\documentclass[12pt]{article}
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\begin{document}
\begin{displaymath}
P_{\rm select}^{\rm dihed}\left(\phi_{g_1,f,p,q}(k)\right) = 
{
{
{{p_{\rm trial}^{\rm ideal}\left(\phi_{g_1,f,p,q}(k)\right)}\over{ p_{\rm trial}^{\rm arb}\left(\phi_{g_1,f,p,q}(k)\right)}}
w_{\rm bias}^{\rm fe}(k)
\exp\left[-\beta \sum_{a=1}^{n_{\rm g}(n)} u_{\rm con}\left(\phi_{g_a,f,p,q}(k)\right)\right]
}
\over
{
W_{\rm dihed}(j)
}
}
\end{displaymath}
\end{document}