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Topic: Problem with Newton Raphson Method for Two Variables
Replies: 5   Last Post: Jun 25, 2013 2:27 AM

 Messages: [ Previous | Next ]
 Torsten Posts: 1,639 Registered: 11/8/10
Re: Problem with Newton Raphson Method for Two Variables
Posted: Jun 24, 2013 8:33 AM

> Hello,
>
> I have two functions, f1(x1)=a1*exp(b1*x1)
> and f2(x2)=a2*exp(-((x2-b2)/c2)^2)
> where a1, b1, a2, b2 and c2 are constants. x1 and x2 are dependent variables.
> I need to solve the two functions using newton raphson's method. Can anyone help me?
>
> Thanks
> Shubha
>
>
> On Wednesday, October 12, 2011 2:50:30 PM UTC+5:30, Emre wrote:

> > clc,clear
> >
> > % Newton Raphson solution of two nonlinear algebraic equations
> >
> > % set up the iteration
> >
> > error1 = 1.e8;
> >
> > xx(1) = 0; % initial guesses
> >
> > xx(2) = 0.5;
> >
> > iter=0;
> >
> > maxiter=40
> >
> > % begin iteration
> >
> > while error1>1.e-12
> >
> > iter=iter+1;
> >
> > x = xx(1);
> >
> > y = xx(2);
> >
> > % calculate the functions
> >
> > f(1) = 4*x^2-20*x+0.25*y^2+8;
> >
> > f(2) = 0.5*x*y^2+2*x+8;
> >
> > % calculate the Jacobian
> >
> > J(1,1) = 8*x-20;
> >
> > J(1,2) = 0.5*y;
> >
> > J(2,1) = 0.5*y^2+2;
> >
> > J(2,2) = x*y;
> >
> > % solve the linear equations
> >
> > y = -J\f';
> >
> > % move the solution, xx(k+1) - xx(k), to xx(k+1)
> >
> > xx = xx + y';
> >
> > % calculate norms
> >
> > error1=sqrt(y(1)^2+y(2)^2);
> >
> > error(iter)=sqrt(f(1)^2+f(2)^2);
> >
> > ii(iter)=iter;
> >
> > if (iter > maxiter)
> >
> > error1 = 0;
> >
> > s=sprintf('****Did not converge within %3.0f iterations.****',maxiter);
> >
> > disp(s)
> >
> > end
> >
> > % check if error1 < 1.e-12
> >
> > end
> >
> > x = xx(1);
> >
> > y = xx(2);
> >
> > f(1) = 4*x^2-20*x+0.25*y^2+8;
> >
> > f(2) = 0.5*x*y^2+2*x+8;
> >
> > % print results
> >
> > f
> >
> > xx
> >
> > iter
> >
> > % plot results
> >
> > semilogy(ii,error)
> >
> > xlabel('iteration number')
> >
> > ylabel('norm of functions')
> >
> > clear ii
> >
> > clear error
> >
> >
> >
> > what am i doing wrong here, it doesn't converge and find the correct values for x and y?
> >
> > Is this part
> >
> > "error1=sqrt(y(1)^2+y(2)^2);
> >
> > error(iter)=sqrt(f(1)^2+f(2)^2);" right?
> >

Except for trivial cases, both functions f1 and f2 don't have a zero.
Moreover, f1 only depends on x1 and f2 only depends on x2.
So what exactly are you trying to do with Newton-Raphson ?

Best wishes
Torsten.

Date Subject Author
10/12/11 Emre
10/12/11 Torsten
6/24/13 shubha22@gmail.com
6/24/13 Torsten
6/24/13 shubha22@gmail.com
6/25/13 Torsten