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This code line:

\[ f \ominus s2 \thickapprox (f \ominus s1) \ominus s1 \]

gives this error:

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.
3
  • Have you added \usepackage{amssymb} in your document preamble?
    – CarLaTeX
    Jan 26, 2017 at 6:56
  • Could you add a MWE to see the packages you use? Jan 26, 2017 at 6:59
  • Search for \thickapprox in The Comprehensive LaTeX Symbol List and you'll see it forms part of the AMS package distribution...
    – Werner
    Jan 26, 2017 at 7:06

1 Answer 1

1

As Carla said: you have to load the package amssymb.

This MWE works:

\documentclass[openright,twoside]{book}

\usepackage{amssymb}

\begin{document}

\[ f \ominus s2 \thickapprox (f \ominus s1) \ominus s1 \]
\end{document}

And its result:

enter image description here

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