3

In this answer on math I wrote

$\frac 12=0.50000\dots =0.499999\dots $

The dots after the 50000 came out higher on the screen than the ones after the 499999. Changing it to \ldots works fine. Why are they higher with \dots in one case than another?

5
  • 2
    that is the whole point about \dots it uses \ldots or \cdots depending on what comes next. Jul 17, 2016 at 16:43
  • @DavidCarlisle: so it is the following equals sign that got the cdots? I did verify that the height is the same as cdots. Jul 17, 2016 at 16:47
  • yes, see the amsmath documentation. Jul 17, 2016 at 16:49
  • 1
    I've re-tagged your posting since the issue you describe can arise -- in a LaTeX document, that is -- only if the amsmath package is loaded.
    – Mico
    Jul 17, 2016 at 16:54
  • On the subject of writing \frac12 rather than \frac{1}{2}, see also Is it wrong to use \frac1{}?
    – Mico
    Jul 17, 2016 at 17:17

1 Answer 1

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Observe that the issue you describe does not arise in a LaTeX document if the amsmath package is not loaded:

enter image description here

\documentclass{article}
\begin{document}
$\frac12=0.50000\dots=0.499999\dots$
\end{document}

Compare this output with what results if the amsmath package is, in fact, loaded:

enter image description here

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\frac12=0.50000\dots=0.499999\dots$
\end{document}

The meanings of the two instances of \dots are quite evidently being interpreted differently. In the first instance, amsmath appears to judge that \cdots is appropriate (possibly because it thinks that there's a multiplicative relationship between 0.50000 and =), whereas it appears to judge that \ldots is appropriate in the second instance.


What to do -- if anything?

Quoting from section 4.3 of the user guide of the amsmath package:

For preferred placement of ellipsis dots (raised or on-line) in various contexts there is no general consensus. It may therefore be considered a matter of taste. By using the semantically oriented commands

  • \dotsc for "dots with commas"
  • \dotsb for "dots with binary operators/relations
  • \dotsm for "multiplication dots"
  • \dotsi for "dots with integrals"
  • \dotso for "other dots" (none of the above)

instead of \ldots and \cdots, you make it possible for your document to be adapted to different conventions on the fly, in case (for example) you have to submit it to a publisher who insists on following house tradition in this respect.

Put differently, you have three choices -- assuming you're working with a LaTeX document and load the amsmath package:

  • Use \dots and hope that your intended usage will coincide with how amsmath interprets the situation. Most of the time, this approach will work just fine.

  • Use \ldots and \cdots ("lower dots" and "center dots") explicitly and don't give LaTeX and amsmath a chance to make decisions about the type of ellipsis to be used. The downside of this approach is that you may be misinterpreting what's optimal (or at least conventional) from a typographic standpoint.

  • Use the "semantically oriented" \dots[x] commands instead of \dots. For the case at hand, you should probably be using \dotsc in both instances.

enter image description here

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\frac12=0.50000\dotsc=0.499999\dotsc$
\end{document}
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    The first \dots becomes \cdots because it is followed by = that is a binary relation. The \dots[x] commands are provided as helpers when the decision cannot be made according to current status (the rules are a bit complex), for instance when there is a trailing ellipsis at the end of a comma separated list (\dotsc) or at the end of a non-ending summation (\dotsb).
    – egreg
    Jul 17, 2016 at 18:09

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