\documentclass[12pt]{article}
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\begin{document}
\begin{displaymath}
P_{\rm select}^{\rm nb}\left({\bf r}_{1},...,{\bf r}_{n_g(n)}\left(i\right)\right) = 
{
{
\exp\left(-\beta {\sum_{a=1}^{n_{\rm g}\left(n\right)}} u_{\rm nb}^{\rm part}\left({\bf r}_a\left(i\right)\right)\right)
{
W_{\rm pre-nb}(i)
}
}\over{
W_{\rm nb}\left(n\right)
}
}
\end{displaymath}
\end{document}