# non-linear system solving for non square matrix

Discussion in 'Numerical Analysis' started by Richard, Sep 21, 2005.

1. ### RichardGuest

hey

it is a long time since i have had to put my maths to use, so forgive
stupid questions.

anywho, i have a system of k non-linear eqns in n variables. i have been
reminding myself of the newton-raphson method for root finding, but all
the literature i have looked at is for systems on n eqns in n variables.

is it possible to apply the newton root finding procedure to such a
system? if not, can anyone point me in the right direction?

thanks.

Richard, Sep 21, 2005

2. ### Peter SpellucciGuest

if k>n, you might minimize sum_{i=1 to k} F_i(x)^2 def =f(x)
using e.g. the gauss -newton or levenberg marquardt methods,
see
http://plato.la.asu.edu/topics/problems/nlolsq.html
for codes (minpack1, lmder, lmdif, elsunc,...)
if however k<n, then the solution, if existing at all, will not be unique
and you may want to compute the solution of minimal norm, which is
also possible using a modified Newtons method. see work of Graves in Duke Math.
J. 17, (1950) and b.T.Polyak in USSR Comp Math Phys4(1964)
(in principle the constructive application of the proof of the implicit function
theorem)
hth
peter

Peter Spellucci, Sep 21, 2005

3. ### Ivan OseledetsGuest

Well, if you write your equations as a overdetermined system of equations as
f(x) = 0 you can solve it just as in the regular case:

x_{k+1} = x_{k} - Df(x)^{+}f(x_k), where Df(x)^{+} is a pseudoinverse of the Jacobian matrix.
It is called "Newton method for overdetermined systems" and its convergence properties are not so well studied; see for example a paper by Didier and Schub.
You do not need to evaluate second order derivatives, what happens if you treat the problem as a nonlinear least squares; The convergence speed is now not quadratic in general, but!
If there is a root y f(y) = 0 the convergence is still quadratic. If there is an approximate root f(y) = eps than the convergence is linear but the speed q
is proportional to eps.
This method has proved to be rather efficient in some applications of mine.

Ivan Oseledets, Oct 9, 2005
4. ### carlosGuest

To get back to speed, read Fletchers Practical Methods of Optimization
2nd ed (1987) Ch 6. After 18 years IMO still by far the best book
to learn residual minimization methods for solving NL equations
when n<=k. Fletcher has that unique British gift of blending
theory and practice in the right proportion while writing complete
sentences with the correct grammar. He favors Gauss-Newton,
and I do too.

Chapters are largely self contained so you wont waste too much time.
If k<n you need n-k constraints; that falls under constrained
minimization, which is covered in the 2nd part of that book.

NR Chs 9-10 might also be consulted but offer no theory.
For open source code see minpack in netlib.

carlos, Oct 9, 2005