[GAP Forum] Error given by StructureDescription function

[Alexander Hulpke]

Dear Forum,

On Nov 9, 2010, at 9:12 PM, krishna mohan wrote:
> 
> gap> A:=[[0,1,0],[0,0,1],[1,0,0]]; B:=[[0,0,1],[0,1,0],[-1,0,0]]; 
> C:=[[E(4),0,0],[0,E(4)^(-1),0],[0,0,1]];
> gap> g2:=GroupWithGenerators([A,B,C]);       
> <matrix group with 3 generators>
> gap> StructureDescription(g2);
> recursion depth trap (5000)

Thank you for reporting this bug. We will correct this in the next release.

>  How can I bypass 
> this problem?

The error is triggered by the routine testing for finiteness of the matrix group. You can avoid it by telling GAP (immediately after creating the group) that it is finite:

SetIsFinite(g2,true);

with this the calculation will work fine.

Finally, let me add the remark that you are doing a potentially rather costly calculation (StructureDescription) in a representation (matrix group over number field) that does not have very efficient algorithms available. You will be able to speed up calculations by first converting to a permutation group:

hom:=IsomorphismPermGroup(g2);
p2:=Image(hom);

Best,

   Alexander Hulpke

-- Colorado State University, Department of Mathematics,
Weber Building, 1874 Campus Delivery, Fort Collins, CO 80523-1874, USA
email: hulpke at math.colostate.edu, Phone: ++1-970-4914288
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