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Re: Trouble with Integrate

"Marko Vojinovic" <vojinovi at> wrote:
> Consider the function:
> f = Sqrt[1+x^4] -x^2
> Upon asking to
> Integrate[f,{x,0,Infinity}]
> Mathematica 4.0 answers:
> -Infinity
> which is not correct.

Alas, so does my Version for Windows.

> However,
> NIntegrate[f,{x,0,Infinity}]
> gives the correct (numerical) answer:
> 1.23605
> The correct (analytical, i.e.. exact) answer to the integral is:
> Gamma[1/4] Gamma[1/4] / 6 Sqrt[Pi]
> which can be obtained after some paperwork.

Using Integrate[f,{x,0,a}], I got an answer involving EllipticF. Its limit
as a -> Infinity is correct, although Mathematica cannot find that limit.

> However, if I ask
> Integrate[1/(Sqrt[1+x^4] + x^2),{x,0,Infinity}]
> (this integrand is equivalent to f) one gets a complicated answer in
> terms of EllipticF.

Strange. My later version doesn't give an answer, merely rewriting the
problem in 2D form.

> Meanwhile, when I ask Mathematica 3.0 the same set of
> questions, I get correct answers,

Aargh! Now I wish that I'd left version 3 on my computer!

Please report these problems to the proper authorities, if you haven't
already done so.


> and analytical integration gives answer
> in terms of Gamma. Two questions:
> 1) Why does version 4.0 give so fairly incorrect result "-Infinity" for
> the first integral?
> 2) How can I 'switch off' the use of elliptic functions and/or 'force'
> Mathematica to use Gamma?

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