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Question

有没有办法使用align、alignat等numcases环境来产生如下所示的对齐和编号的方程式：

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

非常感谢任何建议。

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看起来 cases 包中的 numcases 环境可以完成这项工作。但它似乎不允许在 align 环境中使用。以下最小示例无法编译。使用：

\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}

我得到：

(/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}

?

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}