@unpublished{PaneahSchulze2008,
  author  = {Boris Paneah and Bert-Wolfgang Schulze},
  title   = {On the existence of smooth solutions of the dirichlet problem for hyperbolic : differential equations},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(1998) 05},
  year    = {2008},
}