Using align inside cases

Question

I was trying to achieve an alignment inside a cases environment. Here is what I was trying to achieve:

I actually was able to achieve the above using the following MathJax code:

$$
f(x,y) = 
\begin{cases} 
\begin{align*}
x&=1, & y&=1 \\
x&=10, & y&=20
\end{align*}
\end{cases}
$$

I realize that $$ is not proper LaTeX syntax, but I'm not sure what to substitute it with.

Now, when I try to put this inside a LaTeX document, it gives me a few errors and doesn't line everything up properly into two columns (there is only one column).

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$$
f(x,y) = 
\begin{cases} 
\begin{align*}
x&=1, & y&=1 \\
x&=10, & y&=20
\end{align*}
\end{cases}
$$

\end{document}

Here is something else I tried, which reduced the graphical errors, but still puts it inside only one column:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
f(x,y) = 
\begin{cases} 
u &= \ln x & v' &= 1 \\
u' &= \frac{1}{x} & v &= x \\
\end{cases}
\end{align*}

\end{document}

I would also like to control the spacing with the alignat environment, but I need to find out how to do this first before I can start thinking about that :)

texdoc amsmath ? align is a display environment (you can use aligned for subterms. — David Carlisle, Sep 24 '17 at 16:10
the latex equivalent to $$..$$ (which is also the mathjax default) is \[..\] — David Carlisle, Sep 24 '17 at 16:12
MathJax is not LaTeX; it (wrongly) allows constructions that are illegal as LaTeX code and this is an example. — egreg, Sep 24 '17 at 16:29

score 6 · Answer 1 · answered Sep 24 '17 at 16:14

6

align is for complete display math, aligned is the version for subterms.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
f(x,y) = 
\left\{
\begin{aligned}
x&=1, & y&=1 \\
x&=10, & y&=20
\end{aligned}
\right.
\]

\end{document}

answered Sep 24 '17 at 16:14

David Carlisle

757,742

score 2 · Accepted Answer · answered Sep 24 '17 at 16:23

The simplest is to use empheq + alignat*. Needless to load amsmath, since empheq loads mathtools, which does it for you.

\documentclass{article}%
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{empheq}

\begin{document}

\begin{empheq}[left={f(x, y)=\empheqlbrace}]{alignat*=2}
    x &=1, &\quad y &=1 \\
    x &=10, & y &=20
\end{empheq}

\end{document}

Using align inside cases

2 Answers2