Post facts that make mathfags seethe.

I'll start

Let $\delta : \mathbb{R} \rightarrow \mathbb{R}$ such that:
$$\delta (x) =<br /> \begin{cases}<br /> 1 &amp; x = 0 \\<br /> 0 &amp; elsewhere<br /> \end{cases}$$

And most importantly:

$$\int\limits_{- \infty}^{+ \infty} \delta (x) dx = 1[/math]$$