how to solve implicit equation with two variables

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xueqi 2013-7-31

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https://ww2.mathworks.cn/matlabcentral/answers/83636-how-to-solve-implicit-equation-with-two-variables

Dear fellows,

I have a function like this pf=f(p1,p2). The problem is "pf" is a vecotor so in fact there are two output for this function, pf(1) and pf(2). And there is not explicit expression for function f. It is a big function and involves a lot of logic compare, etc inside it. I want to solve for p1 and p2 such that pf(1)=1000 and pf(2)=1000. Do you know how to realize this?

Cheers, Xueqi

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Walter Roberson 2013-7-31

You are right, sum() could result in accidental zeros. The difficulty with sum(f2(p).^2) is that it will not have 0 crossings, just a single 0 (round-off permitting); if that is acceptable then fminbnd() would likely be a better choice than fsolve()

xueqi 2013-8-1

As I see, I think the problem comes from the function are far from smooth. It has numerous local minimum. Probably this is the reason it stuck at the initial point...So I think we should look for some way to get rid of this problem. How does fminbnd work better in this aspect?

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