3

enter image description here

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

Torbjørn T.
  • 206,688
tiankonghewo
  • 311
  • 1
    What is the question? Does \documentclass{article} \usepackage{amsmath,amsfonts} \DeclareMathOperator*{\err}{err} \begin{document} \[\err_{\mathcal{D}}(f)\] \end{document} serve you? –  Nov 04 '15 at 09:02
  • yes， this is a way， – tiankonghewo Nov 04 '15 at 09:03

2 Answers2

6

First a glimpse from amsmath manual (page 17):

enter image description here

What you need is 

\DeclareMathOperator*{\err}{err}

To have \mathscr, load mathrsfs too.

\documentclass{article}
\usepackage{amsmath,mathrsfs}
\DeclareMathOperator*{\err}{err}
\begin{document}
  \[\err_{\mathscr{D}}(f)\]
\end{document}

enter image description here

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

\DeclareMathOperator*{\err}{\mathfrak{err}}

enter image description here

  • 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 04 '15 at 14:24
  • @barbarabeeton: Thanks. Obviously you have better eyes than me. I have added a note accordingly. –  Nov 04 '15 at 14:52
1

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:

\[
  \underset{\mathcal{D}}{\err}
\]
Philipp
  • 5,078
  • 10
  • 30
  • 32