# What formatting should I use on 'd' when writing an integral? [duplicate]

Possible Duplicate:
What’s the proper way to typeset a differential operator?

What formatting should I use on 'd' when writing an integral?

I normally use

$$\int_{-\infty}^{infty} e^{-x^2} \mathoperator{d}\!x = \pi$$


which produces

$\int_{-\infty}^{infty} e^{-x^2} dx = \pi$


Is there a better formatting for dx?