Re: Inverse Matrix Function appears a bit wonky

[Ada Cheng]

Your matrix is singular.
Ada
On Wed, 3 Nov 2004, Robert A. Macy wrote:

> I tried a simple test of the inverse function using a "made
> up" matrix...
>
> testmatrix =
> 1 2 3 4
> 2 2 3 3
> 1 2 1 2
> 3 2 2 1
>
> forgive the wraparound results...
> testmatrixinv = inverse(testmatrix) =
> -3.37769972052787e+15 4.5035996273705e+15
> 1.12589990684262e+15 -2.25179981368525e+15
>
> 3.37769972052787e+15 -4.5035996273705e+15
> -1.12589990684262e+15 2.25179981368525e+15
>
> 3.37769972052787e+15 -4.5035996273705e+15
> -1.12589990684262e+15 2.25179981368525e+15
>
> -3.37769972052787e+15 4.5035996273705e+15
> 1.12589990684262e+15 -2.25179981368525e+15
>
> inverse function merrily calculated the above inverse
> matrix with no special notes.
>
> but when I multiply the two, the results are...
> testmatrixinv*testmatrix =
> 1 0 1 -1
> 0 1 0 1.5
> -1 -0.5 1 1
> 1 0.5 0.5 0.25
>
> which is definitely NOT the identity matrix
>
> the testmatrix was just an arbitrary matrix to test the
> process.  It is very disconcerting that the first,
> arbitrary test, fails so miserably.  What happened?
>
> What do I do to make certain octave doesn't do these
> erroneous inverse calculations the next time I go to use
> the inverse function?
>
>            - Robert -
>
>
>
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============================================================================
 Ada Cheng                              address@hidden
 Assistant Professor                    http://www.kettering.edu/~acheng
 Department of Science and Mathematics
 Kettering University
 1700 West Third Avenue
 Flint, Michigan 48504-4898
 U.S.A.
============================================================================



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