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Fix error in code handling subgroups of finitely presented groups with generators of order 2 which could lead to wrong results #5747

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Jun 17, 2024
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Fix error in code handling subgroups of finitely presented groups with generators of order 2 which could lead to wrong results #5747

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8 changes: 8 additions & 0 deletions lib/sgpres.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -2978,6 +2978,10 @@ local m,offset,rels,ri,ccr,i,r,ct,A,a,w,n,DATA,p,dr,
for i in ri do
r:=i;
while not r in ccr[offset+r[1]] do
# replace order 2 gens
for j in [1..Length(r)] do
if -r[j] in ordertwo then r[j]:=-r[j];fi;
od;
AddSet(ccr[offset+r[1]],Immutable(r));
r:=Concatenation(r{[2..Length(r)]},r{[1]});
od;
Expand Down Expand Up @@ -3191,9 +3195,12 @@ local m,offset,rels,ri,ccr,i,r,ct,A,a,w,n,DATA,p,dr,
i:=i+1;
od;


NEWTC_Compress(DATA,true); # always compress at the end
DATA.index:=DATA.n;

Info(InfoFpGroup,3,"found index ",DATA.index);

if Length(collapse)>0 then
Info(InfoFpGroup,3,DATA.defcount," definitions");
# which collapses gave at least 1%
Expand Down Expand Up @@ -3242,6 +3249,7 @@ local freegens,freerels,subgens,aug,trace,e,ldc,up,bastime,start,bl,bw,first,tim
trace:=arg[5];
fi;
start:=timerFunc();

if aug and trace=false then
e:=NEWTC_DoCosetEnum(freegens,freerels,subgens,aug,trace);
if e=fail then return fail;fi;
Expand Down
5 changes: 5 additions & 0 deletions tst/testbugfix/2024-06-17-MTC.tst
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@@ -0,0 +1,5 @@
# Fix #5744
gap> f := FreeGroup("a","b");;
gap> g := f / [ f.1*f.1,f.1*f.2*f.1*f.2*f.1*f.2,f.1*f.2*f.2*f.1*f.2^-1 ];;
gap> Size(g);
1
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