Generalized nonconvex nonsmooth low-rank minimization

As surrogate functions of L0-norm, many nonconvex penalty functions have been proposed to enhance the sparse vector recovery. It is easy to extend these nonconvex penalty functions on singular values of a matrix to enhance low-rank matrix recovery. However, different from convex op-timization, solving the nonconvex low-rank minimization problem is much more challenging than the nonconvex sparse minimization problem. We observe that all the ex-isting nonconvex penalty functions are concave and mono-tonically increasing on [0,∞). Thus their gradients are decreasing functions. Based on this property, we propose an Iteratively Reweighted Nuclear Norm (IRNN) algorithm to solve the nonconvex nonsmooth low-rank minimization problem. IRNN iteratively solves a Weighted Singular Value Thresholding (WSVT) problem. By setting the weight vector as the gradient of the concave penalty function, the WSVT problem has a closed form solution. In theory, we prove that IRNN decreases the objective function value monoton-ically, and any limit point is a stationary point. Extensive experiments on both synthetic data and real images demon-strate that IRNN enhances the low-rank matrix recovery compared with state-of-the-art convex algorithms. 1