@article{QuaisserAlpers1998,
  author    = {Quaisser, Erhard and Alpers, Karsten},
  title     = {Lattices in the Pseudoeuclidean plane},
  year      = {1998},
  language  = {en}
}
@article{Alpers1994,
  author    = {Alpers, Karsten},
  title     = {{\"U}ber zweidimensionale diskrete Bewegungsgruppen der pseudoeuklidischen Ebene},
  year      = {1994},
  language  = {de}
}