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Re: Unable to write algebraic equations in correct Oct. syntax

From: Ivan Sutoris
Subject: Re: Unable to write algebraic equations in correct Oct. syntax
Date: Wed, 7 Jan 2009 18:10:17 +0100

2009/1/7 Gene Partlow <address@hidden>:
> Hi all...
> I am having terrible trouble forming correct Octave syntax.  I am
> sending this to you in hopes that someone might help me with re-
> phrasing rather standard nonlinear algebraic equations into a form
> suitable for the online Octave solver website at
> [Yes... I know that many of you would urge that I get hold of the new
> Octave 3.0.3 instead of using the old on-line site... but regardless,
> when I try to put the equations in 0 = f(x,y,v) form, whatever I do
> results in "errors".]


Indeed, I would recommend you to download regular Octave installation
:) Anyway, I think what you are looking for is function "fsolve",
which takes two arguments: first, a function "f", which for given
vector of variables X returns vector of deviations from your
equalities (so you need to rewrite your equations in f(X)==0 form),
and a starting vector X0.

For example, to solve system

x^2 + y^2 == 1
y == x^2

f could be written like this:

function y = f(u)
  %first equation
  y(1) = u(1)^2 + u(2)^2 - 1;
  %second equation
  y(2) = u(1)^2 - u(2);

and solution would be obtained by

u0 = [2,2];
sol = fsolve("f", u0)

It is important that f has only one input vector, which includes all
your variables (so in the example above, x becomes u(1), y becomes

Ivan Sutoris

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