fsolve for 2 equation with 2 variables

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Miroslav Mitev 2017-12-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/372758-fsolve-for-2-equation-with-2-variables

评论： Star Strider 2017-12-13

Here is my function, in order to shorten the expression here I add A and B, but in my original function they are inside F(1) and F(2) (i.e. that is not the problem), this is the error I get: Objective function is returning undefined values at initial point.

function F = myfun(x)
N=32;
K=5;
s=1;
b=0.1;
P=5;
A=-2*s^4*x(1)*N+2*s^4*x(1)*K+3*s^2*x(2)*log(2);
B=sqrt(8)*x(2)*s^2*log(2);
g=exprnd(1,1,N-K); 
F(1) = sum((g.*(1-b*x(1))/(x(2)*log(2)))-1./g)+(N-K)*((A+sqrt(A^2-B^2))/((4*s^2/sqrt(8))*B))-N*P;
F(2) = b*(sum(log2((g*(1-b*x(1)/x(2)*log(2))))))-(N-K)*log2(1+(2*A^2+2*A*sqrt(A^2-B^2)-B^2)/((4/sqrt(8))*B*(A+(4/sqrt(8))*B+sqrt(A^2-B^2))));
end

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采纳的回答

Star Strider 2017-12-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/372758-fsolve-for-2-equation-with-2-variables#answer_296071

What is the initial point you chose? Your function returns finite values for random non-zero arguments. It returns [NaN NaN] for [0 0] as an input.

The solution is most likely to use an initial point other than [0 0]. I would use rand(2,1).

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Matt J 2017-12-13

Since it is only a function of 2 variables, you could do a coarse surf() plot of norm(F) and find visually where the roots approximately lie. This would give you a better initial guess than simply randomizing.

Star Strider 2017-12-13

My pleasure.

It might be useful for you to post the symbolic expression you are coding as well as your code for it as a new Question, since that seems to be the problem.

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