Incompatibility between split and empheq?

Question

I have an issue using split inside an empheq environment.

When combining both environments, the alignment of the empheq is broken.

Any clues ?

If this is a duplicate or an obvious/known incompatibility, my apologies in advance.

\documentclass[]{article}

\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{empheq}


\begin{document}

Equations that can fit
\begin{empheq}[left=\empheqlbrace]{align}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\end{empheq}

Equations that can't
\begin{empheq}[left=\empheqlbrace]{align}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\end{empheq}

So I split the first equation
\begin{empheq}[left=\empheqlbrace]{align}
& \begin{split}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
\end{split}\\
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
a\end{empheq}

\end{document}

in the first two you have the (strange) setting with only a right hand side so all the terms are after a & but in the third equation there is no & before bbbbb so it comes to the left of the aaaa as the split is after a & — David Carlisle, Aug 17 '18 at 20:51
You'll get the same with algin instead of empheq, it is caused by the missing & as @DavidCarlisle mentioned. And you'll have to remove the & prior to split. — Skillmon, Aug 17 '18 at 20:53
That's precisely the difference between the split and the aligned syntax. — Bernard, Aug 17 '18 at 21:35
@Bernard, now I just found https://tex.stackexchange.com/questions/187938/whats-the-difference-between-split-and-aligned and https://tex.stackexchange.com/questions/192069/why-cant-split-with-curly-brace-be-inside-align-but-it-can-be-inside-gather. I guess i will remove my post... — BambOo, Aug 17 '18 at 21:47
@BambOo: I don't think it's necessary, as it's sometimes hard to question the (answered) question which match exactly yours. I'd rather add a link to these questions as a complement. — Bernard, Aug 17 '18 at 21:50

score 1 · Answer 1 · answered Aug 17 '18 at 20:57

Adding the missing & and removing the one prior to split does solve your issue:

\documentclass[]{article}

\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{empheq}


\begin{document}

Equations that can fit
\begin{empheq}[left=\empheqlbrace]{align}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\end{empheq}

Equations that can't
\begin{empheq}[left=\empheqlbrace]{align}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\end{empheq}

So I split the first equation
\begin{empheq}[left=\empheqlbrace]{align}
 \begin{split}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
\end{split}\\
&bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
a\end{empheq}
\end{document}

score 1 · Accepted Answer · edited Aug 17 '18 at 21:19

It is also compatible with aligned, which can replace split. Note that empheq loads mathtools, which loads amsmath, so you only have to load the first of these packages.

\documentclass[]{article}

\usepackage{empheq}

\begin{document}

Equations that can fit
\begin{empheq}[left=\empheqlbrace]{align}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\end{empheq}

Equations that can't
\begin{empheq}[left=\empheqlbrace]{align}
& aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
& bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\end{empheq}

It works with `aligned`:

\begin{empheq}[left=\empheqlbrace]{align}
& \begin{aligned}[b]
  & aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\\
  & aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
\end{aligned}\\
 & bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\end{empheq}

\end{document}

Yes, indeed it is working with aligned. +1 for the answer, however it doesn't really solve the issue between split and empheq. Thank you anyway — BambOo, Aug 17 '18 at 21:24
I very rarely use split, so I didn't even try to understand what was happening there, as most of the time, aligned (or alignedat) can replace it — Bernard, Aug 17 '18 at 21:32

Incompatibility between split and empheq?

2 Answers2