function y=newtonsys(f,dfdx,x,varargin)
%NEWTONSYS Root finding, Newton method, systems of equations.
%   X=NEWTONSYS(F,DFDX,X0) attempts to find the solution of the nonlinear
%   system of equations F(X)=0 near X0 where DFDX is the derivative (Jacobian
%   matrix) of the function F using Newton method.
%
%   X=NEWTONSYS(F,DFDX,X0,P1,P2,...) passes the parameters P1,P2,... to F.

%   Francisco J. Beron-Vera,2006/09/18 

y=x+1;
k=0;
while norm(y-x)>eps*norm(y)
   k=k+1;
   if k>50
      warning('Tolerance not satisfied.')
      break
   end
   y=x;
   x=x-dfdx(x,varargin{:})\f(x,varargin{:});
en