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


I have the following block of TeX:

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 \\

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:

enter image description here

\usepackage{amsmath}% http://ctan.org/pkg/amsmath
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 

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}

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.

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