# Separate labels in cases

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


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

-

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:

\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

Using only the amsmath package and no additional packages, it is possible with some box manipulation:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\newsavebox{\mycases}% Store case "title" and brace
\begin{align}
\sbox{\mycases}{$\displaystyle f(x)=\left\{\begin{array}{@{}c@{}}\vphantom{1\ x\geq0}\\\vphantom{0\ x<0}\end{array}\right.\kern-\nulldelimiterspace$}
\end{align}
\end{document}

-

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

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}