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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Alexander Hulpke, 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
##
## This file contains the operations for cosets of permutation groups
##
#############################################################################
##
#F MinimizeExplicitTransversal( <U>, <maxmoved> ) . . . . . . . . . . local
##
InstallGlobalFunction( MinimizeExplicitTransversal, function( U, maxmoved )
local explicit, lenflock, flock, lenblock, index, s;
if IsBound( U.explicit )
and IsBound( U.stabilizer ) then
explicit := U.explicit;
lenflock := U.stabilizer.index * U.lenblock / Length( U.orbit );
flock := U.flock;
lenblock := U.lenblock;
index := U.index;
ChangeStabChain( U, [ 1 .. maxmoved ] );
for s in [ 1 .. Length( explicit ) ] do
explicit[ s ] := MinimalElementCosetStabChain( U, explicit[ s ] );
od;
Sort( explicit );
U.explicit := explicit;
U.lenflock := lenflock;
U.flock := flock;
U.lenblock := lenblock;
U.index := index;
fi;
end );
#############################################################################
##
#F RightTransversalPermGroupConstructor( <filter>, <G>, <U> ) . constructor
##
MAX_SIZE_TRANSVERSAL := 100000;
# so far only orbits and perm groups -- TODO: Other deduced actions
InstallGlobalFunction(ActionRefinedSeries,function(G,U)
local o,A,ser,act,i;
o:=List(Orbits(U,MovedPoints(G)),Set);
SortBy(o,Length);
A:=G;
ser:=[A];
act:=[0]; # dummy entry
i:=1;
while i<=Length(o) and Size(A)>Size(U) do
A:=Stabilizer(A,o[i],OnSets);
if Size(A)<Size(Last(ser)) then
Add(ser,A);
Add(act,[o[i],OnSets]);
fi;
i:=i+1;
od;
if Size(A)>Size(U) then
Add(ser,U);
Add(act,fail);
fi;
# refine large step?
for i in [1..Length(ser)-1] do
if IndexNC(ser[i],ser[i+1])>MAX_SIZE_TRANSVERSAL then
A:=IntermediateGroup(ser[i],ser[i+1]:cheap);
if A<>fail then
# refine with action
o:=ActionRefinedSeries(ser[i],A);
ser:=Concatenation(ser{[1..i]},o[1]{[1..Length(o[1])-1]},
ser{[i+1..Length(ser)]});
act:=Concatenation(act{[1..i]},o[2]{[1..Length(o[2])-1]},
act{[i+1..Length(act)]});
else
# no refinement, next step
i:=i+1;
fi;
else
# no refinement needed, next step
i:=i+1;
fi;
od;
# make ascending like AscendingSeries
return [Reversed(ser),Reversed(act)];
end);
BindGlobal( "RightTransversalPermGroupConstructor", function( filter, G, U )
local GC, UC, noyet, orbs, domain, GCC, UCC, ac, nc, bpt, enum, i,
actions,nct;
GC := CopyStabChain( StabChainImmutable( G ) );
UC := CopyStabChain( StabChainImmutable( U ) );
noyet:=ValueOption("noascendingchain")<>true;
if not IsTrivial( G ) then
orbs := ShallowCopy( OrbitsDomain( U, MovedPoints( G ) ) );
Sort( orbs, function( o1, o2 )
return Length( o1 ) < Length( o2 ); end );
domain := Concatenation( orbs );
GCC:=GC;
UCC:=UC;
while Length( GCC.genlabels ) <> 0
or Length( UCC.genlabels ) <> 0 do
#Print(SizeStabChain(GCC),"/",SizeStabChain(UCC),":",
# SizeStabChain(GCC)/SizeStabChain(UCC),"\n");
if noyet and (
(SizeStabChain(GCC)/SizeStabChain(UCC) >MAX_SIZE_TRANSVERSAL) or
(Length(UCC.genlabels)=0 and
SizeStabChain(GCC)>MAX_SIZE_TRANSVERSAL)
) then
# first get a factorization through actions
ac:=ActionRefinedSeries(G,U);
actions:=ac[2];
ac:=ac[1];
# go in biggish steps through the chain
nc:=[ac[1]];
nct:=[actions[1]];
for i in [3..Length(ac)] do
if Size(ac[i])/Size(Last(nc))>MAX_SIZE_TRANSVERSAL then
Add(nc,ac[i-1]);
Add(nct,actions[i-1]);
fi;
od;
Add(nc,Last(ac));
Add(nct,Last(actions));
if Length(nc)>2 then
ac:=[];
for i in [Length(nc),Length(nc)-1..2] do
Info(InfoCoset,4,"Recursive [",Size(nc[i]),",",Size(nc[i-1]));
Add(ac,RightTransversal(nc[i],nc[i-1]
# do not try to factor again
:noascendingchain));
od;
return FactoredTransversal(G,U,ac);
fi;
noyet:=false;
fi;
bpt := First( domain, p -> not IsFixedStabilizer( GCC, p ) );
ChangeStabChain( GCC, [ bpt ], true ); GCC := GCC.stabilizer;
ChangeStabChain( UCC, [ bpt ], false ); UCC := UCC.stabilizer;
od;
fi;
AddCosetInfoStabChain(GC,UC,LargestMovedPoint(G));
MinimizeExplicitTransversal(UC,LargestMovedPoint(G));
enum := Objectify( NewType( FamilyObj( G ),
filter and IsList and IsDuplicateFreeList
and IsAttributeStoringRep ),
rec( group := G,
subgroup := U,
stabChainGroup := GC,
stabChainSubgroup := UC ) );
return enum;
end );
#############################################################################
##
#R IsRightTransversalPermGroupRep( <obj> ) . right transversal of perm group
##
DeclareRepresentation( "IsRightTransversalPermGroupRep",
IsRightTransversalRep,
[ "stabChainGroup", "stabChainSubgroup" ] );
InstallMethod( \[\],
"for right transversal of perm. group, and pos. integer",
true,
[ IsList and IsRightTransversalPermGroupRep, IsPosInt ], 0,
function( cs, num )
return CosetNumber( cs!.stabChainGroup, cs!.stabChainSubgroup, num );
end );
InstallMethod( PositionCanonical,
"for right transversal of perm. group, and permutation",
IsCollsElms,
[ IsList and IsRightTransversalPermGroupRep, IsPerm ], 0,
function( cs, elm )
return NumberCoset( cs!.stabChainGroup,
cs!.stabChainSubgroup,
elm );
end );
#############################################################################
##
#M RightTransversalOp( <G>, <U> ) . . . . . . . . . . . . . for perm groups
##
InstallMethod( RightTransversalOp,
"for two perm. groups",
IsIdenticalObj,
[ IsPermGroup, IsPermGroup ], 0,
function( G, U )
return RightTransversalPermGroupConstructor(
IsRightTransversalPermGroupRep, G, U );
end );
#############################################################################
##
#F AddCosetInfoStabChain( <G>, <U>, <maxmoved> ) . . . . . . add coset info
##
InstallGlobalFunction( AddCosetInfoStabChain, function( G, U, maxmoved )
local orb, pimg, img, vert, s, t, index,
block, B, blist, pos, sliced, lenflock, i, j,
ss, tt,t1,t1lim,found,tl,vimg,
sel,shortsel,gorpo,pisel,prepi,transinv;
# iterated image
vimg:=function(point,list)
local i;
for i in list do
point:=point^i;
od;
return point;
end;
Info(InfoCoset,5,"AddCosetInfoStabChain [",
SizeStabChain(G),",",SizeStabChain(U),"]");
if IsEmpty( G.genlabels ) then
U.index := 1;
U.explicit := [ U.identity ];
U.lenflock := 1;
U.flock := U.explicit;
else
AddCosetInfoStabChain( G.stabilizer, U.stabilizer, maxmoved );
# U.index := [G_1:U_1];
U.index := U.stabilizer.index * Length( G.orbit ) / Length( U.orbit );
Info(InfoCoset,5,"U.index=",U.index);
# block := 1 ^ <U,G_1>; is a block for G.
block := OrbitPerms( Concatenation( U.generators,
G.stabilizer.generators ), G.orbit[ 1 ] );
U.lenblock := Length( block );
lenflock := Length( G.orbit ) / U.lenblock;
# For small indices, permutations are multiplied, so we need a
# multiplied transversal.
if IsBound( U.stabilizer.explicit )
and U.lenblock * maxmoved <= MAX_SIZE_TRANSVERSAL
and U.index * maxmoved <= MAX_SIZE_TRANSVERSAL * lenflock then
U.explicit := [ ];
U.flock := [ G.identity ];
tt := [ ]; tt[ G.orbit[ 1 ] ] := G.identity;
for t in G.orbit do
tt[ t ] := tt[ t ^ G.transversal[ t ] ] /
G.transversal[ t ];
od;
fi;
# flock := { G.transversal[ B[1] ] | B in block system };
blist := BlistList( G.orbit, block );
pos := Position( blist, false );
while pos <> fail do
img := G.orbit[ pos ];
B := block{ [ 1 .. U.lenblock ] };
sliced := [ ];
while img <> G.orbit[ 1 ] do
Add( sliced, G.transversal[ img ] );
img := img ^ G.transversal[ img ];
od;
for i in Reversed( [ 1 .. Length( sliced ) ] ) do
for j in [ 1 .. Length( B ) ] do
B[ j ] := B[ j ] / sliced[ i ];
od;
od;
Append( block, B );
if IsBound( U.explicit ) then
Add( U.flock, tt[ B[ 1 ] ] );
fi;
#UniteBlist( blist, BlistList( G.orbit, B ) );
UniteBlistList(G.orbit, blist, B );
pos := Position( blist, false, pos );
od;
G.orbit := block;
# Let <s> loop over the transversal elements in the stabilizer.
U.repsStab := List( [ 1 .. U.lenblock ], x ->
BlistList( [ 1 .. U.stabilizer.index ], [ ] ) );
U.repsStab[ 1 ] := BlistList( [ 1 .. U.stabilizer.index ],
[ 1 .. U.stabilizer.index ] );
index := U.stabilizer.index * lenflock;
s := 1;
# For large indices, store only the numbers of the transversal
# elements needed.
if not IsBound( U.explicit ) then
# If the stabilizer is the topmost level with explicit
# transversal, this must contain minimal coset representatives.
MinimizeExplicitTransversal( U.stabilizer, maxmoved );
# if there are over 200 points, do a cheap test first.
t1lim:=Length(G.orbit);
if t1lim>200 then
t1lim:=50;
fi;
sel:=Filtered([1..Maximum(G.orbit)],x->IsBound(U.translabels[x]));
shortsel:=Length(sel)<t1lim;
if shortsel then
# inverse transversal elements
t1:=ShallowCopy(G.generators);
Add(t1,G.identity);
vert:=List(t1,x->x^-1);
# inverses of transversal, stored compact
# TODO: Instead of `Position`, use translabels entry
#transinv:=List(G.transversal,x->vert[Position(t1,x)]);
transinv:=[];
for i in [1..Length(G.transversal)] do
if IsBound(G.transversal[i]) then
t:=Position(t1,G.transversal[i]);
if t=fail then
Add(t1,G.transversal[i]);
Add(vert,G.transversal[i]^-1);
t:=Length(t1);
fi;
transinv[i]:=vert[t];
fi;
od;
# Position in orbit
gorpo:=[];
for i in [1..Length(G.orbit)] do
gorpo[G.orbit[i]]:=i;
od;
fi;
orb := G.orbit{ [ 1 .. U.lenblock ] };
pimg := [ ];
while index < U.index do
pimg{ orb } := CosetNumber( G.stabilizer, U.stabilizer, s,
orb );
t := 2;
if shortsel then
# test for the few wrong values, mapping backwards
pisel:=Filtered([1..Length(pimg)],
x->IsBound(pimg[x]) and pimg[x] in sel);
while t <= U.lenblock and index < U.index do
# For this point in the block, find the images of the
# earlier points under the representative.
vert := G.orbit{ [ 1 .. t-1 ] };
img := G.orbit[ t ];
tl:=[];
while img <> G.orbit[ 1 ] do
Add(tl,transinv[img]);
img := img ^ G.transversal[ img ];
od;
prepi:=pisel;
for t1 in [Length(tl),Length(tl)-1..1] do
prepi:=OnTuples(prepi,tl[t1]);
od;
# If $Ust = Us't'$ then $1t'/t/s in 1U$. Also if $1t'/t/s
# in 1U$ then $st/t' = u.g_1$ with $u in U, g_1 in G_1$
# and $g_1 = u_1.s'$ with $u_1 in U_1, s' in S_1$, so
# $Ust = Us't'$.
#if ForAll( [ 1 .. t-1 ], i -> not
# vert[ i ] in prepi ) then
if not ForAny(gorpo{prepi},x->x>=1 and x<=t-1) then
U.repsStab[ t ][ s ] := true;
index := index + lenflock;
fi;
t := t + 1;
od;
else
while t <= U.lenblock and index < U.index do
# do not test all points first if not necessary
# (test only at most t1lim points, if the test succeeds,
# test the rest)
# this gives a major speedup.
t1:=Minimum(t-1,t1lim);
# For this point in the block, find the images of the
# earlier points under the representative.
vert := G.orbit{ [ 1 .. t1 ] };
img := G.orbit[ t ];
while img <> G.orbit[ 1 ] do
vert := OnTuples( vert, G.transversal[ img ] );
img := img ^ G.transversal[ img ];
od;
# If $Ust = Us't'$ then $1t'/t/s in 1U$. Also if $1t'/t/s
# in 1U$ then $st/t' = u.g_1$ with $u in U, g_1 in G_1$
# and $g_1 = u_1.s'$ with $u_1 in U_1, s' in S_1$, so
# $Ust = Us't'$.
if ForAll( [ 1 .. t1 ], i -> not IsBound
( U.translabels[ pimg[ vert[ i ] ] ] ) ) then
# do all points
if t1<t-1 then
img := G.orbit[ t ];
if t<=10*t1lim then
vert := G.orbit{ [ 1 .. t - 1 ] };
while img <> G.orbit[ 1 ] do
vert := OnTuples( vert, G.transversal[ img ] );
img := img ^ G.transversal[ img ];
od;
found:=ForAll( [ t1+1 .. t - 1 ], i -> not IsBound
( U.translabels[ pimg[ vert[ i ] ] ] ) );
else
# avoid calculating tons of images of a long list
# instead calculate images on the fly
# this implicitly assumes that, if we get to so
# long a list, failure will happen quickly.
tl:=[];
while img <> G.orbit[ 1 ] do
Add(tl,G.transversal[img]);
img := img ^ G.transversal[ img ];
od;
found:=ForAll( [ t1+1 .. t - 1 ], i -> not IsBound
( U.translabels[ pimg[ vimg(G.orbit[ i ],tl) ] ] ) );
fi;
if found then
U.repsStab[ t ][ s ] := true;
index := index + lenflock;
fi;
else
U.repsStab[ t ][ s ] := true;
index := index + lenflock;
fi;
fi;
t := t + 1;
od;
fi;
s := s + 1;
od;
# For small indices, store a transversal explicitly.
else
for ss in U.stabilizer.flock do
Append( U.explicit, U.stabilizer.explicit * ss );
od;
while index < U.index do
t := 2;
while t <= U.lenblock and index < U.index do
ss := U.explicit[ s ] * tt[ G.orbit[ t ] ];
if ForAll( [ 1 .. t - 1 ], i -> not IsBound
( U.translabels[ G.orbit[ i ] / ss ] ) ) then
U.repsStab[ t ][ s ] := true;
Add( U.explicit, ss );
index := index + lenflock;
fi;
t := t + 1;
od;
s := s + 1;
od;
Unbind( U.stabilizer.explicit );
Unbind( U.stabilizer.flock );
fi;
fi;
end );
#############################################################################
##
#F NumberCoset( <G>, <U>, <r> ) . . . . . . . . . . . . . . coset to number
##
InstallGlobalFunction( NumberCoset, function( G, U, r )
local num, b, t, u, g1, pnt, bpt;
if IsEmpty( G.genlabels ) or U.index = 1 then
return 1;
fi;
# Find the block number of $r$.
bpt := G.orbit[ 1 ];
b := QuoInt( Position( G.orbit, bpt ^ r ) - 1, U.lenblock );
# For small indices, look at the explicit transversal.
if IsBound( U.explicit ) then
return b * U.lenflock + Position( U.explicit,
MinimalElementCosetStabChain( U, r / U.flock[ b + 1 ] ) );
fi;
pnt := G.orbit[ b * U.lenblock + 1 ];
while pnt <> bpt do
r := r * G.transversal[ pnt ];
pnt := pnt ^ G.transversal[ pnt ];
od;
# Now $r$ stabilises the block. Find the first $t in G/G_1$ such that $Ur
# = Ust$ for $s in G_1$. In this code, G.orbit[ <t> ] = bpt ^ $t$.
num := b * U.stabilizer.index * U.lenblock / Length( U.orbit );
# \_________This is [<U,G_1>:U] = U.lenflock_________/
t := 1;
pnt := G.orbit[ t ] / r;
while not IsBound( U.translabels[ pnt ] ) do
num := num + SizeBlist( U.repsStab[ t ] );
t := t + 1;
pnt := G.orbit[ t ] / r;
od;
# $r/t = u.g_1$ with $u in U, g_1 in G_1$, hence $t/r.u = g_1^-1$.
u := U.identity;
while pnt ^ u <> bpt do
u := u * U.transversal[ pnt ^ u ];
od;
g1 := LeftQuotient( u, r ); # Now <g1> = $g_1.t = u mod r$.
while bpt ^ g1 <> bpt do
g1 := g1 * G.transversal[ bpt ^ g1 ];
od;
# The number of $r$ is the number of $g_1$ plus an offset <num> for
# the earlier values of $t$.
return num + SizeBlist( U.repsStab[ t ]{ [ 1 ..
NumberCoset( G.stabilizer, U.stabilizer, g1 ) ] } );
end );
#############################################################################
##
#F CosetNumber( <arg> ) . . . . . . . . . . . . . . . . . . number to coset
##
InstallGlobalFunction( CosetNumber, function( arg )
local G, U, num, tup, b, t, rep, pnt, bpt, index, len;
# Get the arguments.
G := arg[ 1 ]; U := arg[ 2 ]; num := arg[ 3 ];
if Length( arg ) > 3 then tup := arg[ 4 ];
else tup := false; fi;
if num = 1 then
if tup = false then return G.identity;
else return tup; fi;
fi;
# Find the block $b$ addressed by <num>.
if IsBound( U.explicit ) then
index := U.lenflock;
else
index := U.stabilizer.index * U.lenblock / Length( U.orbit );
# \_________This is [<U,G_1>:U] = U.lenflock_________/
fi;
b := QuoInt( num - 1, index );
num := ( num - 1 ) mod index + 1;
# For small indices, look at the explicit transversal.
if IsBound( U.explicit ) then
if tup = false then
return U.explicit[ num ] * U.flock[ b + 1 ];
else
return List( tup, t -> t / U.flock[ b + 1 ] / U.explicit[ num ] );
fi;
fi;
# Otherwise, find the point $t$ addressed by <num>.
t := 1;
len := SizeBlist( U.repsStab[ t ] );
while num > len do
num := num - len;
t := t + 1;
len := SizeBlist( U.repsStab[ t ] );
od;
if len < U.stabilizer.index then
num := PositionNthTrueBlist( U.repsStab[ t ], num );
fi;
# Find the representative $s$ in the stabilizer addressed by <num> and
# return $st$.
rep := G.identity;
bpt := G.orbit[ 1 ];
if tup = false then
pnt := G.orbit[ b * U.lenblock + 1 ];
while pnt <> bpt do
rep := rep * G.transversal[ pnt ];
pnt := pnt ^ G.transversal[ pnt ];
od;
pnt := G.orbit[ t ];
while pnt <> bpt do
rep := rep * G.transversal[ pnt ];
pnt := pnt ^ G.transversal[ pnt ];
od;
return CosetNumber( G.stabilizer, U.stabilizer, num ) / rep;
else
pnt := G.orbit[ b * U.lenblock + 1 ];
while pnt <> bpt do
tup := OnTuples( tup, G.transversal[ pnt ] );
pnt := pnt ^ G.transversal[ pnt ];
od;
pnt := G.orbit[ t ];
while pnt <> bpt do
tup := OnTuples( tup, G.transversal[ pnt ] );
pnt := pnt ^ G.transversal[ pnt ];
od;
return CosetNumber( G.stabilizer, U.stabilizer, num, tup );
fi;
end );
#############################################################################
##
#M AscendingChainOp(<G>,<pnt>) . . . approximation of
##
InstallMethod( AscendingChainOp, "PermGroup", IsIdenticalObj,
[IsPermGroup,IsPermGroup],0,
function(G,U)
local s,c,mp,o,i,step,a;
s:=G;
c:=[G];
repeat
mp:=MovedPoints(s);
o:=ShallowCopy(OrbitsDomain(s,mp));
SortBy(o,Length);
i:=1;
step:=false;
while i<=Length(o) and step=false do
if not IsTransitive(U,o[i]) then
Info(InfoCoset,2,"AC: orbit");
o:=ShallowCopy(OrbitsDomain(U,o[i]));
SortBy(o,Length);
# union of same length -- smaller index
a:=Union(Filtered(o,x->Length(x)=Length(o[1])));
if Length(a)=Sum(o,Length) then
a:=Set(o[1]);
fi;
s:=Stabilizer(s,a,OnSets);
step:=true;
elif Index(G,U)>NrMovedPoints(U)
and IsPrimitive(s,o[i]) and not IsPrimitive(U,o[i]) then
Info(InfoCoset,2,"AC: blocks");
s:=Stabilizer(s,Set(MaximalBlocks(U,o[i]),Set),
OnSetsDisjointSets);
step:=true;
else
i:=i+1;
fi;
od;
if step then
Add(c,s);
fi;
until step=false or Index(s,U)=1; # we could not refine better
if Index(s,U)>1 then
Add(c,U);
fi;
Info(InfoCoset,2,"Indices",List([1..Length(c)-1],i->Index(c[i],c[i+1])));
return RefinedChain(G,Reversed(c));
end);
InstallMethod(CanonicalRightCosetElement,"Perm",IsCollsElms,
[IsPermGroup,IsPerm],0,
function(U,e)
return MinimalElementCosetStabChain(MinimalStabChain(U),e);
end);
InstallMethod(\<,"RightCosets of perm group",IsIdenticalObj,
[IsRightCoset and IsPermCollection,IsRightCoset and IsPermCollection],0,
function(a,b)
# for permutation groups the canonical rep is the smallest element of the
# coset
if ActingDomain(a)<>ActingDomain(b) then
return ActingDomain(a)<ActingDomain(b);
fi;
return CanonicalRepresentativeOfExternalSet(a)
<CanonicalRepresentativeOfExternalSet(b);
end);
InstallMethod(Intersection2, "perm cosets", IsIdenticalObj,
[IsRightCoset and IsPermCollection,IsRightCoset and IsPermCollection],0,
function(cos1,cos2)
local H1, H2, x1, x2, shift, sigma, listMoved_H1, listMoved_H2,
listMoved_sigma, U, repr, H1_sigma, H2_sigma, H12, swap, rho, diff;
# We set cosInt = cos1 \cap cos2 = H1 x1 \cap H2 x2
H1:=ActingDomain(cos1);
H2:=ActingDomain(cos2);
x1:=Representative(cos1);
x2:=Representative(cos2);
if H1=H2 then
if cos1=cos2 then
return cos1;
else
return [];
fi;
fi;
# We are using that
# H1*x1 \cap H2*x2 = (H1 \cap H2*x2/x1)*x1 = (H1 \cap H2*sigma)*shift,
# where shift and sigma are defined as below:
shift:=x1;
sigma:=x2 / x1;
# Reducing as much as possible in advance by using various relatively
# cheap to compute criteria
while true do
listMoved_H1:=MovedPoints(H1);
listMoved_H2:=MovedPoints(H2);
listMoved_sigma:=MovedPoints(sigma);
# If the coset intersection is non-empty, then there is h1 \in H1 and
# h2 \in H2 such that h1 = h2*sigma. Therefore sigma is contained in
# the group generated by H1 and H2. A necessary condition for this is
# that the points moved by sigma are a subset of the points moved by
# H1 and H2.
if not IsSubset(Union(listMoved_H1, listMoved_H2), listMoved_sigma) then
return [];
fi;
# Suppose x is an element of H1 \cap H2*sigma. Then for any positive
# integer n, we know that n^x is contained in n^H1 but also in
# (n^H2)^\sigma. Thus if the intersection of n^H1 and (n^H2)^\sigma is
# empty, then the intersection of the cosets is also empty. Clearly
# the orbit intersection contains n whenever n is fixed by sigma, so
# we only have to consider this for n moved by sigma.
if ForAny(listMoved_sigma, n -> IsEmpty(Intersection(Orbit(H1,n), OnTuples(Orbit(H2,n)
,sigma)))) then
return [];
fi;
# If there are points that are moved by sigma and by H2 but not by H1,
# then there must be an element in H2 which matches the action of sigma
# on these points, or else the intersection is empty
diff:=Difference(Intersection(listMoved_H2, listMoved_sigma), listMoved_H1);
if Length(diff) > 0 then
repr:=RepresentativeAction(H2, diff, OnTuples(diff, Inverse(sigma)), OnTuples);
if repr=fail then
return [];
fi;
# Since repr is in H2, we can replace sigma by repr*sigma
# without changing the coset H2*sigma. The new sigma then fixes
# all points in diff, as does H1. Hence replacing H2 by the
# stabilizer in H2 of diff does not change H1 \cap H2 sigma.
sigma:=repr * sigma;
H2:=Stabilizer(H2, diff, OnTuples);
continue;
fi;
# Mirror to the previous check:
# If there are points that are moved by sigma and by H1 but not by H2,
# then there must be an element in H1 which matches the action of sigma
# on these points, or else the intersection is empty
diff:=Difference(Intersection(listMoved_H1, listMoved_sigma), listMoved_H2);
if Length(diff) > 0 then
repr:=RepresentativeAction(H1, diff, OnTuples(diff, sigma), OnTuples);
if repr=fail then
return [];
fi;
# Again this is similar to the case before, except that we are adjusting
# H1 now and thus also need to take `shift` into account. The situation
# is as follows:
#
# cosInt = (H1 \cap H2 sigma) shift
# = (H1 sigma^{-1} \cap H2) sigma shift
# = (Stab_{H1}(diff) repr sigma^{-1} \cap H2) sigma shift
# = (Stab_{H1}(diff) \cap H2 sigma repr^{-1} ) repr shift
H1:=Stabilizer(H1, diff, OnTuples);
sigma:=sigma / repr;
shift:=repr * shift;
continue;
fi;
# easy termination criterion: reduction to group intersection
if sigma in H2 then
U:=Intersection(H1, H2);
return RightCoset(U, shift);
fi;
# easy termination criterion: reduction to group intersection
if sigma in H1 then
# cosInt = (H1 \cap H2 sigma) shift
# = (H1 sigma^{-1} \cap H2) sigma shift
U:=Intersection(H1, H2);
return RightCoset(U, sigma * shift);
fi;
# any element of H1 which moves points not moved by sigma or anything
# in H2 can not be in the intersection, so we may as well remove them
# by a stabilizer computation
diff:=Difference(listMoved_H1, Union(listMoved_H2, listMoved_sigma));
if Length(diff) > 0 then
H1:=Stabilizer(H1, diff, OnTuples);
continue;
fi;
# the same but with the roles of H1 and H2 reversed
diff:=Difference(listMoved_H2, Union(listMoved_H1, listMoved_sigma));
if Length(diff) > 0 then
H2:=Stabilizer(H2, diff, OnTuples);
continue;
fi;
# More general but more expensive than previous check
H1_sigma:=ClosureGroup(H1, sigma);
if not IsSubgroup(H1_sigma, H2) then
H2:=Intersection(H1_sigma, H2);
continue;
fi;
H2_sigma:=ClosureGroup(H2, sigma);
if not IsSubgroup(H2_sigma, H1) then
H1:=Intersection(H2_sigma, H1);
continue;
fi;
# No more reduction tricks available
break;
od;
# A final termination criterion
H12:=ClosureGroup(H1, H2);
if not sigma in H12 then
return [];
fi;
# We are now inspired by the algorithm from
# Lazlo Babai, Coset Intersection in Moderately Exponential Time
#
# We use the algorithm from Page 10 of coset analysis and we reformulate
# it here in order to avoid errors:
# --- The naive algorithm for computing H1 \cap H2 sigma is to iterate
# over elements of H1 and testing if one belongs to H2 sigma. If we find
# one such z then the result is the coset RightCoset(U, z). If not
# then it is empty.
# --- Since the result is independent of the cosets U, what we can
# do is iterate over the RightCosets(H1, U). The algorithm is the
# one of Proposition 3.2
# for r in RightCosets(H1, U) do
# if r in H1*sigma then
# return RightCoset(U, r * shift)
# fi;
# od;
# --- (TODO for future work): The question is how to make it faster.
# One idea is to use an ascending chain between U and H1.
# Section 3.4 of above paper gives statement related to that but not a
# useful algorithm. The question deserves further exploration.
#
# We select the smallest group for that computation in order to have
# as few cosets as possible
if Order(H2) < Order(H1) then
# cosInt = (H1 \cap H2 sigma) shift
# = (H1 sigma^{-1} \cap H2) sigma shift
swap:=H1;
H1:=H2;
H2:=swap;
shift:=sigma * shift;
sigma:=Inverse(sigma);
fi;
# So now Order(H1) <= Order(H2)
U:=Intersection(H1, H2);
for rho in RightTransversal(H1, U) do
if rho / sigma in H2 then
return RightCoset(U, rho * shift);
fi;
od;
return [];
end);
#############################################################################
##
#F FactorCosetAction( <G>, <U>, [<N>] ) operation on the right cosets Ug
## with possibility to indicate kernel
##
BindGlobal("DoFactorCosetActionPerm",function(arg)
local G,u,op,h,N,rt,ac,actions,hom,i,q;
G:=arg[1];
u:=arg[2];
if Length(arg)>2 then
N:=arg[3];
else
N:=false;
fi;
if IsList(u) and Length(u)=0 then
u:=G;
Error("only trivial operation ? I Set u:=G;");
fi;
if IsSubset(u, G) then
return DoFactorCosetAction(G, u, G);
fi;
if N=false then
N:=Core(G,u);
fi;
ac:=ActionRefinedSeries(G,u);
actions:=ac[2];
ac:=ac[1];
hom:=false;
for i in [2..Length(ac)] do
if actions[i-1]<>fail
# allow 2GB memory use for writing down orbit
and SIZE_OBJ(actions[i-1][1])*IndexNC(ac[i],ac[i-1])<2*10^9 then
op:=rec();
h:=Orbit(ac[i],actions[i-1][1],actions[i-1][2]:permutations:=op);
if IsBound(op.permutations) then
rt:=List(op.permutations,PermList);
q:=Group(rt);
SetSize(q,IndexNC(G,N));
h:=GroupHomomorphismByImagesNC(ac[i],Group(rt),
op.generators,rt);
else
h:=ActionHomomorphism(ac[i],h,actions[i-1][2],"surjective");
fi;
else
rt:=RightTransversal(ac[i],ac[i-1]);
if not IsRightTransversalRep(rt) then
# the right transversal has no special `PositionCanonical' method.
rt:=List(rt,i->RightCoset(ac[i-1],i));
fi;
h:=ActionHomomorphism(ac[i],rt,OnRight,"surjective");
fi;
Unbind(op);
Unbind(rt);
if i=2 then
hom:=h;
else
hom:=KuKGenerators(ac[i],h,hom);;
q:=Group(hom);
StabChainOptions(q).limit:=Size(ac[i]);
hom:=GroupHomomorphismByImagesNC(ac[i],q,GeneratorsOfGroup(ac[i]),hom);;
fi;
od;
op:=Image(hom,G);
SetSize(op,IndexNC(G,N));
# and note our knowledge
SetKernelOfMultiplicativeGeneralMapping(hom,N);
AddNaturalHomomorphismsPool(G,N,hom);
return hom;
end);
InstallMethod(FactorCosetAction,"by right transversal operation",
IsIdenticalObj,[IsPermGroup,IsPermGroup],0,
function(G,U)
return DoFactorCosetActionPerm(G,U);
end);
InstallOtherMethod(FactorCosetAction,
"by right transversal operation, given kernel",IsFamFamFam,
[IsPermGroup,IsPermGroup,IsPermGroup],0,
function(G,U,N)
return DoFactorCosetActionPerm(G,U,N);
end);
