[GAP Forum] StructureDescription

Bartosz Putrycz Bartosz.Putrycz at math.univ.gda.pl
Tue Jul 8 09:47:06 BST 2008

> Dear Andries,
> Several points:
> 1.  StructureDescription works quickly (see next point also) with
>     groups called from the SmallGroups library.  It tends to be
>     much slower with groups described otherwise (e.g., some random
>     presentation).
> 2.  A slightly modified version of the program works substantially
>     faster and with less memory; it's my understanding it will be in
>     the next version of gap.  (Thanks Burkhard!)
> 3.  Crashing:  This turned out to be a combination of things:  First,
>     some sort of memory problem in our machine.  That's fixed now.
>     I can highly recommend the programs that the gap group makes
>     available for testing memory.
>     Second:  However, in computing large automorphism groups, gap
>     continued to crash the machine.  It appears that this was caused
>     by some interaction with the operating system.  We switch to 
>     Scientific Linux (~ RedHat Enterprise 5) last week & the problem
>     seems to have vanished (at least I'm using up to 50+G and the
>     machine hasn't crashed yet).

Dear Keith,

I also noticed problems with StructureDescription, with groups of high rank.
For example extraspecial 2-group of order 128:
gap> StructureDescription(SmallGroup([128, 2327]));
exceeded the permitted memory (`-o' command line option) at

With change of memory settings it is possible to handle it, but it takes 
a lot of time on my machine.

And maybe more challenging:
StructureDescription(SmallGroup([512, 10494212]));


> 4.  Which group(s) of order 1024?  I'd be intersted in seeing if the
>     new version of StructureDescription can handle them.
> So for the StructureDescription problem, for my uses,I'm quite happy
> at the moment.  The help I received was fantastic.  Thanks everyone!
> I still have a problem with computing automorphism groups for groups
> of order 256, but I'll send a note about that at lome point later.
> Keith
>> The GAP manual advertises:
>> "The method for StructureDescription exhibits the structure of
>>  the given group to some extend using the strategy outlined below.
>>  The idea is to return a possibly short string which gives some insight
>>  in the structure of the considered group and can be computed
>>  reasonably quickly."
>> (manual typo: s/extend/extent/)
>> It seems to me that the idea fails. Maybe it is meant to be used only
>> on baby groups, say with fewer than 100 elements, but used on
>> rather small groups it usually crashes after many hours.
>> Reactions are: recursion depth overflow (at recursion depth 5000), or
>> memory overflow (needs more than the 3GB allowed on the command line), or
>> missing info in Holt/Plesken library.
>> This is on a 4GB machine, and Google shows me the complaint of
>> R. Keith Dennis who tried groups of order 256 on a 128GB machine,
>> and found that GAP crashed, so I need not be surprised that GAP
>> fails for me on groups of order 1024.
>> Other functions reveal the structure of the groups involved rather quickly.
>> I wonder what StructureDescription tries to do that is so extremely expensive.
>> Maybe the present behaviour should be regarded as buggy.
>> It does not live up to what the manual promises.
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