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How to right-side annotate a bracketed-together set of items in the align environment, cases sub-environment?

E.g. how to put an annotation such as "(Fermat)" where the following code would put a number for the equation pair:

\begin{align}
\begin{cases}
first equation\\
second equation\\
\end{cases}
 \end{align}

? (I realise to get rid of the number, I have to change align to align*)

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2  
Please consider to upvote answers you find useful. –  lockstep Jan 21 '12 at 18:27
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3 Answers

up vote 3 down vote accepted

if you're using amsmath, don't bother adding *; instead, put \tag{Fermat} just before \end{align} and that will be placed where the equation number usually goes.

actually, you should just use equation instead of align since cases is a single entity.

you also don't need (or want) the second \\ before \end{cases}.

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Thanks. tag{Fermat} before \end{align} - shortest solution of all so far! :) –  Harry Macpherson Jan 21 '12 at 18:26
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You could be use rcases from the mathtools package, or you could use the \left. \right\} with amsmath (already included by the mathtools package) to obtain the curly brace on the right, and use \tag to add the annotation:

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{align}
\begin{rcases}
 first equation\\ \tag{Fermat}
 second equation\\
\end{rcases}
\end{align}
%
\begin{align}
\left.
\begin{aligned}
  first equation\\ 
  second equation\\
\end{aligned}
\right\} \tag{Fermat}
\end{align}
\end{document}
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Thanks for this, but where should I put the annotation? –  Harry Macpherson Jan 21 '12 at 18:17
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Automatic number of any line in a math environment can be overridden by a \tag command:

\begin{equation}
\label{eq:fermat}\tag{Fermat}
\begin{cases}
first equation\\
second equation
\end{cases}
\end{equation}

In this way the tag will go in place of the equation number. If you want instead to add also the annotation, which might be even better,

\begin{equation}
\label{eq:fermat}
\begin{cases}
first equation\\
second equation
\end{cases}
\qquad\text{(Fermat)}
\end{equation}

Don't use align when you're not doing an alignment (the surrounding spacing is excessive) and don't use a final \\ in amsmath environments (that causes an erroneous vertical space).

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1  
with amsmath, don't need \text with \tag since it's not in math mode. –  barbara beeton Jan 21 '12 at 18:21
    
Thanks! (I am doing an alignment, but dropped it for this minimal example). I've now used \label{eq:fermat}\tag{Fermat}. Interestingly it works even with align*; the * must take out the number but not what replaces it. Thanks for the `\` tip too. –  Harry Macpherson Jan 21 '12 at 18:22
    
Yes, \tag works also in *-environments. With \eqref{eq:fermat} you'll get the tag, of course, that is, (Fermat). –  egreg Jan 21 '12 at 18:24
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