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:
