What is the Fourier transform of $f(x)=e^{-x^2}$? I remember there is a special rule for this kind of function, but I can't remember what it was. 
Does anyone know?

Caveat: I'm using the normalization $\hat f(\omega) = \int_{-\infty}^\infty f(t)e^{-it\omega}\,dt$.
A cute way to to derive the Fourier transform of $f(t) = e^{-t^2}$ is the following trick: Since $$f'(t) = -2te^{-t^2} = -2tf(t),$$
taking the Fourier transfom of both sides will give us
$$i\omega \hat f(\omega) = -2i\hat f'(\omega).$$
Solving this differential equation for $\hat f$ yields
$$\hat f(\omega) = Ce^{-\omega^2/4}$$
and plugging in $\omega = 0$ finally gives
$$ C = \hat f(0) = \int_{-\infty}^\infty e^{-t^2}\,dt = \sqrt{\pi}.$$
I.e. $$ \hat f(\omega) = \sqrt{\pi}e^{-\omega^2/4}.$$
