# Residue Theorem

• Apr 18th 2009, 11:56 AM
Residue Theorem
I cant figure out where the singularities are on this Q!

Q: Use a large semicircular contour and the residue theorem to evaluate:
$\displaystyle \int_{-\infty}^{\infty} \frac{x^2}{x^4 + 1} dx$.

Lets call the integral above I.

Then I = $\displaystyle 2 \pi i \sum$ residues of $\displaystyle \frac{z^2}{z^4 + 1}$.

Then when i find these residues i sub them into $\displaystyle \frac{z^2}{4z^3} = \frac{1}{4z}$ to get the residues but i cant figure out what z should be. It SHOULD be $\displaystyle e^{\frac{\pi i}{4}}$ and $\displaystyle e^{\frac{3 \pi i}{4}}$ but i have no idea how to get those. I got $\displaystyle \sqrt{i}$ and $\displaystyle i \sqrt{i}$. Are those equivalent?!?
• Apr 18th 2009, 12:24 PM
Opalg
If $\displaystyle z^4+1=0$ then $\displaystyle z^4=-1$. So you are looking for fourth roots of –1. How do you find them? (Answer: De Moivre's theorem. Write –1 as $\displaystyle e^{i\pi}$ and raise it to the power 1/4.)