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From: "Bruno Luong" <b.luong@fogale.findmycountry>
Newsgroups: comp.soft-sys.matlab
Subject: Re: How to solve XA + B = 0?
Date: Thu, 2 Jul 2009 18:08:02 +0000 (UTC)
Organization: FOGALE nanotech
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"Matt " <xys@whatever.com> wrote in message <h2iq66$21p$1@fred.mathworks.com>...
> "Tao Shen" <tshen@mathworks.com> wrote in message <h2ink4$acj$1@fred.mathworks.com>...
> > Hello all,
> > 
> > I need to solve a equation which is XA + B =0. It is a least squares 
> > problem.
> > 
> > The X is a 2*N matrix, A is N*2, and B is 2*2. Because A is N*2, it seems we 
> > can't find its right inverse matrix but only left inverse matrix. However it 
> > is not useful in this case.
> > 
> > Is there any method to solve this problem?
> 
> It's an under-determined system (4 equations in 2*N unknowns). You'll have to decide how  you'd like to distinguish among possibly many solutions.

A now Matt has pointed to this aspect, I wonder if OP can specified "least square" in his first post. If each individual row of X is minimum 2-norm solution, this is pseudo inverse solution (and using "/" is wrong way to go). 

I have no idea (yet) how to find the X with minimum (global) 2-norm.

Bruno