(3) as an ILP by introducing indicator (binary) variables x i,j , y i,j , i ̸ = j and z i for i, j ∈ V .
z i = 1 indicates that unit i is selected, i.e., i ∈ S, x i,j = 1 indicates that i ∈ S but j / ∈ S, and y i,j = 1 indicates both i and j are selected.