\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} P_{\rm sample}({\bf r}) \propto { { \int_{{\bf r}=0}^{{\bf r}_{\rm max}} { \exp[-\beta U({\bf r}) ] } d{\bf r} } } \end{displaymath} \end{document}
\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} P_{\rm sample}({\bf r}) \propto { { \int_{{\bf r}=0}^{{\bf r}_{\rm max}} { \exp[-\beta U({\bf r}) ] } d{\bf r} } } \end{displaymath} \end{document}