       Re: (revision) NIntegrate that upper limit is infinite

• To: mathgroup at smc.vnet.net
• Subject: [mg71952] Re: [mg71930] (revision) NIntegrate that upper limit is infinite
• From: Daniel Lichtblau <danl at wolfram.com>
• Date: Tue, 5 Dec 2006 06:04:57 -0500 (EST)
• References: <200612041139.GAA02940@smc.vnet.net>

```Evanescence wrote:
> Dear all:
> I am very sorry for my article contain a lot of mistake before.
> So I check it and state my questions again.
> I am really sorry that cause a lot of confuse for everybody.
> My questions is as follows:
> First I definite a function that is:
> G[a_] := ((-(a -Cos[0.5])^(-3/2))/(3*(10)^2))*(Sin[0.5])^(2) +
> (1/(1-0.1*0.1))*((0.005*(1 + Cos[0.5]) + 0.001/1000)*(1 +
> Cos[0.5])^(1/2)*(ArcTan[(a - Cos[0.5])^(1/2)/(1 + Cos[0.5])^(1/2)] -
> Pi/2) + (0.005*(1 - Cos[0.5]) - 0.001/1000)*(1 -
> Cos[0.5])^(1/2)*(ArcTanh[(1 - Cos[0.5])^(1/2)/(a - Cos[0.5])^(1/2)]))
>
> another function is:
> P[a_]:=Re[N[LegendreP[(-1/2)+I,a]]]       where I=(-1)^(1/2)
>
> then
> NIntegrate[G[a]*P[a], {a, 1, Infinity}]
>
> but get error message is:
> NIntegrate's singularity handling has failed at point
> {a}={2.7013362243395366*(10^150)}for the specified
> precision goal.
> Try using larger values for any of \$MaxExtraPrecision
> or the options WorkingPrecision, or SingularityDepth and MaxRecursion.
>
> NIntegrate::"inum":
> "Integrand G[a]*P[a] is not numerical at {a}=
> {2.7013362243395366`*(10^150)}.
>
> Please solve my confuse ,thank you very much!
>
> By the way
> G[Infinity]*P[Infinity]=0 ï¿½A G is singular pointï¿½AP=1
> and I try Plot the graph for G[a]ï¿½AP[a]ï¿½Aand G[a]*P[a] ï¿½Afrom the
> graph of  G[a]*P[a]
> it should convergence for a-->Infinity.

I don't think this converges.

gg = (100*(ArcTanh[Sqrt[1 - Cos[1/2]]/Sqrt[a - Cos[1/2]]]*
(-1/1000000 + (1 - Cos[1/2])/200)*Sqrt[1 - Cos[1/2]] +
(-Pi/2 + ArcTan[Sqrt[a - Cos[1/2]]/Sqrt[1 + Cos[1/2]]])*
Sqrt[1 + Cos[1/2]]*(1/1000000 + (1 + Cos[1/2])/200)))/99 -
Sin[1/2]^2/(300*(a - Cos[1/2])^(3/2));

pp = LegendreP[(-1/2)+I,a];

Now we check the leading terms of the series expansion at infinity. We
ignore Re[]. Probably does not matter because it appears to be real valued.

Normal[Series[gg*pp, {a,Infinity,1}]]

((2^(1/2 + I)*(-1667/330000 +
(4999*Sqrt[-((-1 + E^(I/2))^2*(1 - Cos[1/2]))/(2*E^(I/2))])/990000 -
(10001*Cos[1/2])/990000 -
(Sqrt[-((-1 + E^(I/2))^2*(1 - Cos[1/2]))/(2*E^(I/2))]*Cos[1/2])/198 -
Cos[1/2]^2/198)*Gamma[-2*I])/(a*Gamma[1/2 - I]^2) +
(2^(1/2 - I)*(-1667/330000 +
(4999*Sqrt[-((-1 + E^(I/2))^2*(1 - Cos[1/2]))/(2*E^(I/2))])/990000 -
(10001*Cos[1/2])/990000 -
(Sqrt[-((-1 + E^(I/2))^2*(1 - Cos[1/2]))/(2*E^(I/2))]*Cos[1/2])/198 -
Cos[1/2]^2/198)*Gamma[2*I])/(a^(1 - 2*I)*Gamma[1/2 + I]^2))/a^I

This looks like it goes as 1/a in magnitude. There is an oscillatory
factor from imaginary powers of a, but too slow (I think) to allow for
convergence.

Daniel Lichtblau
Wolfram Research

```

• Prev by Date: Re: Re: Reduction of Radicals
• Next by Date: Re: Re: NETLink:
• Previous by thread: (revision) NIntegrate that upper limit is infinite
• Next by thread: Re: (revision) NIntegrate that upper limit is infinite