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Re: problems with NMinimize

  • To: mathgroup at smc.vnet.net
  • Subject: [mg109376] Re: problems with NMinimize
  • From: Ingolf Dahl <ingolf.dahl at physics.gu.se>
  • Date: Sat, 24 Apr 2010 04:01:07 -0400 (EDT)

Is there any serious Mathematica user in this forum, which has not stumbled
on this problem one or several times, when NMinimize, or some other
function, tries to evaluate a numeric function symbolically first, before
inserting the numbers? Sometimes it would be nice to have an option,
SymbolicEvaluation->False, which could be set for NMinimize in these cases.
It is not always so convenient to have to define an extra function just to
take care of this. I think there is a whole group of commands acting
similarly to NMinimize.

Ingolf Dahl
Sweden



-----Original Message-----
From: Bob Hanlon [mailto:hanlonr at cox.net] 
Sent: den 23 april 2010 09:51
To: mathgroup at smc.vnet.net
Subject: [mg109376] [mg109370] Re: problems with NMinimize


Define a function that only evaluates for numeric arguments

f[a_?NumericQ, b_?NumericQ] :=
 Abs[NIntegrate[Sin[x], {x, -a, b}]]^2

NMinimize[f[a, b], {a, b}]

{7.56317*10^-19,{a->0.131888,b->-0.131888}}


Bob Hanlon

---- Neil Broderick <ngb at ecs.soton.ac.uk> wrote: 

=============
Hi,
I am trying to use NMinimize to find the solutions to various numerical
equations and I keep getting error
messages concerning non-numerical values. For example consider the
following:

In[2]:= NMinimize[Abs[NIntegrate[Sin[x], {x, -a, b}]]^2, {a, b}]

During evaluation of In[2]:= NIntegrate::nlim: x = -1. a is not a valid
limit of integration. >>

During evaluation of In[2]:= NIntegrate::nlim: x = -1. a is not a valid
limit of integration. >>

During evaluation of In[2]:= NIntegrate::nlim: x = -1. a is not a valid
limit of integration. >>

During evaluation of In[2]:= General::stop: Further output of
NIntegrate::nlim will be suppressed during this calculation. >>

Out[2]= {5.08978*10^-24, {a -> 0.0994414, b -> 0.0994414}}

My actual problem involves taking Fourier transforms of lists of numbers but
you get the picture. Why is
NMinimize putting variables into the function rather than just numbers and
is the likely to cause a problem
in some cases?

regards,
Neil=





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