Stacking symbols

LATEX allows you to set one symbol above another through the \stackrel command. This command accepts two arguments, and sets the first centrally above the second.

\begin{mathegs}
\verb@$X \stackrel{f^*}{\rightarrow}Y$@ & $X \stackrel{f^*}{\rig...
...l{\triangle}{=} x^2 + 1$@ & $f(x) \stackrel{\triangle}{=} x^2 + 1$
\end{mathegs}