Authors will probably not cross-reference every equation in text. When a
numbered equation needs to be referred to in text by its number, the
\label{#1} and \ref{#1} commands should be used. The
\label{#1} command is used within the equation or the eqnarray line
to be referenced:
input:
\begin{equation}
A=B \label{pauli}
\end{equation}
... It follows from Eq.\ (\ref{pauli})
that this is the case ...
output:
... It follows from Eq. (1) that this is the case ...
Please note the parentheses surrounding the command. They are necessary for
proper output. You can also label individual lines in an eqnarray. Numbers
produced with \eqnum{#1} can also be cross-referenced: just follow
the \eqnum{#1} command with a \label{#1} command.
Using a \label{#1} after \begin{mathletters} will allow you
to reference the general number of the equations in the
mathletters environment. For example, if
\begin{mathletters}
\label{allequations} % notice location
\begin{equation}
E=mc^2,\label{equationa}
\end{equation}
\begin{equation}
E=mc^2,\label{equationb}
\end{equation}
\begin{equation}
E=mc^2,\label{equationc}
\end{equation}
\end{mathletters}
gives the output
then Eq.\ (\ref{allequations}) gives ``Eq. (2)''.
Note: incorrect cross-referencing will result if \label{#1} is
used in an unnumbered single line equation (i.e., within the \[ and
\] commands), or if \label{#1} is used on a line of an
eqnarray that is not being numbered (i.e., a line that has a
\nonumber).
Please see Sec. vi for further information about cross-referencing.
