DeclareMathOperator in bold

Question

Ho can I declare a math operator which will appear in bold font?

I tried the following, but it didn't work:

\DeclareMathOperator{\Div}{Div}
\newcommand{\Divxk}{\mathbf{\Div}_{X/k}}

Answer 1

The \mathbf around \Div does nothing, because \Div is translated internally to something like "choose the \mathrm font and typeset ‘Div’”. Math fonts don't inherit features from the context: \mathrm always corresponds to the upright text font in medium weight (or boldface) if \boldmath is in force.

You can check that

$\mathbf{\mathrm{x}}$

produces a medium weight ‘x’.

So you have to define

\DeclareMathOperator{\Div}{\mathbf{Div}}
\newcommand{\Divxk}{\Div_{X/k}}

The definition of \Div becomes “choose the \mathrm font and typeset ‘\mathbf{Div}’”, so, for the same reason as before, the boldface font will be chosen.

Answer 2

Try this:

\DeclareMathOperator{\Div}{\mathbf{Div}}

Answer 3

An alternative approach is to use \boldsymbol:

\documentclass{article}

\usepackage{amsmath}
\DeclareMathOperator{\Div}{Div}
\newcommand{\Divxk}{\mathop{\boldsymbol{\Div}}_{X/k}}

\begin{document}
\( \Div \)

\( \Divxk \)
\end{document}