RE: [Lapackpp-devel] LaLinearSolve not totally exact

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

The numbers aren't even close, either 0, or 1e-300.   

I tested, for example, the following.

A= 
[1  1  1
3  3  1
3  2  7
3  2  7]

X is a blank 3x3 matrix

B=A=
[1  1  1
3  3  1
3  2  7
3  2  7]

The resulting X is all 0's; when it should be a 3x3 identity matrix.  Am I
possibly using the function wrong or something? 

PS.  For testing, I've been doing a lot of copying/pasting into matlab and
maple; it would be nice if a flag could be set to tell the operator<< call
to output the display in various formats.  

Jack
-----Original Message-----
From: Christian Stimming [mailto:sti...@tu...] 
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