# Homework Help: The Alternate form of the Dirac Delta Function

1. Sep 29, 2011

### WilcoRogers

Hello,

I am trying to show that:
$\delta(x) = \lim_{\epsilon \to 0} \frac{\sin(\frac{x}{\epsilon})}{\pi x}$
Is a viable representation of the dirac delta function. More specifically, it has to satisfy:
$\int_{-\infty}^{\infty} \delta(x) f(x) dx = f(0)$

I know that the integral of sin(x)/x over the reals is $\pi$, and so far as I can tell, it doesn't depend on epsilon. What I've tried so far is integration by parts, which leads me to:
$f(x) - \int_{-\infty}^{\infty} f'(x) dx$

Which isn't really getting me somewhere, and the limit drops off due to the integral not caring what epsilon is. Is there another way of approaching this? Or am I on the right track, I just can't pull out an f(0) from this.

Any help is appreciated,

Thanks.

2. Sep 29, 2011

### obafgkmrns

Try substituting z = x/epsilon.

3. Sep 29, 2011

### vela

Staff Emeritus
Are you familiar with complex analysis and the residue theorem?