Split-like environment inside cases environment

Question

I'd like to have a cases environment with some cases that are too long to fit on one line. I tried this:

\begin{cases}
  \begin{split}
    long expression \\ second line of long expression
  \end{split} & condition \\
  ...

but I get the warning

Package amsmath Warning: Cannot use 'split' here; trying to recover with 'aligned' on input line 201.

It automatically replaces the split with an aligned environment, which looks like this:

Is there any way to get these long expressions to behave like they were in split or multline environments, with the top line flush left and the next line indented a little?

Answer 1

Well, I'd follow amsmath's suggestion to use aligned instead of split:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\[
  \text{left hand side} =
  \begin{cases}
    \!\begin{aligned}%[b]
       & \text{a very long expression} \\
       & + \text{that continues on the next line}
    \end{aligned}           & \text{1st condition} \\%[1ex]
    \text{short expression} & \text{2nd condition}
  \end{cases}
\]
\end{document}

Does this yield the output you want? (Note that the \! in front of aligned is needed since that environment adds a \, we have to cancel out.)

EDIT: If you want to achieve alignment and spacing as Niel suggests (@Niel: I would want that; good catch), then just remove the two % in the code.

Answer 2

Here is an alternative construction to Hendrik's, which aligns the 1st condition with the bottom-most line of the long expression (which I would suggest as good practise generally for readability). I also add vertical space between the two formulae to make it easier to separate the two visually.

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\[
  \text{left hand side} =
    \left\{
    \begin{aligned}
       & \text{a very long expression} \\
       & + \text{that continues on the next line} && \text{1st condition}
    \\[1ex]
       & \text{short expression}                  && \text{2nd condition}
    \end{aligned}
    \right.
\]
\end{document}

I omit the cases environment here, as all of the alignment work that it does is now better performed by aligned. The alignment tabs are chosen to get uniform alignment of the conditions, and left-alignment for everything involved; they can be changed to obtain right-alignment where desired.

Answer 3

ShareLatex

Just to give concrete example rather than only putting text assume that you have written the following cases equation

\begin{equation*}
L_{t}= 
\begin{cases}
    A_{t} & \text{if }A_{t}\leq L_{t-1}e^{g} \\ 
    L_{t-1}e^{g} & \text{if }L_{t-1}e^{g}<A_{t}\leq
    L_{t-1}e^{g}+E_{t-1}e^{g}+B_{t-1} \\ 
    \begin{split}
    L_{t-1}e^{g} \\
    +\alpha \delta(1-b)(A_{t}-L_{t-1}e^{g}-E_{t-1}e^{g}-B_{t-1})  
    \end{split} & \text{if } A_{t}>L_{t-1}e^{g}+E_{t-1}e^{g}+B_{t-1}
\end{cases}
\end{equation*}

As you may see it is not fitted well then you may write the following

\begin{equation*}
  L_{t}= 
  \left\{
    \begin{aligned}
        & A_{t} && \text{if }A_{t}\leq L_{t-1}e^{g} \\
        & L_{t-1}e^{g} && \text{if }L_{t-1}e^{g}<A_{t}\leq L_{t-1}e^{g}+E_{t-1}e^{g}+B_{t-1} \\
        & L_{t-1}e^{g} \\
       & \quad + \alpha \delta(1-b)(A_{t}-L_{t-1}e^{g}-E_{t-1}e^{g}-B_{t-1})   && \text{if } A_{t}>L_{t-1}e^{g}+E_{t-1}e^{g}+B_{t-1} \\
    \end{aligned}
    \right.
\end{equation*}

Which will be printed as