@unpublished{Popivanov2008,
  author  = {Petăr R. Popivanov},
  title   = {Lorenz transformations and creation of logarithmic singularities to the solutions of some nonstrictly hyperbolic semilinear systems with two space variables},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(2004) 07},
  year    = {2008},
}