@ARTICLE{LeNa05a,
  author = {Robert Michael Lewis and Stephen G. Nash},
  title = {Model Problems for the Multigrid Optimization of Systems Governed
	by Differential Equations},
  journal = {{SIAM} Journal on Scientific Computing},
  year = {2005},
  volume = {26},
  pages = {1811--1837},
  number = {6},
  doi = {10.1137/S1064827502407792}
}