The c r s are the expansion coefficients. The coefficients c r s form a self-adjoint matrix, since ⟨ ⟨ ψ ψ n s | B | ψ ψ n r ⟩ ⟩ = c r s . Next step would be to diagonalize the matrix c r s . To do so, we sum over all r with g constants d r . So,