Sparse Principal Component of a Rank-deficient Matrix
  We consider the problem of identifying the sparse principal component of a rank-deficient matrix. We introduce auxiliary spherical variables and prove that there exists a set of candidate index-sets (that is, sets of indices to the nonzero elements of the vector argument) whose size is polynomially bounded, in terms of rank, and contains the optimal index-set, i.e. the index-set of the nonzero elements of the optimal solution. Finally, we develop an algorithm that computes the optimal sparse principal component in polynomial time for any sparsity degree. 
Megasthenis Asteris, Dimitris S. Papailiopoulos, and George N.   Karystinos
cs.IT cs.LG cs.SY math.IT math.OC