 On a combinatorial problem by Nicolaas Govert de Bruijn and Paul Erdős

By Nicolaas Govert de Bruijn and Paul Erdős. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen51,1277–1279 (also Indagationes Mathematicae10,421–423). Primary scan: users.renyi.hu/~p_erdos/1948-01.pdf

Theorem1 gives the finite unique-pair line-count bound and identifies the near-pencil and projective-plane equality configurations.