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}] -- Space - The final frontier