Sometimes you need to place text above arrows. Let's say you need to write transitions for a labelled transition system. In fact in LaTeX this is not so obvious, especially if you want to have text above and under the arrow. I know of two easy ways to do it.
$$ E \xrightarrow{\alpha} F $$
The second method.
$$ \mathop{\longrightarrow}^_{\alpha} $$