I have a very simple question (I'm a beginner on mathematica, and didn't find the answer here). When I try to maximize a simple function like Maximize[x - 0.5*x^(4/3), x] I obtain an error message "the function value... is not a real number". When I replace "0.5" by "1/2" I get the correct anwer. I also get an answer with any integer, but never with a decimal number. Eg "2.5" doesnt work, but "5/2" works. What is the problem please ?
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2 Answers
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If you restrict x to be positive it works as expected:
NMaximize[{x - 0.5*x^(4/3), x > 0}, x]
(* {0.84375, {x -> 3.375}} *)
answered Mar 19, 2018 at 13:30
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$\begingroup$ Additionally, one might want to apply
Rationalize[]to the exponent. $\endgroup$ Commented Mar 19, 2018 at 14:50 -
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Maximize[{x - 0.5*x^(4/3), FunctionDomain[x - 0.5*x^(4/3), x]}, x]$\endgroup$ Commented Mar 19, 2018 at 15:15
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you can also do this:
Maximize[x - c*x^(4/3), x] /. c -> .5
{0.84375, {x -> 3.375}}
as a bit of explanation, when you give Maximize everything exact it uses analytic methods to find the value. As soon as you give it a float (inexact) value it switches to numerical methods which produce complex results for negative x.
answered Mar 19, 2018 at 15:15