Using numcases with cases

Question

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

Answer 1

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

\documentclass{article}
\usepackage{empheq} % also loads amsmath

\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).

Answer 2

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{adjustwidth}{6em}{6em}
\begin{numcases}
    {\begin{cases}
    Eqn1 = 0,\\
    Eqn2 = 0,
    \end{cases} \iff}
  Eqn3 = 0,\\
  Eqn4 = 0,\\
  Eqn5 = 0.
\end{numcases}
\end{adjustwidth}

\end{document}