where the ζ and d a are the same as used in Eq. 35a. The algorithm in Eq. 35a and Eq. 35b is equivalent to the original condition in Eq. 28, but significantly reduces the computational cost from order O(K 2 ) to order O(2K), and there is no need to generate and store the reduced density matrix, μρ (n) of order O(K 2 ).