[Networkit] LAMG not converning for small tolerance

Simon Adrian simon.adrian at tum.de
Wed Nov 2 19:04:22 CET 2016


I have played around with the test example cpp/numerics/test/LAMGGTest.cpp  of 
the LAMG implementation.

It works well when the I use the default tolerance (i.e., 
status.desiredResidualReduction = 1e-6* b.length() / 
(hierarchy.at(0).getLaplacian() * x - b).length();)

When I decrease the tolerance, it stops working at around 1e-10 for the "wing" 
graph, that is, it does not converge and the output for the final residual is 
"-nan".

I'm not an expert in the LAMG algorithm, but it seems to me that this is not 
the correct behavior (given that there is some distance to machine precision 
from the tolerance).

I compiled a slightly modified version without the googletest environment (see 
attachment); I presume that this should not effect the observation.

Thank you very much for your help!

Best regards,
Simon
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