# [Tex/LaTex] Label one line in align* environment

aligncross-referencing

I have the following block of TeX:

\begin{align*}
m_n^+ = \dfrac{1}{n}\sum\limits_{k=N+1}^{n} =& \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(a_k-L+L\right) \\
<& \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(\dfrac{\epsilon}{2}+L\right) \\
<& \dfrac{1}{n}\sum\limits_{k=1}^{n}\left(\dfrac{\epsilon}{2}+L\right) = \dfrac{\epsilon}{2}+L \\
\end{align*}


How could I set a label for the last line in this block so that I may reference it later?

#### Best Answer

You can either \tag the line in an (unnumbered) align* or \nonumber each equation you don't want in a (numbered) align. Here's a snapshot of both:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align*}
m_n^+ = \dfrac{1}{n}\sum\limits_{k=N+1}^{n} &= \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(a_k-L+L\right) \\
&< \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(\dfrac{\epsilon}{2}+L\right) \\
&< \dfrac{1}{n}\sum\limits_{k=1}^{n}\left(\dfrac{\epsilon}{2}+L\right) = \dfrac{\epsilon}{2}+L
\stepcounter{equation}\tag{\theequation}\label{myeq1}\\
\end{align*}
See~\eqref{myeq1}.

\begin{align}
m_n^+ = \dfrac{1}{n}\sum\limits_{k=N+1}^{n} &= \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(a_k-L+L\right) \nonumber \\
&< \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(\dfrac{\epsilon}{2}+L\right) \nonumber \\
&< \dfrac{1}{n}\sum\limits_{k=1}^{n}\left(\dfrac{\epsilon}{2}+L\right) = \dfrac{\epsilon}{2}+L \label{myeq2}
\end{align}
See~\eqref{myeq2}.
\end{document}


The approach depends on the ease of use, or perhaps the size (number/quantity) of the set of the equations.

The former \tag-method uses the same counter (equation), although it requires manually stepping. In both instances, \label works as usual to capture the number for later use through \eqref.