When I type
$$ A_n\ =\ \left\{\begin{array}{ccc}
[\hskip.017in 0,1\hskip.017in] & {} & n\ {\rm is\ even} \\
[\hskip.017in -1,0\hskip.017in] & {} & n\ {\rm is\ odd} \\
\end{array}\right.\ , $$
I get an error message claiming "illegal unit of measure" and "missing number." I can't find anything wrong with the code. What am I missing?
The problem is that the first [ follows \\ and is mistaken for introducing the optional argument. Use \relax or, better, use the proper tool.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$ A_n\ =\ \left{\begin{array}{ccc}
[\hskip.017in 0,1\hskip.017in] & {} & n\ {\rm is\ even} \ \relax
[\hskip.017in -1,0\hskip.017in] & {} & n\ {\rm is\ odd} \
\end{array}\right.\ , $$
[
A_n=\begin{cases}
[0,1] & \text{$n$ is even,} \
[-1,0] & \text{$n$ is odd,}
\end{cases}
]
\end{document}
I have little doubts about which rendering is better both for input and output. If you want to align the two intervals to the right, add \hphantom{-}:
\[
A_n=\begin{cases}
\hphantom{-}[0,1] & \text{$n$ is even,} \\
[-1,0] & \text{$n$ is odd,}
\end{cases}
\]
I'd avoid the comma hanging from nowhere.
Note that \rm has been deprecated for about 30 years and $$ since LaTeX version 1, about 40 years ago.
\textup, just in case this is used within an italic environment like a theorem.
– barbara beeton
Jun 16 '22 at 23:21
\text, all by itselt, follows the style of the surrounding environment.
– barbara beeton
Jun 16 '22 at 23:24
[\hphantom{-}0,1], so that the left brackets align. I don't know if that would be better.
– Teepeemm
Jun 17 '22 at 13:39
the [is being seen as an optional argument, which takes a length, \\[5pt] so use
....\\ \relax
[....]
to hide the [ from \\
Note $$ and\hskip are TeX primitives that should not be used in LaTeX, use \[ and \hspace (but avoid \hspacein math) \rm has not been defined by default in latex since 1993, use \textrm{ is odd}