Recall that det(ℱ ) is the product of the inverse of 𝜆 𝑖 (𝐂) (see Eq. 8). Since ℱ is a positive semidefinite matrix for our designs, 𝜆 𝑖 ≥ 0. In addition, numerical methods on matrices required for optimization, such as inversion and Gaussian elimination, require a nonzero matrix determinant.