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+\documentclass[12pt]{article}
+
+\begin{document}
+
+$$
+   P_{IJ} = \frac{\sum_{k}^{N} m_k v_{k_I} v_{k_J}}{V} + 
+   \frac{\sum_{k}^{N} r_{k_I} f_{k_J}}{V}
+$$
+
+\end{document}