Command “&” is not working

Question

I am new to TeX and I am trying to write the following

\begin{equation}
N_{a,0}\begin{cases}
R_{0,0} & a=0\\
N_{a-1,0}e^{-m} & 1\leq a\leq a_{max}-1\\
N_{a-1,0}e^{-m}/1-e^{-m} & a=a_{max}
\end{cases}
\end{equation}

but I get an error because & is not being recognized as a command, it does not even have the green color as the rest of the commands do. Any ideas on how to fix it?

Answer 1

Whether & is recognized as a command as far as syntax coloring is concerned depends on the editor you're using.

In any case, in order to use the cases environment you need to load amsmath.

Minimal example:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
N_{a,0}=
\begin{cases}
  R_{0,0} & a=0\\
  N_{a-1,0}e^{-m} & 1\leq a\leq a_{\max}-1\\
  N_{a-1,0}e^{-m}/1-e^{-m} & a=a_{\max}
\end{cases}
\end{equation}

\end{document}

Note \max to get the letters upright.

Here's what I get in the editor I use most commonly (Aquamacs)

This is how TeXStudio colorizes the same input: