4

I can align several equations inside a single brace with the following code.

\begin{equation}
    \left\lbrace
        \begin{aligned}
            k \cdot E &= 0 \\
            k \cdot B &= 0
        \end{aligned}
    \right.
\end{equation}

And I know how to use subequation numbering when I don't put the brace.

\begin{subequations}
    \begin{align}
        k \cdot E &= 0 \\
        k \cdot B &= 0
    \end{align}
\end{subequations}

But I do not manage to do both. The following code compiles but does not produce the expected result.

\begin{equation}
    \left\lbrace
        \begin{subequations}
            \begin{aligned}
                k \cdot E &= 0 \\
                k \cdot B &= 0
            \end{aligned}
        \end{subequations}
    \right.
\end{equation}

Expected:

     / k . E = 0     (10a)
    {
     \ k . D = 0     (10b)

Actual result:

     / k . E = 0
    {                 (11)
     \ k . D = 0

And there is nothing with the tag 10.

If I move the subequations outside (that is, in order: subequations, equation, \left, aligned), then I get the single tag (10).

If I move subequations inside aligned, it does not compile.

5

Use empheq (but first think again if you really need separate numbers):

\documentclass{article}
\usepackage{amsmath}
\usepackage{empheq}

\begin{document}
\setcounter{equation}{9} % just for the example

\begin{subequations}
\begin{empheq}[left=\empheqlbrace]{align}
k \cdot E &= 0 \\
k \cdot B &= 0
\end{empheq}
\end{subequations}

\end{document}

enter image description here

  • Thank you! How does it play with cleveref and hyperef? – Niriel Oct 31 '14 at 9:41
  • @Niriel Since it internally uses amsmath, there should be no problem, but as I rarely use cleveref I can't say. Please report any problem. – egreg Oct 31 '14 at 9:57

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