RE: [Lapackpp-devel] LaLinearSolve not totally exact

[Jacob \(Jack\) G. <jg...@cs...>]

I believe I solved the problem.

The in the linslv.cc function, Xtmp should be a copy of B; however, at no
point is an actual copy made.  Xtmp is left at 0's before making the dgels
call.

I replaced line 219 with the line Xtmp=B; and it seems to fix the problem.

The output result appears to be correct, at least for an AX=A style problem,
where X should return the identity matrix.

Although it actually returns I + some small values in the order of 10e-17;
whereas CLAPACK returns 0's in these places for the same problem.  Is this
only something that displays with higher precision, or is there actually a
10e-17 error intruduced somewhere?

Jack

-----Original Message-----
From: lap...@li...
[mailto:lap...@li...] On Behalf Of Christian
Stimming
Sent: August 10, 2004 4:46 PM
To: Jacob (Jack) Gryn
Cc: lap...@li...
Subject: Re: [Lapackpp-devel] LaLinearSolve not totally exact

Am Dienstag, 10. August 2004 21:38 schrieb Jacob \(Jack\) Gryn:
> I tried it out again with both square and non-square matricies on both 
> complex and doubles:
>
> Results are the same for both Complex matricies and Doubles.
>
> With square matrices, the results are more viable.  The equation AX=B, 
> with A and B being the same, X should return the identity matrix; 
> however it does not.  Although through multiplying AX, the result still is
B.
>
> With rectangular matrices, the results are the same in both complex 
> and doubles, but AX != B;  X is usually all 0's, or very close to 0's 
> (10e-300).

When you say AX != B, how large is the actual difference, that is D=(AX-B)
and what is the Mat_Norm2(D)? If that's in fact in the region of 10e-15 or
smaller, then you are only seeing some rounding effects due to the floating
point representation -- I wouldn't consider this a problem. If you have
matrices with numbers in normal regions, then simply consider anything below
10^-10 as zero and you're fine. Did I misunderstand something?

Christian



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