Esta linha de código:
\[ f \ominus s2 \thickapprox (f \ominus s1) \ominus s1 \]
dá esse erro:
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.
Responder1
Como disse a Carla: você tem que carregar o pacote amssymb.
Este MWE funciona:
\documentclass[openright,twoside]{book}
\usepackage{amssymb}
\begin{document}
\[ f \ominus s2 \thickapprox (f \ominus s1) \ominus s1 \]
\end{document}
E seu resultado:
