       Re: How to deal with this integral

• To: mathgroup at smc.vnet.net
• Subject: [mg97837] Re: How to deal with this integral
• From: m.r at inbox.ru
• Date: Mon, 23 Mar 2009 04:03:59 -0500 (EST)
• References: <gq2eva\$edb\$1@smc.vnet.net>

```paperkite rainyday wrote:
> hi everybody,
>
> I want to deal with this integral with Mathematica:
>
> Integrate[a*b*Exp[-(a+b)]*DiracDelta[1/(4*a)+1/(4*b)+c-t],{a,0,Infinity},{b,0,Infinity},{c,0,Infinity},Assumptions->t>0],
>
> Mathematica 6.0 gives 0. But it is wrong, the correct result is
> (1/2)*x^2*Exp[-x]*((1+x)*BesselK[0,x]+(2+x+2/x)*BesselK[1,x]),
> where x=1/(2*t).
>
> What's wrong?
>
> Thanks,
>
>
> paperkite

It's not as fun when you know the answer, but you can evaluate the
integral by making the change of variables u == a + b, v == a b. The
(u, v) region is given by

In:= Reduce[t > 0 && b > a > 0 && 1/a + 1/b < 4 t && a + b == u &&
a b == v,
{u, v}, {a, b}, Reals]

Out= t > 0 && u > 1/t && u/(4 t) < v < u^2/4

(we're integrating over b > a, where the substitution function is one-
to-one).

In:= Assuming[t > 0, 2 Integrate[v E^-u/Sqrt[u^2 - 4 v],
{u, 1/t, Infinity}, {v, u/(4 t), u^2/4}]] /.
HoldPattern@ MeijerG[{{}, {a_}}, {{b_, c_}, {}}, z_] :>
z^b E^-z HypergeometricU[a - c, b - c + 1, z] // FullSimplify

Out= (E^(-(1/(2 t))) (BesselK[1, 1/(2 t)] + (1 + 2 t) BesselK[2, 1/
(2 t)]))/(16 t^3)

Maxim Rytin
m.r at inbox.ru

```

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