此代码行：
\[ f \ominus s2 \thickapprox (f \ominus s1) \ominus s1 \]
给出此错误：
l.663 \[ f \ominus s2 \thickapprox
(f \ominus s1) \ominus s1 \]
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
答案1
正如卡拉所说：你必须加载包裹amssymb。
此 MWE 的工作原理如下：
\documentclass[openright,twoside]{book}
\usepackage{amssymb}
\begin{document}
\[ f \ominus s2 \thickapprox (f \ominus s1) \ominus s1 \]
\end{document}
其结果如下：
