Re: matrix determinant calculation

[A Scotte Hodel]

Others have explained why the computed result is reasonable.

If you're interested in further details on numerical roundoff and the 
associated problems, I recommend "Matrix Computations" by Golub and Van 
Loan.  It's an excellent text and reference.  I strongly recommend it.

When testing for singularity on a computer, it's better to use the 
singular value decomposition (developed by Prof Golub in the 50's) than 
a determinant calculation.  The ratio of largest singular value over 
the smallest singular value is called the two-norm condition of a 
matrix, and is a good measure of how hard it is to accurately invert a 
matrix on a computer.  Being *close* to singular is just as bad as 
*being* singular, on a computer.

I hope this is useful to you.

On May 26, 2005, at 12:49 AM, Hui Zhang wrote:

> hi, i am not sure the following problem is already reported or not.
> It happens both version 2.1.50 (sparc-sun-solaris2.8) and
> version 2.1.57 (i686-pc-linux-gnu).
>
> octave:1> a=[1,2,3;4,5,6;7,8,9]
> a =
>
>   1  2  3
>   4  5  6
>   7  8  9
>
> octave:2> det(a)
> ans =  6.6613e-16
>
> THE ANS is WRONG.
>
>
> Best regards,
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Hui Zhang     Ph.D. Candidate & Research Assistant
> VLSI System Design Lab
> Department of Electrical and Computer Engineering
> State University of New York at Stony Brook
> Stony Brook, NY 11794-2350
> Tel: 631-632-1068  Email: address@hidden
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



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