\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} p_{\rm trial}^{\rm arb}(r) = { {2} \over { \left[ {\rm Erf}\left({{r_{\rm max}-\mu}\over{\sigma \sqrt{2}}}\right) - {\rm Erf}\left({{0-\mu}\over{\sigma \sqrt{2}}}\right) \right] \sigma \sqrt{2 \pi} } } \exp\left(-{{(r - \mu)^2}\over{2 \sigma^2}}\right) \end{displaymath} \end{document}