Umlauts in math mode

Question

I'm trying to use German umlauts in math mode but get the known error: LaTeX Warning: Command \" invalid in math mode on input line ##.

So I could think of two possible solutions:

1. Use \text instead of \mathrm. But this wouldn't be the right solution because this text will be changed according to the surrounding text and the text I try to write ("Empfänger", German word for "receiver") should appear as a superscript to a field variable.
2. Replace the letter ä with "a. But this leads to LaTeX not recognizing the ligature of fä.

Since I'm using the lmodern package, the letter ä is still shown, so the resulting PDF is what I would expect.

Are there some elegant and correct solutions to this merely cosmetic problem?

---

My code for this would be:

\documentclass{scrartcl}
% Kodierung
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
% Sprache (neue deutsche Rechtschreibung)
\usepackage[ngerman]{babel}
% Mathematik
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}

% reference
Empfänger

% creates warning but is correct
\begin{align}
    \mathrm{Empfänger}
\end{align}

% creates no warning but has no ligature
\begin{align}
    \mathrm{Empf"anger}
\end{align}

\end{document}

Answer 1

As you load the ams packages, you can use \textnormal:

\documentclass{scrartcl}
% Kodierung
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
% Sprache (neue deutsche Rechtschreibung)
\usepackage[ngerman]{babel}
% Mathematik
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}

Test
\begin{align}
    \textnormal{Empf"anger}
\end{align}
test

\itshape Test
\begin{equation}
    \textnormal{Empf"anger}
\end{equation}
test

\end{document}

As the example shows, this is immune to the surounding text changes.

Answer 2

In Text mode \"o begets a lower case O with two dots above. Any upper or lower case letter, or, interestingly enough, number, as well as some characters can be umlauted. This character sequence does not work in Math Mode.

In Math Mode (in between $ $) use \ddot{o}. Again, you can substitute any letter, number or some characters for the o in the foregoing character string.

FYI using the Windows's character map alt+0246 for example, does not work. Neither does copying and pasting out of MSWord. I've tried it to no avail.

Answer 3

Instead of using \mathrm you should use \text to write text in a math environment. That should not have any problems with umlauts.

Answer 4

Maybe not the most elegant solution, but definitely package-independent is to define a new command:

\newcommand{\aumlaut}{\text{\textit{\"a}}}

If you have inputenc with UTF-8 for instance, you could of course have ä straightaway.