# Using numcases with cases

I have an equation like so:

   $\begin{cases} Eqn1 &= 0,\\ Eqn2 &= 0, \end{cases} \iff \begin{cases} Eqn3 &= 0,\\ Eqn4 &= 0,\\ Eqn5 &= 0. \end{cases}$


This works all nice and good, however, I would like to numerate the "right side" of those (in this case 3) equations. Naively, I thought I could use numcases for the second "block", but of course this is good as numcases for some reason cannot be between  or the-like. Is there a way to do this?

Patrick

• Where did you get numcases? It isn't in amsmath or mathtools. – John Kormylo Feb 23 '19 at 15:29

I'm not sure your readers will understand what the numbers refer to.

\documentclass{article}

\begin{document}

\begin{empheq}[
left=
{\begin{cases}
Eqn1 = 0,\\
Eqn2 = 0,
\end{cases}} \iff
\empheqlbrace
]{align}
Eqn3 &= 0,\\
Eqn4 &= 0,\\
Eqn5 &= 0.
\end{empheq}

\end{document}


The additional braces around \begin{cases}...\end{cases} are needed because there are commas in the code. Note that you shouldn't use &= inside cases (thanks to Bernard for noting). To disambiguate to which equalities the numbers refer to, one may use theadjustwidth environment, to have the numbers closer. Also, one can nest cases in the left-side argument of numcases:

\documentclass{article}
\usepackage{cases}
\usepackage{changepage}
\usepackage{showframe}
\renewcommand{\ShowFrameLinethickness}{0.3pt}

\begin{document}

\begin{numcases}
{\begin{cases}
Eqn1 = 0,\\
Eqn2 = 0,
\end{cases} \iff}
Eqn3 = 0,\\
Eqn4 = 0,\\
Eqn5 = 0.
\end{numcases}
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