Re: Integral no longer evaluated in Version 7, 8
- To: mathgroup at smc.vnet.net
- Subject: [mg114430] Re: Integral no longer evaluated in Version 7, 8
- From: Daniel Lichtblau <danl at wolfram.com>
- Date: Sat, 4 Dec 2010 06:13:27 -0500 (EST)
John Jowett wrote:
> Hello,
> With Mathematica Version 7, the integral
>
> Integrate[(x^2/2)*BesselK[5/3, x], {x, 0, Infinity}]
>
> correctly evaluated to (8*Pi)/(9*Sqrt[3]). In Mathematica 7 or 8, it
> gives the message
>
> Integrate::idiv: Integral of x^2 BesselK[5/3,x] does not converge on
> {0,\[Infinity]}. >>
>
> I haven't been able to find any way to get this to work (NIntegrate
> works fine). Termwise integration of the asymptotic form of the
> integrand works but does not appear to converge.
>
> Can anybody explain why Mathematica lost this capability? It may have
> something to do with no longer recognising cancellations among
> expressions involving the Gamma function. Any ideas for getting the
> integral to work ?
>
> Thanks,
> John Jowett
It's a known bug, caused by a bad series expansion at infinity for the
antiderivative of that integrand.
i1 = (x^2/2)*BesselK[5/3, x];
i2 = Integrate[i1, x];
i3 = Normal[Series[i2, {x, Infinity, 3}]];
Now compare plots (the first is to show that it very likely is
convergent based on integrand behavior).
Plot[i1, {x, 2, 20}]
Plot[i2, {x, 2, 20}]
Plot[i3, {x, 2, 20}]
Daniel Lichtblau
Wolfram Research