full
 # structure of G/G', as the latter is more expensive.
 mintry:=Maximum(List(Collected(Factors(Size(G)/Size(DerivedSubgroup(G)))),x->x[2]));
 mintry:=Maximum(mintry,2);
 if mintry=Length(gens) then return gens;fi;
 i:=Maximum(2,mintry);
 while i<=mintry+1 and i<Length(gens) do
 # try to find a small generating system by random search
 j:=1;
 while j<=5 and i<Length(gens) do
 U:=Subgroup(G,List([1..i],j->Random(G)));
 if isok(U) then
 gens:=Set(GeneratorsOfGroup(U));
 fi;
 j:=j+1;
 od;
 i:=i+1;
 od;
 fi;

 # try to delete generators
 i := 1;
 while i <= Length(gens) and Length(gens)>mintry do
 # random did not improve much, try removing generators one by one
 U:=Subgroup(G,gens{Difference([1..Length(gens)],[i])});
 if isok(U) then
 gens:=Set(GeneratorsOfGroup(U));
 else
 i:=i+1;
 fi;
 od;
 return gens;
end);

#############################################################################
##
#M GeneratorsSmallest(<G>) . . . . . . . . . . . . . for permutation groups
##
DeclareGlobalFunction( "GeneratorsSmallestStab" );

InstallGlobalFunction( GeneratorsSmallestStab, function ( S )
 local gens, # smallest generating system of <S>, result
 gen, # one generator in <gens>
 orb, # basic orbit of <S>
 pnt, # one point in <orb>
 T, # stabilizer in <S>
 o2,img,oo,og; # private orbit algorithm

 # handle the anchor case
 if Length(S.generators) = 0 then
 return [];
 fi;

 # now get the smallest generating system of the stabilizer
 gens := GeneratorsSmallestStab( S.stabilizer );

 # get the sorted orbit (the basepoint will be the first point)
 orb := Set( S.orbit );
 SubtractSet( orb, [S.orbit[1]] );

 # handle the other points in the orbit
 while Length(orb) <> 0 do

 # take the smallest point (coset) and one representative
 pnt := orb[1];
 gen := S.identity;
 while S.orbit[1] ^ gen <> pnt do
 gen := LeftQuotient( S.transversal[ pnt / gen ], gen );
 od;

 # the next generator is the smallest element in this coset
 T := S.stabilizer;
 while Length(T.generators) <> 0 do
 pnt := Minimum( OnTuples( T.orbit, gen ) );
 while T.orbit[1] ^ gen <> pnt do
 gen := LeftQuotient( T.transversal[ pnt / gen ], gen );
 od;
 T := T.stabilizer;
 od;

 # add this generator to the generators list and reduce orbit
 Add( gens, gen );

 #SubtractSet( orb,
 # Orbit( GroupByGenerators( gens, S.identity ), S.orbit[1] ) );

 # here we want to be really fast, so use a private orbit algorithm
 o2:=[S.orbit[1]];
 RemoveSet(orb,S.orbit[1]);
 for oo in o2 do
 for og in gens do
 img:=oo^og;
 if img in orb then
 Add(o2,img);
 RemoveSet(orb,img);
 fi;
 od;
 od;

 od;

 # return the smallest generating system
 return gens;
end );

InstallMethod(GeneratorsSmallest,"perm group via minimal stab chain",
 [IsPermGroup],
function(G)
 # call the recursive function to do the work
--> --------------------

--> maximum size reached

--> --------------------

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