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When does extra space get inserted by \operatorname? In the example below it appears extra spacing is inserted in the second and third case, but not the first. Why?

\documentclass{report}
\usepackage{amsmath}
\begin{document}
\[\operatorname{T}\{a,b\}\]
\[\operatorname{T}X\]
\[\operatorname{T}\big\{a,b\big\}\]
\end{document}
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    Last one should be \bigl\{a,b\bigr\}.
    – Manuel
    Feb 1, 2015 at 11:04

1 Answer 1

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\operatorname makes the command use the \mathop primitive to get operator spacing like \log.

TeX assigns a "class" to every atom in a math list and then adds space of various sizes between atoms of different classes. A mathop followed by an ordinary character gets a thin space but a mathop followed by a math-open (like ( ) does not. This is so that \log x gets a thin space separating the operator from the argument which is not needed in \log(x).

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  • 1
    There is a thin space in the last example because \big\{ builds an ordinary symbol and not an opening.
    – egreg
    Feb 1, 2015 at 11:11
  • @egreg, is it a bug? \big\{ should be opening, shouldn't?
    – Sigur
    Feb 1, 2015 at 11:15
  • 2
    @Sigur no it's a documented feature, that is why there is a \bigl command. Feb 1, 2015 at 11:16
  • 3
    @Sigur \bigl\{ does \mathopen{\big\{} and \bigr\} does \mathclose{\big\}}. There's also \bigm, where \bigm| does \mathrel{\big|}. Economy is the only reason for the existence of \big, which should never be used.
    – egreg
    Feb 1, 2015 at 11:20

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