" fsolve" to solve a nonlinear system with two unknowns - wrong results!

Hello,

I am trying to solve a system of nonlinear equations with two unknowns. I am using "fsolve" to do so (and I know what results/answers I should get). However, the values I get are wrong! The code containing the function of nonlinear equations is added below(pfun). Am I doing something wrong in passing parameters, etc? In the command line I first give an initial guess by entering: x0 = [0,0] and then I enter: S = fsolve(@pfun, x0) then I get wrong values in S. Why is that so?

Here is the code in the function:

function F = pfun(x)
w = 31;
n = 10; 
m = 6;
F = [2.*(1-2.*x(1)).*(1-x(1).^(m+1))./ (1-2.*x(1)).* (1-x(1).^(m+1)) + w.*(1-x(1)).*(1-(2.*x(1)).^ (m+1)) - x(2); 1-(1-x(2).^(n-1))-x(1)];

I highly appreciate your help. Thank you.

Answer 1

[Alan Weiss]

When I tried I get the following:

Equation solved.

fsolve completed because the vector of function values is near zero
as measured by the default value of the function tolerance, and
the problem appears regular as measured by the gradient.

<stopping criteria details>

S =

      1.0003    1.0000

Then I tried checking the answer:

pfun(S)

ans =

     1.0e-12 *

     -0.3917
      0.0209

This looks correct to me. If you want a different answer, try a different starting point, or double-check your equations to ensure that they are doing what you think.

One more thing: it would help if you would format your code using the {} Code button. But your code was short enough that I was able to format it for myself.

Alan Weiss

MATLAB mathematical toolbox documentation

[Atekeh]

Hi Alan, I double double checked the equations. You're right. I'm sorry .. there was a mistake in parentheses (one pair less!). Although I had checked it before posing the question, I don't know how I had not noticed that! Thank you.