Latex: Expression under summation on multiple lines

Purpose

Describe the sum symbol like this:

zebk=1Nk∑i∈Rkj∈W(i)∩Rkmax(fi−fj)

Solution code

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