Re: Wrong results for "Large" simultaneous equation calculation

[Ben Abbott]

On Jan 15, 2010, at 5:58 PM, David Parman 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?

Unfortunately if you don't give us an example that doesn't work, no one will be 
able to help.

Fortunately, you can easily provide your example :-)

If the matrix is only 120x120, try  ...

        save example.mat A b

... and attach example.mat to your reply.

Ben