@book{PettersLevineWambsganss2001,
  author    = {Arlie O. Petters and Harold Levine and Joachim Wambsganß},
  title     = {Singularity theory and gravitational lensing},
  volume    = {21},
  isbn      = {0-8176-3668-4},
  year      = {2001},
}