cases
I am trying to compile a simple cases example, but I'm having trouble with the tabular separator &. Here's my code:
\begin{equation}
f(x) =
\begin{cases}
x + 1 & \mbox{blabla} \\
x & \mbox{otherwise}
\end{cases}
\end{equation}
I am getting the following errors:
Misplaced alignment tab character &. x + 1 &
Misplaced alignment tab character &. x &
What could be wrong?
The package mathtools (that also loads amsmath) provides dcases and dcases*:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\alpha_{mn} =
\begin{dcases*}
\frac{16\mu^2}{mn\pi^2[\pi^2(m^2/a^2 + n^2/b^2) - \mu^2]} &
for $m$ and $n$ odd;\\
0\vphantom{\frac{0}{0}} & for $m$ or $n$ even;
\end{dcases*}
\end{equation}
\end{document}
The difference between cases and dcases is that display style is used in the latter, but not in the former. The dcases* form sets the second part in text mode, making it more convenient to type textual conditions.
The \vphantom{\frac{0}{0}} is an adjustment to make the construction more balanced. Note that you don't need \left and \right in the denominator, which would make the brackets too big.
I think that you want to use \left\{...\right. around your equation. To be able to align your equations inside these you can use the split environment from the amsmath package.
EDIT
In the comments both above and below the consensus is that the aligned environment is superior to split in this situation. Perhaps I am missing it but I can't see the difference in this example, however, as people more knowledgeable than I are advocating using the aligned environment there must be cases where it does matter.
Here is a comparison of the two environments for this example:
This produces:
\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator\vol{\text{vol}}
\begin{document}
\[\left\{
\begin{aligned}
\pi_n (i) &= {n^2}/{\vol(G_n)} \cr
\pi_n (j) &= {n}/{\vol(G_n)} \cr
q^{(n)}_{ij} &= {1}/{n^2 } \cr
q^{(n)}_{ji} &= {1}/{n}
\end{aligned}\right.\tag{\textbf{aligned}}
\]
\[\left\{
\begin{split}
\pi_n (i) &= {n^2}/{\vol(G_n)} \cr
\pi_n (j) &= {n}/{\vol(G_n)} \cr
q^{(n)}_{ij} &= {1}/{n^2 } \cr
q^{(n)}_{ji} &= {1}/{n}
\end{split}\right.\tag{\textbf{split}}
\]
\end{document}
Best Answer
Short answer
The
casesenvironment requires theamsmathpackage:Long answer
For historical reasons, LaTeX defines a
\casesmacro: on page 232 of Lamport's manual one readsand
\casesis indeed a Plain TeX command which has quite a different syntax than thecasesenvironment. It would be possible to use it without loadingamsmath, but the features of this package are much better than the Plain TeX commands: more flexible and with a uniform syntax.By the way, here's how one could input the equation with this method:
It's evident that the syntax is foreign to LaTeX: the second column is typeset in text mode and not math mode and the lines must be terminated by
\cr. Most important, the alignment must go between braces: this is the main cause of the error.Don't use it and stick to
\usepackage{amsmath}.