## ganeshie8 one year ago Evaluate $\large \int\limits_0^{\infty} \dfrac{\cos x}{x^2+1}\,dx$

1. Astrophysics

Evaluate $\large \int\limits\limits_0^{\infty} \dfrac{\cos x}{x^2+1}\,dx$

2. ganeshie8

I found a nice solution few days back while attempting SithsAndGiggle's latest problem It's really nice, doesn't use complex analysis/series...

3. Astrophysics

Lol I was thinking series

4. ganeshie8

But the one I have is rather lenthy I have posted this here to see alternative ways... series might work nicely too..

5. Jhannybean

Can it be like a power series?

6. Empty

This looks fun and interesting, I really wanna work on this one and find a pretty answer

7. ganeshie8

it sure feels like multiple approaches are possible...

8. Empty

Well it's less than $$\pi/2$$ I guess that's the best guess, maybe like half of that, like $$\pi/4$$ would be a fun guess.

9. ganeshie8

$$|\cos x|\le 1$$ so replacing $$\cos x$$ by $$1$$ is it ?

10. ganeshie8

thats not too crude it seems, http://www.wolframalpha.com/input/?i=%5Cint%5Climits_0%5E%7B%5Cinfty%7D+cos%28x%29%2F%28x%5E2%2B1%29

11. Empty

Yeah, it's just a guess though haha

12. Empty

Oh wow, that e is unexpected, but $$e \approx 2$$ hahaha

13. Astrophysics

Can't you use Euler's equation otherwise this seems tedious xD

14. Empty

I'm trying to see some kind of way to do differentiation under the integral sign or somehow back out into a double integral and flip the order of integration but I can't really think of anything.

15. Empty

For what it's worth maybe someone can do something with this: $I(y) = \int_0^\infty \frac{ \arctan(xy)\cos x}{x} dx$ $I'(1) = \int_0^\infty \frac{\cos x dx}{1+x^2}$ I think this is true, not completely sure.

16. Owlcoffee

$\int\limits_{0}^{\infty}\frac{ cosx }{ x^2+1 }dx$ What you can do, is assign a constant that tends to infinity on the superior number of the integral, this way, you can create a finite integral, solvable with Barrows rule: $\lim_{b \rightarrow \infty}\int\limits_{0}^{b}\frac{ cosx }{ x^2+1 }dx$

17. Jhannybean

$\int_0^{\infty}\frac{1}{x^2+1}dx \cdot \int_0^{\infty}\cos(x)dx$ Would this make it more complicated? I have feeling Kai already wrote this in a different form...lol.

18. IrishBoy123

$\mathcal F \left\{ \frac{1}{t^2+1} = \pi \ e^{-2 \pi |f|}\right\}$ this is an even function, $$\hat f = \hat f_c$$ but we only want $$\hat f_c/2$$ so that's $\frac{1}{2} \pi \ e^{(-2 \ \pi (\frac{1}{2 \pi})) }$ pretty crap way to do it, eh?!

19. IrishBoy123

oops $\mathcal F \left\{ \frac{1}{t^2+1} \right\} = \pi \ e^{-2 \pi |f|}$

20. anonymous

We can use l'hopital's rule , it ll be much quicker right ?

21. anonymous

u=x^2+1 du=2xdx dx=du/2x x=sqrt(u-1) integral (cos x)/x . 1/2u du hmmm how this is related to cosine integral

22. Jhannybean

Ohhhh... integration by parts :P