\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} W_{\rm bend}^{B}(n,b) = \sum_{i=1}^{\bf nch\_bend\_b} \left[ {{p_{\rm trial}^{\rm ideal}\left(\phi_{g_b,f,g_a}(i)\right)}\over{ p_{\rm trial}^{\rm arb}\left(\phi_{g_b,f,g_a}(i)\right)}} \exp\left[-\beta u_{\rm bend}\left(\theta_{g_b,f,g_a}(i)\right)\right] \exp\left[-\beta u_{\rm imp}\left(\phi_{p,f,g_a,g_b}(i)\right)\right] \right] \end{displaymath} \end{document}