@article{Fritzsche.Frommer.Szyld.07,
  author = {David Fritzsche and Andreas Frommer and Daniel B. Szyld},
  title = {Extensions of Certain Graph-based Algorithms for
  Preconditioning},
  year = 2007,
  volume=29,
  journal={SIAM Journal on Scientific Computing},
  pages={2144--2161}
}