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From: "John D'Errico" <woodchips@rochester.rr.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: underdetermined linear equation system
Date: Sun, 11 Oct 2009 17:02:06 +0000 (UTC)
Organization: John D'Errico (1-3LEW5R)
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"arnold szilagyi" <john.doe.nospam@mathworks.com> wrote in message <hasu5b$di4$1@fred.mathworks.com>...
> hello,
> 
> I have a big an very urgent problem. I have to solve an equation system in this way:
> 
> I have 50 unknows, and 8-15 equations. I have an initiative solution, so I'm looking for an iterative method to solve the system.
> 
> Since I'm not a mathematician, I googled a lot for an appropiate method, but I couldn't find nothing. 
> I tried the mldivide function of the matlab, but it is not good for me. the solutions must be all positive, and matlab gives me a lot of zeros (I know that's the way it solves the system with mldivide).
> 
> So I'm stuck.
> can anybody help me please?
> 
> bets regards
> arnold

There are infinitely many solutions to your problem.
So MATLAB chooses an arbitrary one. If you have
additional information (i.e., that the solution must
be non-negative) then you must supply that piece
of information!

Use a tool that will allow you to do this. One such
tool is linprog. A second is lsqnonneg. I'd expect to
see different solutions from the two, but since there
are infinitely many solutions, what else can you
expect?

Note that if you use linprog, you will need to provide
a linear objective function.

John