nested equations numbering

Question

Is there a way of playing with align, alignat, numcases etc environments that produces aligned and numbered equations which look like this :

Blahblah text before equations :

      A = B  \iff  C = D             (1)

                   { C_1 = D_1       (2)
             \iff  {
                   { C_2 = D_2       (3)

             \iff  E = F             (4)

Blahblah text after equations

Any suggestion greatly appreciated.

it looks like the numcases environment from the cases package would do the job. but it seems not allowed inside an align environment. the following minimal example won't compile. with :

\documentclass{article}
\usepackage{amsmath,cases}
\begin{document}
\begin{align}
A = B &\iff C=D\\
&\iff 
 \begin{numcases}{}
    C_1 = D_1\\
    C_2 = D_2
  \end{numcases}\\
&\iff E=F
\end{align}
\end{document}

i get :

(/usr/share/texmf-dist/tex/latex/base/article.cls
Document Class: article 2007/10/19 v1.4h Standard LaTeX document class
(/usr/share/texmf-dist/tex/latex/base/size10.clo))
(/usr/share/texmf-dist/tex/latex/amsmath/amsmath.sty
For additional information on amsmath, use the `?' option.
(/usr/share/texmf-dist/tex/latex/amsmath/amstext.sty
(/usr/share/texmf-dist/tex/latex/amsmath/amsgen.sty))
(/usr/share/texmf-dist/tex/latex/amsmath/amsbsy.sty)
(/usr/share/texmf-dist/tex/latex/amsmath/amsopn.sty))
(/usr/share/texmf-dist/tex/latex/cases/cases.sty) (./attempt.aux)
! Missing \endgroup inserted.
<inserted text> 
                \endgroup 
l.12 \end{align}

?

I'm not sure why two equations to be considered simultaneously should receive a number each. — egreg, Oct 17 '12 at 15:22

score 10 · Answer 1 · edited Jun 09 '17 at 17:32

10

\documentclass[reqno]{amsart}
\begin{document}
\begin{align}
A = B &\iff C = D \\
&\makebox[0pt][l]{\smash{\raisebox{-.5\baselineskip}{${}\iff\!\left\{\rule{0pt}{1.1\baselineskip}\right.$}}}
 \hphantom{{}\iff{}}\;\;E=F
\\
&\hphantom{{}\iff{}}\;\;G=H\\
&\iff I=J
\end{align}
\end{document}

enter image description here

edited Jun 09 '17 at 17:32

David Carlisle

757,742

answered Oct 17 '12 at 15:36

egreg

1,121,712

This is just too complicated back then. Now after three years, is there a new solution? If I only have equation (2)(3) as in the above and I want: the left bracket, the align, the numbering? – Troy Woo Jan 28 '15 at 18:58
2

@TroyWoo Look at the empheq package that has just what you're looking for. – egreg Jan 28 '15 at 21:54

nested equations numbering

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