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.