The Friendship Theorem

Karol Pak. The Friendship Theorem. Formalized Mathematics, 20(3):235-237, 2012. [doi]

@article{Pak12a,
  title = {The Friendship Theorem},
  author = {Karol Pak},
  year = {2012},
  url = {http://www.degruyter.com/view/j/forma.2012.20.issue-3/v10037-012-0028-7/v10037-012-0028-7.xml},
  researchr = {https://researchr.org/publication/Pak12a},
  cites = {0},
  citedby = {0},
  journal = {Formalized Mathematics},
  volume = {20},
  number = {3},
  pages = {235-237},
}