# Integral and limit

During my research this integral has shown up

$\frac{1}{2T} \int_{-T}^T \left( 1 - \frac{|\tau|}{T}\right)e^{-\alpha\tau^2}\cos(2\pi f_0 \tau) d\tau$

I tried to solved by taking the real part of a the complex exponential but it didn't work. Any help?

Cheers,

Mikitov

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already tried CAS ? – Luis H Gallardo Jan 6 '11 at 17:39
I don't think this integral can be expressed in elementary functions (which is why by the way this doesn't look like homework to me); however it is not too difficult to compute it in terms of the error function (see e.g. en.wikipedia.org/wiki/Error_function). If you would like more details on that, math.stackexchange.com is probably a better place to ask. – algori Jan 6 '11 at 17:42
Yes, no homework in my life anymore... – mikitov Jan 6 '11 at 21:22
What exactly do you want with it? It is just the convolution (up to normalization and linear change of variable) of $e^{-x^2}$ and the Fejer kernel. Of course, there is no algebraic formula (unless you consider integration by parts a.k.a. "expressing in terms of error function" a great step forward) but all reasonable questions shouldn't be hard to answer :). – fedja Jan 7 '11 at 4:20

The limit is 0, the integral from $0$ to $T$ is: $$\frac{i \sqrt{\pi } e^{-\frac{\pi ^2 f^2}{a}} \left(\text{erfi}\left(\frac{\pi f-i a t}{\sqrt{a}}\right)-\text{erfi}\left(\frac{\pi f+i a t}{\sqrt{a}}\right)\right)}{4 a^{1/2}}$$ $$-\frac{\pi ^{3/2} f e^{-\frac{\pi ^2 f^2}{a}} \text{erfi}\left(\frac{\pi f-i a t}{\sqrt{a}}\right)+\pi ^{3/2} f e^{-\frac{\pi ^2 f^2}{a}} \text{erfi}\left(\frac{\pi f+i a t}{\sqrt{a}}\right)-2 \pi ^{3/2} f e^{-\frac{\pi ^2 f^2}{a}} \text{erfi}\left(\frac{\pi f}{\sqrt{a}}\right)}{4ta^{3/2}}$$ $$+\frac{\left(1+e^{4 i \pi f t}\right) e^{-t (a t+2 i \pi f)}-2}{4ta},$$ as per Mathematica.
$\lim_{T \to \infty}$ ? It does not make any sense to me for any value of alpha. – mikitov Jan 6 '11 at 21:39
@Simon: I just pasted mathematica's TeXForm. Thank god WZ was there to fix it... @mikitov: The integral converges (for any positive alpha), so when you divide by $T$ it is not at all surprising that the limit is $0.$ – Igor Rivin Jan 6 '11 at 23:21