\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
P_{\rm select}^{\rm bend B}\left(\phi_{g_b,f,g_a}(i)\right) = 
{{
\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_{f,g_a,g_b,p}(i)\right)\right]
\right]
}
\over
{
W_{\rm bend}^{B}(n,b)
}}
\end{displaymath}
\end{document}