Using fsolve inside an other fsolve

2015 3 16
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更新时间：2015 3 16
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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.

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Alan Weiss
Alan Weiss 2015-3-16

0 个投票

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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Vanduy TRAN
Vanduy TRAN 2015-3-16
Hi Alan,
I need to solve two non-linear systems A and B by fsolve. A is describeb by
if true
X=fsolve(@fun(X,a),X0)
end
B is describeb by
if true
[a,Y]=fsolve(@func(Y,X),Y0);
end
I want that when i run fsolve to solve A in na iterations, at each iteration, i take X to pass into fsolve of B as a output, and passing back from fsolve of B parameter a into fsolve of A for the next iteration of A. My problem is programed by the following codes.
if true
X0; % initial guess
a;
for i=1:na,
X(i)=fsolve(@fun(X(i),a),X0)
[a,Y]=fsolve(@func(Y,X(i)),Y0);
X0=X(i); %update X0
Y0=Y; % update Y0
end
Do you have any idea?
Thank you so much :)
Alan Weiss
Alan Weiss 2015-3-16
What you are doing looks reasonable in some sense. But some of the details are not clear to me.
When you call
[a,Y]=fsolve(@func(Y,X(i)),Y0);
the returned Y will almost certainly be a vector of zeros, or a vector with very small elements. Is that what you want?
Alan Weiss
MATLAB mathematical toolbox documentation

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