@Article{PU06,
  author =   "Hartmut Prautzsch and Georg Umlauf",
  title =    "Parametrizations for Triangular $G^k$ Spline Surfaces of Low Degree",
  journal =     ACM ToG,
  year =     2006,
  volume =  25 ,
  number =   "4",
  keyword = "",
  pages =    "1281-1293",
  comment = "hp"}