2
$\begingroup$

I have a couple questions that I'd like answered:

(1) How do I evaluate this integral?

(2) Is it possible to do a substitution? What about $u=e^{i \cdot t}$?

(3) What if 5 and 3 are replaced by other constant integers?

(4) What should be the most effective method to handle question (3), using the fewest calculations (like in numerical methods)? In other words, what is the most effecient way to handle the general case? (Note that it doesn't have to be a numerical method; I just want something fast!)

  • 0
    Say that $y$ and $z$ are naturals, to help.2010-11-29

1 Answers 1

3

This is another contour integral. Replace $y$ by $a$ and $z$ by $b$ to simplify notation. Then your integral is $-i \int_{-\pi}^{\pi}{\cos(ae^{5it} + be^{3it}) \over e^{it}} ie^{it}\,dt$ $= -i \int_{|z| = 1}{ \cos(az^5 + bz^3) \over z}\,dz$ By the residue theorem this integral is $2\pi$ times the residue of $\cos(az^5 + bz^3)$ at $z = 0$. Since $\cos(az^5 + bz^3) = 1$ when $z = 0$, this residue is $1$. So your overall integral is $2\pi$.

Note that it doesn't matter what $a$ or $b$ are, and that $3$ and $5$ can be replaced by any nonnegative integers.

  • 0
    @Zaricuse: Wow! Thanks! This really opens up some doors for me. I may have some similar questions in the future, as I try to explore some ideas relating to this.2010-11-30