Recall that in-line math is produced by using single dollar
signs, $ ... $, and displayed equations are produced
by using double dollar signs, $$ ... $$. To produce
a numbered equation, use the equation environment;
for example,
\begin{equation}
\chi(M_g) = 2-2g,
\end{equation}
produces
Note that the equation environment puts you in math
mode, so there is no need to use $$...$$ around the
\begin{equation} ... \end{equation}.
If you want to refer to a numbered equation, you can put a
\label{ } command between the \begin{equation}
and the \end{equation}. Then the key will be
associated with the number of the equation, and you can
reference the equation by typing, for example,
...as we see from (\ref{eqnkey}).
See Section 6.8 above for explanations of the
\label{ } and \ref{ } commands.