How to properly typeset E_{\mathrm{k}} in beamer

Question

Scientific typography makes an important distinction between italic type and roman (upright) type (exemplary source).

The symbol for kinetisch energy is E_k with E printed in italic type, k printed in roman type.

Normally, I would write that as E_{\mathrm{k}}.

However, that does not work with beamer. The package changes normal math to a sans-serif font, but leaves \mathrm a serif-font. The subscript k will be typeset in a serif-font, but it should simply match the default math font, just being upright.

So how do I get the intended behavior in beamer?

\documentclass{beamer}
\begin{document}
\begin{frame}
  $E_{\mathrm{k}}$
\end{frame}
\end{document}

(Other packages that switch to sans-serif math fonts also redefine mathrm to be sans-serif and, of course, upright, e.g. cmbright, sfmath and arevmath.)

Answer 1

By adding \usefonttheme[onlymath]{serif}, we find the serif font in mathematical formulas.

\documentclass{beamer}
\usefonttheme[onlymath]{serif}
\begin{document}
\begin{frame}
  $E_\mathrm{k}$
\end{frame}
\end{document}

Output:

Answer 2

One could use $E_{\mathsf{k}}$, but that is not very portable.

From the beamer manual:

Note: The command \mathrm will always produce upright (not slanted), serif text and the command \mathsf will always produce upright, sans-serif text. The command \mathbf will produce upright, boldface, sans-serif or serif text, depending on whether mathsans or mathserif is used. To produce an upright, sans-serif or serif text, depending on whether mathsans or mathserif is used, you can use for instance the command \operatorname from the amsmath package. Using this command instead of \mathrm or \mathsf directly will automatically adjust upright mathematical text if you switch from sans-serif to serif or back.

So, the beamer way to do things is to define an operator and then use it. But an operator is not quite the same thing.

In my humble opinion, the beamer package is flawed in that respect.

Answer 3

Imho the problem is that at the time the names for the math alphabets were choosen sans serif math wasn't really on the agenda of math typesetters. And while I do understand why you want \mathrm to give a sans serif upright font, I would find it a bit curious if rm leads to a sf font (and that with e.g. arevmath \mathsf gives a slanted sf font).

So with pdflatex I would use (this assumes that amsmath is loaded) \operatorfont:

\documentclass{beamer} %or article
\usepackage{amsmath}
\usepackage{ifluatex}
\ifluatex
\usepackage{unicode-math}
\setmathfont{Cambria Math}
\else
\def\symup{\mathrm}
\fi
\begin{document}
sin

$\sin{x}\quad{\operatorfont sin}{x}\quad\symup{sin}{x}\quad\mathrm{sin}{x}$
\end{document}

output with beamer + pdflatex

output with article + pdflatex

With unicode math I would probably prefer \symup (I don't quite understand why unicode-math uses a text font for the operators instead of the math font):

output with beamer and lualatex

output with article and lualatex: