How can I label only one line of an equation array?

Question

Consider the following commands to create a labelled multi-line equation array:

\begin{eqnarray}
\label{eqn:1}
X&=&Y+Z\\
&\leq & U \times W.

\end{eqnarray}

It creates a two-line equations with two numbers. How can I associate only one equation number to this two-line equations? or better, how can I associated different equation numbers to each line? (of course, without breaking the equation array into several disjoint equations!)

I have the exact same question. Have to use \eqnarray because that's required by the journal template. — Drumy, Apr 22 '20 at 09:01

score 21 · Accepted Answer · edited Jun 11 '17 at 11:52

21

amsmath provides a similar interface to eqnarray, but is better in terms of it's horizontal spacing/alignment:

enter image description here

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align}
  X &= Y + Z \label{eqn:1} \\
    &\leq U \times W. \nonumber \\
    & = Y + Z \tag{a}
\end{align}
\end{document}

\nonumber removes the number for that line in the align environment. \tag can also be used to create a manual tag.

edited Jun 11 '17 at 11:52

Moriambar

11,466

answered Sep 29 '13 at 06:10

Werner

603,163

1

Though I never use it myself. I still forget when answering questions on this site that it's recommended not to use eqnarray. – A.Ellett Sep 29 '13 at 06:18
2

\tag*{a} for labels without parentheses – Sep 29 '13 at 06:33

score 7 · Answer 2 · edited Jun 16 '17 at 06:51

if only one equation number is wanted for a group, the split environment from amsmath is a reasonable candidate; it must be "wrapped" in an equation environment for the equation number to appear.

if all the equations in a group are to be numbered, then (as pointed out in other answers) the align environment is more suitable.

\begin{equation}
\begin{split}
 X &=Y+Z\\
   &\leq U \times W
\end{split}
 \label{eqn:1}
\end{equation}

\begin{align}
 X &=Y+Z \label{eqn:2}\\
   &\leq U \times W \label{eqn:3}
\end{align}

output of example code

score 2 · Answer 3 · edited Jun 16 '17 at 06:51

2

Use \notag:

\begin{eqnarray}
\label{eqn:1}
X&=&Y+Z\\
\notag
&\leq & U \times W.    
\end{eqnarray}

But, as @werner reminds us, it is not advised to use eqnarray, align does a better job:

\begin{align}
\label{eqn:1}
     X & = Y + Z \\
\notag & \leq  U \times W.    
\end{align}

This requires that you load the package amsmath to have access to \notag

Math environments in LaTeX are not happy with empty lines. If you want each line to have a number, just don't use \notag.

See How to get only one vertically centered equation number in align environment with two equations for how to get one equation number for multiple lines.

edited Jun 16 '17 at 06:51

David Carlisle

757,742

answered Sep 29 '13 at 05:47

A.Ellett

50,533

Thanks for your answer. How can I label each line individually? – Sep 29 '13 at 05:50
2

@VahidShirbisheh -- instead of \notag, put a \label on the second line. each line can be individually labeled for later reference. – barbara beeton Sep 29 '13 at 13:24

How can I label only one line of an equation array?

3 Answers3

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