[GAP Forum] perfect residuum?

[R. K. Dennis]

Dear Colleagues,

I have several gap questions:

1.  If one computes ConjugacyClassesMaximalSubgroups(g) for some groups g
such as PSL(2,17^2), gap stops with an error message
"perfect residuum too large"
and then gives the options to "quit" or "return".  If one takes "return",
then the compution seems to continue as usual, giving the result.
[See Footnote A at the end of this message.]

Assuming this is correct, then why interrupt the computation?

2.  Such interruptions occur for other groups such as PSL(2,p) for
smaller values.  Now one knows what the maximal subgroups are for
PSL(2,p), as well as PSL(2,q), q=p^n, at least up to isomorphism.  One
can use this plus IsomorphicSubgroups(g,M) to find the maximal
subgroups, but that doesn't seem to be very efficient.

Is there a way to do this directly?  Ok, I know how to find the first two
types of maximal subgroups with

Stabilizer(g,1)

for the group which is the semidirect product of the additive group of 
F_q with the group of units squared acting by multiplication.  And

Stabilizer(g,[1,2],OnSets)

for the dihedral group of order q-1 (or 2(q-1) for p = 2).

Anyone know how to get the others?

3.  I assumed that "too large" meant a memory problem, so I tried 
memory_allocated to determine how much was used.  But the memory used
didn't seem very large.  I tried this after my own program, say MyProgram.
As I don't know what memory_allocate actually is applied to, I changed to
putting it inside MyProgram, just after ConjugacyClassesMaximalSubgroups(g),
and then added the value to the tuple of return values.  Didn't seem to
change things much.  Does it work for programs that are user-created?


Thanks very much for any help or suggestions!

Best regards,

Keith Dennis


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Footnote A:


[102,12068640,"A",[1,289],"PSL(2,289)"]
Error, the perfect residuum is too large at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:1368 called from
RepsPerfSimpSub( G, false ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:1488 called from
RepresentativesPerfectSubgroups( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:401 called from
LatticeByCyclicExtension( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:909 called from
LatticeSubgroups( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:208 called from
ConjugacyClassesSubgroups( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/maxsub.gi:369 called from
...  at *stdin*:7
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> memory_allocated;
brk> return;
cc = 7  max = 491029
amt = 1365885
mem = 1.30MB
Time:    0d16h16m47s 16ms

...

[110,16776960,"A",[1,256],"PSL(2,256)"]
Error, the perfect residuum is too large at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:1368 called from
RepsPerfSimpSub( G, false ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:1488 called from
RepresentativesPerfectSubgroups( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:401 called from
LatticeByCyclicExtension( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:909 called from
LatticeSubgroups( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/grplatt.gi:208 called from
ConjugacyClassesSubgroups( G ) at /home/dennis/GAP4.10.1/gap-4.10.1/lib/maxsub.gi:369 called from
...  at *stdin*:7
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>