I'm trying to export a PDF of a LyX document (it has never caused a problem before). Below I have posted pictures of the error message along with the equation LyX highlighted for me. I don't know what \hbox is, and I didn't use that in my code. Can someone please help me edit the equation so that I can export a PDF?

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  • 1
    This \hbox thing is just an example for a typo, has nothing to do with your text!
    – vaettchen
    Feb 1, 2013 at 16:57

1 Answer 1


The command \coloneqq is unknown, you need to load a package for this, such as mathtools, txfonts or pxfonts.

Add to your document preamble:


LyX 2.1 will have direct mathtools support.

  • 1
    Thank you. Where would I type this "\usepackage{mathtools}"? Feb 2, 2013 at 4:21
  • 1
    The document preamble can be edited via the LyX menu: Document/ Settings.
    – Stefan Kottwitz
    Feb 2, 2013 at 8:13
  • 2
    I'm using Lyx 2.2.2 and have to force the use of mathtools from Document / Settings, the auto detect setting seems broken.
    – Nobody
    Dec 15, 2016 at 15:42

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