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corrupted subgroup in a direct product #6000

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#6790
Open
corrupted subgroup in a direct product#6000
#6790
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kind: bugIssues describing general bugs, and PRs fixing themkind: bug: wrong resultIssues describing bugs that result in mathematically or otherwise wrong results, and PRs fixing them

Description

@ThomasBreuer
ThomasBreuer
opened on Jun 3, 2025

The following happens in GAP 4.14.0 as well as in the master branch.

Here is a correct computation.

gap> g1:= ExtraspecialGroup( 2^3, "+" );;
gap> g2:= CyclicGroup( 4 );;
gap> dp:= DirectProduct( g1, g2 );;
gap> img:= Image( Embedding( dp, 2 ) );;
gap> IsCyclic( img );
true
gap> List( GeneratorsOfGroup( img ), Order );
[ 4, 2 ]
gap> List( AllSubgroups( img ), Size );
[ 1, 2, 4 ]

If we ask for the centre of the direct product before creating the image of the 2nd embedding,
we get the following.

gap> g1:= ExtraspecialGroup( 2^3, "+" );;
gap> g2:= CyclicGroup( 4 );;
gap> dp:= DirectProduct( g1, g2 );;
gap> Centre( dp );;                                                  # this line makes the difference
gap> img:= Image( Embedding( dp, 2 ) );;
gap> IsCyclic( img );
true
gap> List( GeneratorsOfGroup( img ), Order );
[ 2, 2 ]
gap> List( AllSubgroups( img ), Size );
[ 1, 2, 2, 2, 4 ]

Activity

  1. ThomasBreuer
    added
    kind: bugIssues describing general bugs, and PRs fixing them
    kind: bug: wrong resultIssues describing bugs that result in mathematically or otherwise wrong results, and PRs fixing them
    on Jun 3, 2025

  2. fingolfin commented on Jun 4, 2025

    @fingolfin
    fingolfin
    on Jun 4, 2025
    Member

    This bug was introduced between GAP 4.11 and 4.12. Using dev/bisect.sh I tracked this down to my commit 0c67c5a from PR #3655.

  3. fingolfin commented on Jun 4, 2025

    @fingolfin
    fingolfin
    on Jun 4, 2025
    Member

    This fixes the issue but I don't understand it (yet):

    diff --git a/lib/pcgsspec.gi b/lib/pcgsspec.gi
index 75a0e5a9a..607a8121f 100644
--- a/lib/pcgsspec.gi
+++ b/lib/pcgsspec.gi
@@ -689,7 +689,7 @@ function( group )
     if HasPcgs(group)  then
         spec := SpecialPcgs( Pcgs( group ) );
     else
-        spec := SpecialPcgs( AttributeValueNotSet( Pcgs, group ) );
+        spec := SpecialPcgs( Pcgs( group ) );
         SetPcgs( group, spec );
     fi;
     SetGroupOfPcgs (spec, group);

  4. fingolfin commented on Jun 6, 2025

    @fingolfin
    fingolfin
    on Jun 6, 2025
    Member

    The reported issue has been fixed for now by PR #6002 but I am leaving this open as I'd like to better understand why things broke -- I am wary of additional related bugs in this code...

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