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Belyi's theorem says that if $X$ is a curve defined over a number field $K$ then $X_{\overline{K}}$ admits a map to $\mathbb{P}^1_{\overline{K}}$ which at most 3 branch points. Must there also exist a map to $\mathbb{P}^1_K$ defined over $K$?

This may be a slightly lazy question, but I figure it's probably got a quick answer.

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    $\begingroup$ The question in the title and the question in the main text are asking for opposite things, so it is unclear to which an answer of "No" would refer. Anyway, the answer to the title question is "No" and the answer to its opposite in the main text (where you must have meant to say "Must there also exist such a map...") is "yes", with all branching over $\{0,1,\infty\}$ (as you must have wanted). See (the end of) Theorem 2.5 in Shinichi Mochizuki's elegant short 2004 paper Non-critical Belyi Maps at his webpage: kurims.kyoto-u.ac.jp/~motizuki/Noncritical%20Belyi%20Maps.pdf $\endgroup$
    – nfdc23
    Commented Jan 22, 2017 at 21:12
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    $\begingroup$ @nfdc23 Fantastic! Thanks! If you'd be so kind as to post this as an answer, I'd be happy to mark this question as answered. $\endgroup$ Commented Jan 22, 2017 at 22:26
  • $\begingroup$ Please feel free to post this information as an answer if you wish to close it off as "answered". I generally prefer not to post official answers when what I have to say fits as a comment (and I don't care about getting points, etc.); that is my version of being "slightly lazy". :) $\endgroup$
    – nfdc23
    Commented Jan 23, 2017 at 15:01

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To record nfdc23's answer:

The following is true:

If $X$ is a curve over a number field $K$, then it admits a map to $\mathbb{P}^1_K$ defined over $K$ branched over at most 3 points.

This follows from Theorem 2.5 in Mochizuki's 2004 paper here.

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