@book{PaneahSchulze1998,
  author    = {Paneah, Boris and Schulze, Bert-Wolfgang},
  title     = {On the existence of smooth solutions of the Dirichlet problem for hyperbolic differential equations},
  series = {Preprint / Universit{\"a}t Potsdam, Institut f{\"u}r Mathematik},
  volume    = {1998, 05},
  journal   = {Preprint / Universit{\"a}t Potsdam, Institut f{\"u}r Mathematik},
  publisher = {Univ.},
  address   = {Potsdam},
  pages     = {11 S.},
  year      = {1998},
  language  = {en}
}
@unpublished{PaneahSchulze1998,
  author    = {Paneah, Boris and Schulze, Bert-Wolfgang},
  title     = {On the existence of smooth solutions of the dirichlet problem for hyperbolic : differential equations},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-25179},
  year      = {1998},
  language  = {en}
}