@Article{HastieStuetzle89,
  author = 	 {T. Hastie and W. Stuetzle},
  title = 	 {Principal curves},
  journal = 	 JASA,
  year = 	 {1989},
  OPTkey = 	 {},
  volume = 	 {84},
  number = 	 {406},
  pages = 	 {502--516},
  OPTmonth = 	 {},
  OPTnote = 	 {},
  OPTannote = 	 {}
}
