\documentclass[24pt]{article}
\pagestyle{empty}
\begin{document}
\begin{eqnarray*}
u^j_{m,m'} = U^j_{m,m'}(0,0,0) + \sum_{r_{ii'} < R_{ii'}}{f_c(r_{ii'}) w_{i'} U^j_{m,m'}(\theta_0,\theta,\phi)}
\end{eqnarray*}
\end{document}