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If err is a operator, I know how to do it, but erris a text, I don't know how to do it.

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    What is the question? Does \documentclass{article} \usepackage{amsmath,amsfonts} \DeclareMathOperator*{\err}{err} \begin{document} \[\err_{\mathcal{D}}(f)\] \end{document} serve you? – user11232 Nov 4 '15 at 9:02
  • yes, this is a way, – tiankonghewo Nov 4 '15 at 9:03

First a glimpse from amsmath manual (page 17):

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What you need is


To have \mathscr, load mathrsfs too.


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As noted by barbara beeton in her comments, the font of err is fraktur. To reproduce the same, load amsfonts package additionally and then define


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  • in the example shown in the question, the "err" is set in a fraktur font. i don't know whether that is important here (sometimes it is), but if so, the "text" of the operator name could be set as {$\mathfrak{err}$}. – barbara beeton Nov 4 '15 at 14:24
  • @barbarabeeton: Thanks. Obviously you have better eyes than me. I have added a note accordingly. – user11232 Nov 4 '15 at 14:52

In addition to Harish Kumar's comment you can also use \underset from the amsmath package to really place it below the err operator. So in addition to declaring the err operator you can set it then like this:

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