@misc{wiki:PtolemysTheorem-2016,
   author = "Wikipedia",
   title = "Ptolemy's theorem --- Wikipedia{,} The Free Encyclopedia",
   year = "2016",
   note = "\url{https://en.wikipedia.org/w/index.php?title=Ptolemy%27s_theorem&oldid=727017817} [Online; accessed 6-August-2016]"
 }

@misc{Harrison,
  author = {John Harrison},
  title = {Ptolemy's Theorem},
  note = {\url{https://github.com/jrh13/hol-light/blob/master/100/ptolemy.ml}}
}

@misc{Wiedijk,
  author = {Freek Wiedijk},
  title = {Formalizing 100 Theorems},
  note = {\url{http://www.cs.ru.nl/~freek/100/}}
}