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Years ago, I read a story where a one-line conjecture in a serious mathematics paper was interpreted as a theorem by some incautious reader. Some embarrassment ensued. The suggestion was that, especially for one-line conjectures, one could put an upside down question mark before and a question mark after; I think this is ordinary usage in written Spanish. Note that this is sometimes done in Unsolved Problems in Number Theory by Richard K. Guy; I'm looking at section A8 on prime gaps, and it is a question on prime gaps that I just answered...

Can someone please tell me how to do this within a displayed equation, meaning double dollar signs before and after? I tried looking at the MathJax website. It would not surprise me if this were available in more recent Latex than the book I own indicates, in which case I would love to know the Latex method.

Will

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A general approach is via the \unicode command (see here in the MathJax documentation). Google informs me that the Unicode number for the upside-down question mark is 191, and indeed, $\unicode{191}$ produces $$\unicode{191}$$

Further Googling leads me to the unicode FF1F for the "fullwidth question mark" $$\unicode{xFF1F}$$ which appears to be a good match stylistically (at least, as I'm seeing it on my computer).


My current font doesn't seem to have unicode 2E2E in it; the command $\unicode{x2E2E}$ just appears as a box to me.

However, the other reversed question mark 061F works fine for me, and it also fits in stylistically, more or less: $\unicode{x061F}$ produces $$\unicode{x061F}$$ which appears on my computer like this.

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  • $\begingroup$ So, Zev, is there a unicode question mark of matching size/font for the end of the equation/inequality? $\endgroup$ – Will Jagy Oct 27 '13 at 19:31
  • $\begingroup$ I think I've found a good match. Answer edited. $\endgroup$ – Zev Chonoles Oct 27 '13 at 19:36
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    $\begingroup$ The actual character displayed depends on a number of factors, going from your MathJax rendering mode, the fonts available, luck and the position of three of Saturn's moons. $\endgroup$ – Mariano Suárez-Álvarez Oct 27 '13 at 19:43
  • $\begingroup$ FF1F, cool! How to do this: U+2E2E ⸮ e2 b8 ae REVERSED QUESTION MARK $\endgroup$ – Will Jagy Oct 27 '13 at 19:45
  • $\begingroup$ For whatever reason, x2E2E works for me. It is just a guess that other people will see the same thing I see; anyway math.stackexchange.com/questions/541727/… MEANWHILE x061F produced the right character but scrambled everything else in the inequality. $\endgroup$ – Will Jagy Oct 27 '13 at 20:17
  • $\begingroup$ On a Mac, the keyboard shortcut for ¿ is Option+Shift+? $\endgroup$ – TBrendle Oct 28 '13 at 14:25
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You can copy them from here :-)

¿¿¿¿¿¿¡¡¡¡¡¡¡

For example, $¿1+1=2?$ is $¿1+1=2?$.

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  • $\begingroup$ Aha! You allow me to do what I wish, while retaining your secrets! $\endgroup$ – Will Jagy Oct 27 '13 at 19:25
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    $\begingroup$ Well, I have a Spanish keyboard, so in a way I am cheating. $\endgroup$ – Mariano Suárez-Álvarez Oct 27 '13 at 19:26
  • $\begingroup$ I used it at the prime gap question. Better with spacing \; between that and the equation/inequality. $\endgroup$ – Will Jagy Oct 27 '13 at 19:28
  • $\begingroup$ youtube.com/watch?v=o5p-uspcgRY $\endgroup$ – Will Jagy Oct 27 '13 at 19:29
  • $\begingroup$ \textquestiondown and \textexclamdown (from textcmds.sty) are somewhat more official ways to get these, but maybe they don't work here. $\endgroup$ – Mariano Suárez-Álvarez Oct 27 '13 at 19:35
  • $\begingroup$ The \textquestiondown did not work, I can see that I spelled it correctly. $\endgroup$ – Will Jagy Oct 27 '13 at 19:49
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I highly recommend the conversion of some unneeded key (a win-key is always good candidate) into a compose key. You can just type [compose] + [?] + [?] then to produce the unicode character ¿. Works in math mode, or wherever you like it.

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