Path: news.mathworks.com!not-for-mail
From: "Bobby Cheng" <bcheng@mathworks.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Ill conditioned matrix makes linsolve crash
Date: Thu, 29 Oct 2009 21:13:47 -0400
Organization: The MathWorks, Inc.
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http://www.mathworks.com/support/bugreports/search_results?search_executed=1&keyword=573283&release_filter=Exists+in&release=168&selected_products=&commit=Search

See bug report 573283.

"Sebastiaan " <s.breedveld@erasmusmc.REMOVE.BOO.BOO.nl> wrote in message 
news:hcc4ma$rso$1@fred.mathworks.com...
> Dear users,
>
> I am trying to solve a very ill conditioned system with linsolve, but it 
> makes Matlab hang.
>
> The matrix A is symmetric and ill-conditioned:
>>> rcond(A)
> ans =
>   5.7435e-21
>
>>> max(max(abs(triu(A)-tril(A)')))
> ans =
>     0
>
> I try to solve this using linsolve (with SYM=false it works):
> linsolvopts =
>       SYM: 1
>    POSDEF: 0
>
> Executing:
>>> [x, rcond] = linsolve(A, b, linsolvopts);
>
> makes Maltab unresponsive. Apparently, the LAPACK subroutine dsysv (which 
> is the one used I think) crashes.
>
> I know I should not try to solve these kind of ill-conditioned matrices, 
> but I use rcond result from linsolve to check if the result may make 
> sense. However, I do not believe dsysv is supposed to crash.
>
> I have tried it with 2009a/b x86_64 on Linux and on another machine with 
> 2009a x68_32, Windows XP, all crash.
>
> The matrix and vector can be downloaded here. It is a moderate 96x96 sized 
> problem (76kiB):
> http://www.xs4all.nl/~pukenmul/downloads/ProblemMat.mat
>
> Thanks!
> Sebastiaan
>