Weird bug in libgap?

[Peter Mueller]

The following code yields a Traceback for no (to me) obvious reason. The same happens on sagecell too. It seems to be a minimal case. For instance removing the line "P = ..." or having less deeply nested loops does not display this problem:

for A in PrimitiveGroups(3):

    for G in A.normal_subgroups():

        for U in G.conjugacy_classes_subgroups():

            P = U.trivial_character()

            for x in G.conjugacy_classes_subgroups():

                for y in x.conjugacy_classes_subgroups():

                    pass

GAPError                                  Traceback (most recent call last)

<ipython-input-1-c79c371f18a8> in <module>()

----> 1 load('./bug2.sage')

/home/mueller/local/sage-9.1/local/lib/python3.7/site-packages/sage/misc/persist.pyx in sage.misc.persist.load (build/cythonized/sage/misc/persist.c:2538)()

    142 

    143     if sage.repl.load.is_loadable_filename(filename):

--> 144         sage.repl.load.load(filename, globals())

    145         return

    146 

[.................. SNIP .....................]

> 2514             sig_on()

   2515             if n == 0:

   2516                 result = CALL_0ARGS(self.value)

GAPError: Error, usage: FreeGroup(<name1>,<name2>..) or FreeGroup(<rank>)

[Dima Pasechnik]

it is not libgap, it is pexpect GAP that is used here - something we should get rid of.

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[Dima Pasechnik]

I double-checked this, and it is in fact a bug in GAP (GAP 4.10).

gap> for A in AllPrimitiveGroups( DegreeOperation,[3]) do
> for G in NormalSubgroups(A) do
> for U in ConjugacyClassesSubgroups(G) do
> P := TrivialCharacter(Representative(U));
> for x in ConjugacyClassesSubgroups(G) do
> Print("x=", x, "\n");
> for y in ConjugacyClassesSubgroups(Representative(x)) do
> Print(y);
> od;od;od;
> od;od;
x=ConjugacyClassSubgroups(A(3),Group( () ))
Error, usage: FreeGroup(<name1>,<name2>..) or FreeGroup(<rank>) at
/home/dimpase/sage/local/share/gap/lib/grpfree.gi:467 called from
FreeGroup( deg - 1, nam ) at
/home/dimpase/sage/local/share/gap/lib/gpprmsya.gi:1753 called from
IsomorphismFpGroup( G, Concatenation( "S_", String( Length(
MovedPoints( G ) ) ), "." ) ) at
/home/dimpase/sage/local/share/gap/lib/gpprmsya.gi:1729 called from
IsomorphismFpGroup( Image( hom ) ) at
/home/dimpase/sage/local/share/gap/lib/grplatt.gi:890 called from
LatticeSubgroups( G ) at
/home/dimpase/sage/local/share/gap/lib/grplatt.gi:208 called from
ConjugacyClassesSubgroups( Representative( x ) ) at *stdin*:36 called from
... at *stdin*:39
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>

[Dima Pasechnik]

On Wed, Aug 19, 2020 at 11:22 AM Dima Pasechnik <dim...@gmail.com> wrote:
>
> I double-checked this, and it is in fact a bug in GAP (GAP 4.10).

And in the current GAP master and in GAP 4.11.0 this example works
just fine. So it's fixed there,
now we need to work on https://trac.sagemath.org/ticket/29314