FIX: IntermediateGroup will not exit quietly if index is large

gap-system · Feb 7, 2021 · ce446ff · ce446ff
1 parent 177da5c
commit ce446ff
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Showing 2 changed files with 15 additions and 1 deletion.
diff --git a/lib/csetgrp.gi b/lib/csetgrp.gi
@@ -330,6 +330,12 @@ local o,b,img,G1,c,m,mt,hardlimit,gens,t,k,intersize;
  return fail;
  fi;
 
+ # try the `AscendingChain` mechanism
+ c:=AscendingChain(G,U:refineIndex:=QuoInt(IndexNC(G,U),2));
+ if Length(c)>2 then
+ return First(c,x->Size(x)>Size(U));
+ fi;
+
  # old code -- obsolete
 
  c:=ValueOption("refineChainActionLimit");
@@ -339,7 +345,10 @@ local o,b,img,G1,c,m,mt,hardlimit,gens,t,k,intersize;
  hardlimit:=1000000;
  fi;
 
- if Index(G,U)>hardlimit then return fail;fi;
+ if Index(G,U)>hardlimit then 
+ Info(InfoWarning,1,
+ "will have to use permutation action of degree bigger than ", hardlimit);
+ fi;
 
  if IsPermGroup(G) and Length(GeneratorsOfGroup(G))>3 then
  G1:=Group(SmallGeneratingSet(G));

diff --git a/tst/testbugfix/2021-02-07-IntermediateGroup.tst b/tst/testbugfix/2021-02-07-IntermediateGroup.tst
@@ -0,0 +1,5 @@
+# IntermediateGroup in large index, reported in Forum (Breuer/Anvita) on 2/7/21
+gap> L:=PSL(2,7^3);;
+gap> S:=SylowSubgroup(L,2);;
+gap> IntermediateGroup(L,S);
+<permutation group of size 344 with 4 generators>