# Using/Forcing upright text within theorem environment

I would like to write an equation within a theorem that involves an integrand and the upright symbol $\textrm{d}$ which I have abbreviated in my preamble with

\newcommand{\dmath}{\textrm{d}}


(and the preamble also contains)

\usepackage{array}
\usepackage{amsthm}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
... etc
\newtheorem{theorem}{Theorem}[section]


I have the following theorem:

\begin{theorem}[Fubini's theorem]
\label{app:thrm:fubini}
Suppose $X$ and $Y$ are measurable spaces. If $f(x,y)$ is integrable ($f(x,y)$ is measurable and $\int_{X,Y} \left| f(x,y)\right| \dmath (x,y) < \infty$) then
\begin{equation*}
\int_{X} \left(\int_{Y} f(x,y) \dmath y \right) \dmath x
=
\int_{Y} \left(\int_{X} f(x,y) \dmath x \right) \dmath y
=
\int_{X,Y} f(x,y) \dmath (x,y).
\end{equation*}
\end{theorem}


and have the following output but would like the \dmath to produce $\textrm{d}$ to produce an upright 'd' character whenever it is called, and would like the following output: Any suggestions on a simple way to achieve the desired result?

I have looked at the following Qs but they don't quite address the issue:

• \newcommand{\dmath}{\mathop{}\!\mathrm{d}} – egreg May 30 '16 at 13:23

Here's a better reimplementation; \mathrm will always choose upright medium, whereas \textrm will retain the settings of the font current outside the formula.

Note some further refinements. See Better automatic spacing of differential d? for more information.

\documentclass{article}

\usepackage{amsthm}
\usepackage{amsmath}
\usepackage{amssymb}

\newtheorem{theorem}{Theorem}[section]

\newcommand{\dmath}{\mathop{}\!\mathrm{d}}

\begin{document}
\begin{theorem}[Fubini's theorem]
\label{app:thrm:fubini}
Suppose $X$ and $Y$ are measurable spaces. If $f(x,y)$ is integrable
\textup{(}$f(x,y)$ is measurable and
$\int_{X,Y} \lvert f(x,y)\rvert \dmath (x,y) < \infty$\textup{)} then
\begin{equation*}
\int_{X} \left(\,\int_{Y} f(x,y) \dmath y \right) \dmath x
=
\int_{Y} \left(\,\int_{X} f(x,y) \dmath x \right) \dmath y
=
\int_{X,Y} f(x,y) \dmath (x,y).
\end{equation*}
\end{theorem}

\end{document}


I wouldn't use \dmath, though, because breqn uses it and you never know if you're going to reuse the code. • Thanks for the improvement, and aslo the heads up about breqn. I had worried that my choice of \dmath might have been used previously. Thanks @egreg – oliversm May 31 '16 at 19:20