@article{JMLR:v10:argyriou09a,
  author  = {Andreas Argyriou and Charles A. Micchelli and Massimiliano Pontil},
  title   = {When Is There a Representer Theorem?  Vector Versus Matrix Regularizers},
  journal = {Journal of Machine Learning Research},
  year    = {2009},
  volume  = {10},
  number  = {87},
  pages   = {2507--2529},
  url     = {http://jmlr.org/papers/v10/argyriou09a.html}
}