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It seems that terms like $q'^a_b$ stopped rendering correctly, since their superscript/subscript are not surrounded by braces (e.g., $q'^{a}_b$ should work). The error shown in preview is Missing open brace for superscript.

See, for instance, CS.SE/2984

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  • $\begingroup$ Must be this update $\endgroup$
    – Gilles 'SO- stop being evil' Mod
    Commented Apr 26, 2016 at 8:17
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    $\begingroup$ Your example of q^a_b seems ok. The question is a special case: primes are treated as superscripts, so you need braces to identify where the subscript belongs (to the prime or to the base). $\endgroup$
    – Peter Krautzberger
    Commented Apr 26, 2016 at 9:21
  • $\begingroup$ @Gilles, it is not the update you indicate, which doesn't alter the expressions that MathJax will display. It was actually the update to MathJax v2.1 back in October of 2012 that caused those expressions to fail to parse. $\endgroup$
    – Davide Cervone
    Commented Apr 26, 2016 at 14:13
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    $\begingroup$ In LaTeX ' is a shortcut for ^{\prime}. So you want $q^{\prime a}_b$. $\endgroup$
    – Yuval Filmus
    Commented Apr 26, 2016 at 17:49
  • $\begingroup$ Please include a screenshot so the post makes sense in the future! $\endgroup$
    – Raphael Mod
    Commented Apr 26, 2016 at 18:21

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As Peter points out in the comments, the issue in the linked post is about combining primes with carets (^). The expression q^'_0 is invalid LaTeX (it produces an error message even in LaTeX). Version 2.0 incorrectly processed this expression, but that was fixed in version 2.1, which was released back in October of 2012 (a few months after the post you cite was written). So the expressions have not parsed correctly for several years. The correct form should be q'_0 not q^'_0. Note that q^a_b is fine, but q^'_0 is not because of the special way that TeX treats the prime.

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