@unpublished{Popivanov2004,
  author    = {Popivanov, Petăr R.},
  title     = {Lorenz transformations and creation of logarithmic singularities to the solutions of some nonstrictly hyperbolic semilinear systems with two space variables},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-26710},
  year      = {2004},
  language  = {en}
}