@incollection {SarkisCopper,
  AUTHOR = {Sarkis, Marcus},
 TITLE = {Two-level Schwarz methods for nonconforming finite elements and discontinuous coeficients},
   booktitle = {Sixth Copper Mountain Conference
on Multigrid Methods},
publisher =  {NASA Conference Publication},
 VOLUME = {3224, Part 2},
 PAGES = {543--365}, 
  YEAR = {1993},
}