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Re: Wrong results for "Large" simultaneous equation calculation


From: Jaroslav Hajek
Subject: Re: Wrong results for "Large" simultaneous equation calculation
Date: Sun, 17 Jan 2010 06:46:58 +0100

On Fri, Jan 15, 2010 at 11:58 PM, David Parman <address@hidden> wrote:
> I am new to Octave, and ran into a problem with my first real application. I
> am working with a large (120x120) set of simultaneous equations. When I
> solve the equations using the A\b nomenclature, I get an answer - no
> complaints from Octave. But the answer is clearly very wrong - all of the
> values are about 6 or more orders of magnitude smaller than they should be.
> I have the same set of calculations in MathCad, which yields an appropriate
> solution. I can also approximate the solution using direct analytical
> approach, which also confirms that the Octave result is way off.
>
> I found a couple of examples online for this type of solution (small 2x2 and
> 3x3 examples, one of which had the wrong answer on the webpage - confirmed
> by Octave and MathCad). These all worked just fine, but they have "nice"
> values (some of my values can be very large numbers, on the order of 10^18).
>
> I have spot checked the matrices and the values appear to be exactly the
> same in Octave as they are in MathCad. Does anybody have an idea what I
> might be missing here?
>
> --
>

For linear equations, Octave use LAPACK, which is about the best
library there is. Chances are good that MathCad is even using the same
under the hood. So it's probable that either you are doing something
wrong, or (not so probable) your Octave is miscompiled. You have
provided too little information to get any more help.
You'd need to describe the problem and share the matrices.

-- 
RNDr. Jaroslav Hajek, PhD
computing expert & GNU Octave developer
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz


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