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Student Support Forum: 'FindRoot vs. FindMinimum' topicStudent Support Forum > General > "FindRoot vs. FindMinimum"

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Ben Dundee
08/21/09 2:57pm

I am trying to numerically minimize a potential, call it f(x,y). Mathematica gives several options to do this, but there is a problem that I don't quite understand.

When I try to minimize f(x,y) with FindRoot, by setting the various derivatives to zero and giving mathematica a starting point, I get one answer. However, when I graph the function, the answer is wrong by about 10% in one of the variables. So, for example, the true minimum of f is at, say (2,1), and mathematica finds a minimum at (2,1.1).

Now, when I take the good value and plug it into the function, then use Minimize, mathematica returns the correct answer. So, for example, when I evaluate

FindMinimum[ f[2,y], ... ]

Mathematica returns

y->1

This is highly frustrating, and I am not sure what is happening. I am evaluating both expressions (FindRoot and FindMinimum) using the following option

WorkingPresicion -> 500

For both functions, I use

Compiled->False.

Also, for FindRoot, I've tried setting PrecisionGoal and AccuracyGoal to ``Infinity''. Finally, I've tried the different Methods in FindRoot, both ``Brent'' and ``Secant'', all with the same result.

Can anyone give me some advice?

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Subject (listing for 'FindRoot vs. FindMinimum')
Author Date Posted
FindRoot vs. FindMinimum Ben Dundee 08/21/09 2:57pm
Re: FindRoot vs. FindMinimum yehuda ben-s... 08/30/09 08:55am
Re: FindRoot vs. FindMinimum Ben Dundee 08/31/09 09:26am
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