Without any theorem-related package (using etoolbox
to patch the LaTeX kernel):
\documentclass{article}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\@begintheorem}{\itshape}{}{}{}
\patchcmd{\@opargbegintheorem}{\itshape}{}{}{}
\makeatother
\newtheorem{pro}{Problem}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}
With amsthm
:
\documentclass{article}
\usepackage{amsthm}
\theoremstyle{definition}
\newtheorem{pro}{Problem}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}
With ntheorem
:
\documentclass{article}
\usepackage{ntheorem}
\theorembodyfont{\normalfont}
\newtheorem{pro}{Problem}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}
With amsthm
plus the thmtools
frontend (can also be used with ntheorem
):
\documentclass{article}
\usepackage{amsthm}
\usepackage{thmtools}
\declaretheoremstyle[bodyfont=\normalfont]{normalbody}
\declaretheorem[style=normalbody,name=Problem]{pro}
\begin{document}
\begin{pro}
This is the first problem.
\end{pro}
\end{document}