[GAP Forum] StructureDescription
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
> 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).
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:
> 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.
>> 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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