@article{nphardnessandinapproximabilityofsparse,
  title = {NP-Hardness and Inapproximability of Sparse PCA},
  author = {Malik Magdon-Ismail},
  year = {2015},
  eprint = {1502.05675},
  archivePrefix = {arXiv},
  url = {http://arxiv.org/abs/1502.05675v2},
}