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Re: Wrong results for "Large" simultaneous equation calculation
From: |
Ben Abbott |
Subject: |
Re: Wrong results for "Large" simultaneous equation calculation |
Date: |
Sat, 16 Jan 2010 22:36:57 -0500 |
On Jan 16, 2010, at 1:09 PM, David Parman wrote:
>>
>> On Fri, Jan 15, 2010 at 4:20 PM, Ben Abbott <address@hidden> wrote:
>> 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
>
> Ben,
>
> Attached is the file. Any advice you can give will be greatly appreciated.
The error looks quite reasonable to me.
octave:111> norm (A*(A\b)-b) / max (b)
ans = 8.0399e-14
I don' t see how Octave's solution can be off by 6 orders of magnitude ...
hmmm, I'm running the developers sources, what version of Octave are you
running?
Ben
p.s. please <reply-all> and respond at the bottom so that others may follow
along.
Re: Wrong results for "Large" simultaneous equation calculation, Jaroslav Hajek, 2010/01/17