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Enhance SimplifiedFpGroup and IsomorphismSimplifiedFpGroup to transfer attributes (such as size, being abelian etc.) to the simplified group #4810

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Mar 3, 2022
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Enhance SimplifiedFpGroup and IsomorphismSimplifiedFpGroup to transfer attributes (such as size, being abelian etc.) to the simplified group #4810

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1 change: 1 addition & 0 deletions lib/ghomfp.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -951,6 +951,7 @@ local H, pres,map,mapi,opt;

# reconvert the Tietze presentation to a group presentation.
H := FpGroupPresentation( pres );
UseIsomorphismRelation( G, H );

if Length(GeneratorsOfGroup(H))>0 then
map:=List(TzImagesOldGens(pres),
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9 changes: 7 additions & 2 deletions tst/testinstall/grpfp.tst
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@@ -1,4 +1,4 @@
#@local a,b,c2,e,f,g,iter,l,s,F,rels,sub
#@local a,b,c2,e,f,g,iter,l,s,F,rels,sub,iso,G
gap> START_TEST("grpfp.tst");
gap> f:= FreeGroup( "a", "b" );; a := f.1;; b := f.2;;
gap> c2:= f / [ a*b*a^-2*b*a/b, (b^-1*a^3*b^-1*a^-3)^2*a ];;
Expand Down Expand Up @@ -105,8 +105,13 @@ gap> SimplifiedFpGroup(F/[GeneratorsOfGroup(F)[1]]);
gap> F:=FreeGroup("a","b","c");;
gap> rels:=ParseRelators(F,"a2,b3,c4,abC");
[ a^2, b^3, c^4, a*b*c^-1 ]
gap> IsomorphismSimplifiedFpGroup(F/rels);
gap> g:=F/rels;;
gap> Size(g);
24
gap> iso:=IsomorphismSimplifiedFpGroup(g);
[ a, b, c ] -> [ c*b^-1, b, c ]
gap> HasSize(Image(iso));
true

# ClosureSubgroupNC will not force a triviality or membership test
# if we do not know anything.
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