Nicolaas Govert de Bruijn worked on combinatorics and mathematical structures. With Paul Erdős, he proved a finite incidence theorem in their 1948 paper: a nontrivial system in which each pair of points lies on exactly one line has at least as many lines as points.[ref]
On a combinatorial problem by Nicolaas Govert de Bruijn and Paul Erdős. Original byline and Theorem 1
The equality cases are highly organized: a near-pencil or a finite projective plane. This shows how a local pair-incidence rule controls both a global counting inequality and the structure of extremal examples.[ref]
On a combinatorial problem by Nicolaas Govert de Bruijn and Paul Erdős. Theorem 1 and equality characterization
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Nicolaas Govert de Bruijn was a Dutch mathematician, known for his contributions to various fields in mathematics, particularly in combinatorics, graph theory, and number theory. He was born on April 3, 1918, and passed away on December 17, 2012.