Equation Icon  ∑ z1 = ij Aij with {i,j | i,j ∈ {0,...,N } ∧ ωi − ωj = ω+}, z2 = ∑ A with {k, l | k,l ∈ {0, ...,N } ∧ ω − ω = ω }. (85 ) ij kl k l −