Fix Extensions for nonsolvable group and trivial cohomology (#5908)

ThomasBreuer · web-flow · commit abb8103901c0 · 2025-01-20T10:07:59.000+01:00
diff --git a/lib/twocohom.gi b/lib/twocohom.gi
@@ -2026,7 +2026,7 @@ InstallOtherMethod(Extensions,"generic method for finite groups",
 function(G,M)
 local coh;
   coh:=TwoCohomologyGeneric(G,M);
-  return List(Elements(VectorSpace(coh.module.field,coh.cohomology)),
+  return List(Elements(VectorSpace(coh.module.field,coh.cohomology,coh.zero)),
     x->FpGroupCocycle(coh,x,true:normalform));
 end);
 
diff --git a/tst/teststandard/twocohom.tst b/tst/teststandard/twocohom.tst
@@ -58,6 +58,9 @@ gap> cp:=CompatiblePairs(g,mo);
 <group of size 240 with 4 generators>
 gap> Length(ExtensionRepresentatives(g,mo,cp));
 2
+gap> mo:= TrivialGModule( g, GF(3) );;
+gap> Length( Extensions( g, mo ) );
+1
 
 # New code for Pc groups
 gap> g:=AbelianGroup([2,2]);;

Original file line number	Diff line number	Diff line change
`@@ -58,6 +58,9 @@ gap> cp:=CompatiblePairs(g,mo);`
`58`	`58`	`<group of size 240 with 4 generators>`
`59`	`59`	`gap> Length(ExtensionRepresentatives(g,mo,cp));`
`60`	`60`	`2`
	`61`	`+gap> mo:= TrivialGModule( g, GF(3) );;`
	`62`	`+gap> Length( Extensions( g, mo ) );`
	`63`	`+1`
`61`	`64`
`62`	`65`	`# New code for Pc groups`
`63`	`66`	`gap> g:=AbelianGroup([2,2]);;`