@article{etna_vol18_pp153-173,
  author  = {Daniela Calvetti and Bryan Lewis and Lothar Reichel and Fiorella Sgallari},
  title   = {Tikhonov regularization with nonnegativity constraint},
  journal = {Electron. Trans. Numer. Anal.},
  volume  = {18},
  year    = {2004},
  pages   = {153--173},
}