I asked a question earlier today, where I learned that the subequations
environment provided by amsmath does not enter math mode: Missing $ inserted error when using \frac inside subequations environment?
According to the documentation however, align should enter math mode (see pg. 10). Then, why does the following MNWE (minimum not working example) not work?
\documentclass[12pt,letterpaper]{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{amssymb}
\begin{document}
\section{Hello World}
\begin{subequations}
\begin{align}
\frac{a}{b} &= 0 \\
x + y &= \frac{a}{b} \\
\end{align}
\end{subequations}
\end{document}
The errors being produced seemingly suggesting once again that align is not a math mode environment?
line 14: Missing $ inserted I've inserted a begin-math/end-math symbol since I think
line 15: Missing \endgroup inserted x + y &
line 15: Missing $ inserted x + y &= \frac{a}{b}
line 15: Extra }, or forgotten $ x + y &= \frac{a}{b}
line 16: Misplaced \cr \end{align}
line 16: Missing $ inserted \end{align}
line 16: Missing } inserted \end{align}
line 16: Misplaced \noalign \end{align}
line 16: Extra }, or forgotten \endgroup \end{align}
line 16: Missing $ inserted \end{align}
line 0: Display math should end with $$
In particular, note the error for line 14: "I think therefore I insert $?". Okay, all jokes aside, what's up here? Am I not understanding the documentation correctly?
Best Answer
This is even more than in your answer (now comment). In any form of math mode,
\par
and equivalent forms, like an empty line, causes an error. The reason is in the Chapter 16, p. 135 of The TeXbook