The above objective function is convex with respect to F and α, respectively. Here, we introduce an EM [38] style algorithm to iteratively optimize F (or α) while keeping α (or F) constant. For a fixed α, we can obtain F directly from Eq. (2). For a fixed F, using tr(αmW) = αmtr(W) and tr(K - W) = tr(K) - tr(W), Eq. (7) can be rewritten as: