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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}

?
share|improve this question
    
I'm not sure why two equations to be considered simultaneously should receive a number each. –  egreg Oct 17 '12 at 15:22
    
Welcome to TeX.SE. –  Peter Grill Oct 17 '12 at 16:42
    
Related: Equation tags for parts of alignments. –  Hendrik Vogt Oct 18 '12 at 10:31
    
@egreg: Why did you delete your answer? It's a hack, but it does what the OP wants! –  Hendrik Vogt Oct 18 '12 at 11:33
    
@HendrikVogt I was waiting for someone else to answer –  egreg Oct 18 '12 at 16:41

1 Answer 1

\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

share|improve this answer
    
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 yesterday
    
@TroyWoo Look at the empheq package that has just what you're looking for. –  egreg yesterday
    
very powerful...thank you. –  Troy Woo yesterday

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