@unpublished{Yagdjian2008,
  author  = {Karen Yagdjian},
  title   = {Geometric optics for the nonlinear hyperbolic systems of Kirchhoff-type},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(2001) 22},
  year    = {2008},
}