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As much as I have tried, it doesn't seem possible to use separate labels when using a casesconstruction (from the amsmath package).

Ideally, I would like to be able to write something like

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
f(x)=\begin{cases}
  1&x\geq0\label{positive}\\
  0&x<0\label{negative}
\end{cases}
\end{equation}
\end{document}

and later refer to both cases at different places. What would be the best way to achieve this result?

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4 Answers 4

up vote 22 down vote accepted

You can use numcases from the cases package if you want each case labelled as a separate equation. There is also the subnumcases environment which gives you an alternative labelling:

enter image description here

\documentclass{standalone}
\usepackage{amsmath}
\usepackage{cases}
\begin{document}
\begin{numcases}{f(x)=}
1&$x\geq0$\label{positive}\\
0&$x<0$\label{negative}
\end{numcases}

See the second case \ref{negative} or the first \ref{positive}

\begin{subnumcases}{f(x)=}
1&$x\geq0$\label{positive-subnum}\\
0&$x<0$\label{negative-subnum}
\end{subnumcases}

See the second case \ref{negative-subnum} or the first \ref{positive-subnum}
\end{document}
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Exactly what I needed! Worth mentioning is the subnum package option that turns every numcases environment into a subnumcases one. –  FelixCQ Oct 19 '11 at 6:23

You can do this by the empheq package from the mh bundle.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{empheq}   % loads »mathtools«, which in turn loads »amsmath«

\begin{document}
  \begin{empheq}[left={f(x)=\empheqlbrace}]{align}
    1 &\quad x\geq0\label{positive}\\
    0 &\quad x<0\label{negative}
  \end{empheq}
\end{document}
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Using only the amsmath package and no additional packages, it is possible with some box manipulation:

enter image description here

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\newsavebox{\mycases}% Store case "title" and brace
\begin{align}
  \sbox{\mycases}{$\displaystyle f(x)=\left\{\begin{array}{@{}c@{}}\vphantom{1\ x\geq0}\\\vphantom{0\ x<0}\end{array}\right.\kern-\nulldelimiterspace$}
  \raisebox{-.5\ht\mycases}[0pt][0pt]{\usebox{\mycases}}1 \quad x\geq0\label{positive} \\
     0 \quad x<0\label{negative}
\end{align}
\end{document}
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You can combine the preceding solution with the subequations environment if you refer to the equations as a group. If you want to refer to the group, you have to introduce a \label just before entering the align environment. Here it is in action:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{heuristica}

\usepackage[overload]{empheq}
\usepackage{cleveref} 

 \begin{document}

\begin{subequations}
\label{Phi:def}
  \begin{align}[left ={Φ(A_n) = \empheqlbrace}]
    & \max \left [ L_n-A_n, 0 \right]\\
    & \max \left [ L_n-A_n, K \right]\label{eq:PayoffStopLosses}\\
    & \max \left [ L_n-A_n, K_1 \right] -\max \left [ A_n-L_n, K_2 \right]
   \end{align}
  \end{subequations}

As we can see in \cref{Phi:def}, and more specifically in \cref{eq:PayoffStopLosses},…

    \end{document} 

enter image description here

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useful concept. please show what references (\eqref) would look like, for the group as a whole, and for one or more of the subequations. –  barbara beeton May 28 at 18:55

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