
From:  Umut Tabak 
Subject:  [Getfemusers] Gmm matrix multiply accuracy issue 
Date:  Wed, 19 Oct 2011 10:29:01 +0200 
Useragent:  Mozilla/5.0 (X11; U; Linux x86_64; enUS; rv:1.9.1.16) Gecko/20101226 Icedove/3.0.11 
Dear all, I have run into a problem today with the gmm++ library. I am not very sure where the problem could be. I coded a symmetric lanczos solver for my problem and used gmm++ and MUMPS (as the linear solver). However, the accuracy of the basic linear algebra operations matrixvector product and gmm::add, scale and so seem to be questionable to me. Well it could be a bit cryptic to understand for you, maybe, but let me rephrase with an example: I am computing some coefficients to form a tridiagonal matrix and these coefficients that I compute in gmm++ start to deviate from the ones I computed in MATLAB. They seem to be the same at the starting iterations then conducting these matrixvector operations(for orthogonalizations), they start to diverge, I am not sure what the problem could be. I went over my code several times, however, the end results tell me that there is indeed a problem in the orthogonalization phase, which is basically a combination of matrixvector operations. Any comments are appreciated on this issue. Best regards, Umut 
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