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Is there a variant of split environment from amsmath package which would allow for two aligned equations in a row? I want to format two pairs of Maxwell equations in two rows and put single equation number between the rows. Unfortunetely, split allows only one alignment tab & in a row, and I need three as in the following example:

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\Div}{div}
\DeclareMathOperator{\Rot}{rot}
\begin{document}
\begin{align}
    \label{18.1:1}
    %\begin{split}
        \Rot\vec{E} &=-\frac{1}{c}\parder{\vec{B}}{t}
        ,
        &
        \Div\vec{B} &=0,
        \\
    \label{18.1:2}
        \Rot\vec{B} &=\frac{1}{c}\parder{\vec{E}}{t}
        +\frac{4\pi}{c}\,\vec{j}
        ,
        &
        \Div\vec{E} &=4\pi\rho_{\varepsilon }
    %\end{split}
    \end{align}
\end{document}

Here every row has its own number.

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1  
Your code is not compilable: \parder is nowhere defined. Also, you may want to use a more descriptive name for your labels, and it's best to precede punctuation marks such commas and full stops by a \,. –  Jubobs Apr 21 '13 at 10:02
1  
The \parder is defined as: \newcommand{\parder}[2]{\frac{\partial {#1}}{\partial {#2}}}. –  m0nhawk Apr 21 '13 at 10:18
    
@m0nhawk, thank you, indeed, i've missed the definition of parder. –  Igor Kotelnikov Apr 22 '13 at 6:25
    
@Jubobs, Sorry, I didn't ask how to format punctuation marks or how to choose names for labels. –  Igor Kotelnikov Apr 23 '13 at 4:03
    
Sorry, I didn't mean to offend you. That was just a side note that I thought could be useful. –  Jubobs Apr 23 '13 at 8:27
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2 Answers

up vote 12 down vote accepted

I think you are after aligned:

Sample output

\documentclass{article}

\usepackage{amsmath}

\DeclareMathOperator{\Div}{div}
\DeclareMathOperator{\Rot}{rot}
\newcommand{\parder}[2]{\frac{\partial {#1}}{\partial {#2}}}

\begin{document}

\begin{align}
  \label{18.1:1}
  \begin{aligned}
    \Rot\vec{E} &=-\frac{1}{c}\parder{\vec{B}}{t},&
    \Div\vec{B} &=0,
    \\
    \Rot\vec{B} &=\frac{1}{c}\parder{\vec{E}}{t}
    +\frac{4\pi}{c}\,\vec{j},&
    \Div\vec{E} &=4\pi\rho_{\varepsilon}.
  \end{aligned}
\end{align}

\end{document}
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Just use split:

\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator{\Div}{div}
\DeclareMathOperator{\Rot}{rot}

\newcommand\parder[2]{\frac{\partial #1}{\partial #2}}

\begin{document}
\begin{align}
\label{18.1:1}
\begin{split}
\Rot\vec{E} &=-\frac{1}{c}\parder{\vec{B}}{t},
\\
\Div\vec{B} &=0,
\end{split}
\\[2ex]
\label{18.1:2}
\begin{split}
\Rot\vec{B} &=\frac{1}{c}\parder{\vec{E}}{t}+\frac{4\pi}{c}\,\vec{j},
\\
\Div\vec{E} &=4\pi\rho_{\varepsilon }
\end{split}
\end{align}
\end{document}

enter image description here

share|improve this answer
    
Interesting solution. –  Igor Kotelnikov Apr 22 '13 at 6:26
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