MATLAB: Using fsolve inside an other fsolve

fsolve

Hi everyone,
How can I use the result of each iteration fsolve as the initial guess for other fsolve. My code is like that: The main file
if true
X0 = rho;  % Make a starting guess at the solution

options = optimoptions('fsolve','Display','iter'); % Option to display output

[X,fval,exitflag,output] = fsolve(@eqDFT,X0,options); % Call solver

rho=X;
end
m -file for the first fsolve
if true
function  F=eqDFT(X)
Y0 = [alpha1 alpha2];  % Make a starting guess at the solution
options = optimoptions('fsolve','TolFun',1.e-12,'Display','iter'); % Option to display output
Y = fsolve(@eqAlpha,Y0,options,X); % Call solver
....
end
and the second fsolve @eqAlpha with Y variable and additionnal parameter X which is defined by rho like that
if true
  function G=eqAlpha(Y,rho)
  ....
  end
end
Thank you so much for reading this question and give me some ideas about it.

Best Answer

I am not sure that I understand you. How many times do you want to run fsolve? I suppose that you have a sequence of equations to solve, where each differs from the previous by a little bit, so the solution to the previous equation is a good starting guess for the next.
All you have to do is to use the previous solution x as the x0 input for fsolve. Something like
 x0 = rho
 for ii = 1:15
   x = fsolve(fun,x0);
   % change the parameters for fun here
   x0 = x; % update x0
 end
Alan Weiss
MATLAB mathematical toolbox documentation
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