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Re: inum error from NIntegrate?


This is not an isolated problem, it also occurs with some other functions,
for example Plot.
The following demonstrates the problem and its solution

lst= {x,x^2};
NIntegrate[{x,x^2}, {x,0,1}]

        {0.5,0.333333}

NIntegrate[lst, {x,0,1}]

        NIntegrate::inum: Integrand {0.5,0.25} is not numerical at {x} =
{0.5}.

        NIntegrate[lst,{x,0,1}]

What happens is that NIntegrate looks at   lst   before it is evaluated
(NIntgegrate has attribute HoldAll), finds that it is not a list and then
works on the assumption that   lst   will become numerical when x is
numerical:
this is false -- it becomes a *list* of two numbers.
The solution is to make   lst   evaluate before NIntegrates inspects its
integrand

NIntegrate[Evaluate[lst], {x,0,1}]

        {0.5,0.333333}

The problem does not arise with Integrate, which evaluates   lst   before
inspecting it.

Integrate[lst, {x,0,1}]

        {1/2, 1/3}

The same kind of problem arises with other functions that hold their
entries, for example Plot: compare

Plot[lst,{x,0,1}];

Plot[Evaluate[lst],{x,0,1}];

--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565

"Carl Gold" <cgold at cns.caltech.edu> wrote in message
news:9b8p5s$7fj at smc.vnet.net...
> Hi - has anyone gotten an error like this before:
>
> I am using NIntegrate and get
>
> NIntegrate[g[t1, t2], {t1, -5 , 5}, {t2, -5 , 5}]
> NIntegrate::"inum": "Integrand \!\({\({0.01831563888873418`}\)}\) is not
> \
> numerical at \!\({t1, t2}\) = \!\({0.`, 0.`}\)."
>
> But if I evaluate the function:
>
> g[0, 0]
> \!\({{1\/\[ExponentialE]\^4}}\)
>
> So the problem is not that the integrand doesn't have a value.
>
> The documentation on the inum error only discusses a case where you hae
> an undefined parameter in your equation, which is not my situation..
> Any ideas what could be causing this error?
>
> Thanks for your help if you have any!
>
> -Carl
>
>




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