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Re: double integral problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112529] Re: double integral problem
  • From: Oliver Jennrich <oliver.jennrich at gmx.net>
  • Date: Sun, 19 Sep 2010 05:36:28 -0400 (EDT)
  • References: <i727k7$eup$1@smc.vnet.net>

Regina <regina.lagrasta at googlemail.com> writes:

> hi there,
> im trying to evaluate the following double integral:
> N[Integrate[Integrate[Exp[-a/(4 x) - 4 x], {x, 0, Q}], {Q, 0, 1}]]
>
> and set a to 1 or some other non-zero number.
>
> but for some reason, mathematica only gives me error messages. what am
> i doing wrong?
> best. regina

On  a very high level - you try to get a numerical value for an
expression that contains a symbolic value (a).

On a lower level- the inner integral already returns unevaluated, you
need to replace Q (and a) by a fixed value, evaluate the inner integral
and only then evaluate the outer:

f = Interpolation[
   Map[{#, NIntegrate[
        Exp[-a/(4 x) - 4 x] /. a -> 1, {x, 0, #}]} /. {a -> 1} &, 
    Range[0, 1, 0.01]]];

NIntegrate[f[Q], {Q, 0, 1}]



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