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From: engstad AT funcom DOT com (Paal-Kristian Engstad)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: 3*3 eigenvalues
Date: 20 Dec 1995 09:45:28 GMT
Organization: Funcom Productions.
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Message-ID: <4b8lvo$ni4@odin.funcom.no>
References: <DJL4wn DOT 2zr AT jade DOT mv DOT net> <4at2kg$n2a AT micro DOT internexus DOT net>
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To: djgpp AT delorie DOT com
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Laszlo Vecsey (master AT micro DOT internexus DOT net) wrote:
: A.Appleyard (A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk) wrote:
: : I have written a C function to work out eigenvalues and eigenvectors of 3*3
: : matrixes quickly and without iterating, if anybody out there is interested.
: 
: Well, now that you got our attention, tell us what they are so we can 
: determine if we're interested! (Or am I the only one that doesn't know 
: what eigen values are and what they are useful for? I hope not)

First, sorry for breaking net-etiquette by explaining this concept, which 
really should be in some other net group, albeit I'm not sure which...

Eigenvalues (and more interesting, the eigenvectors), are mathematical 
concepts of great importance. Briefly explained, imagine you have a set 
of equations:
	y1 = a*x1 + b*x2
	y2 = c*x1 + d*x2
which in "short-hand" using matrix-vector product is
	y = A*x,
where
	    [y1]     [x1]          [ a b ]
        y = [y2] x = [x2], and A = [ c d ].

[This might not seem like a useful equation, but you can use it to rotate 
stuff in two dimentions etc.]

Eigenvectors are defined as those vectors 'm' who solve this equation:
	A*m = k*m,
where 'k' is a scalar (real number). And this number 'k' is called the 
eigenvalue.

PKE.