Describe the sum symbol like this:
z_{ebk}=\frac{1}{N_k}\sum_{\substack{i \in R_k \\ j \in W \left(i \right) \cap R_k}}\max \left( f_i - f_j \right)The key point is the usage of \substack.
Refs:
1. http://tex.stackexchange.com/questions/12344/separate-long-math-text-under-sum-symbol-into-different-lines
2. http://tex.stackexchange.com/questions/80460/expression-under-summation-on-multiple-lines
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Latex: Expression under summation on multiple lines
原文：http://blog.csdn.net/stpallas/article/details/46819797