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 
ight) cap R_k}}max left( f_i - f_j 
ight)

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