= Paul Erdős
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<Paul Erdős> worked on <number theory>, <combinatorics> and extremal problems. His joint 1948 paper with <Nicolaas Govert de Bruijn> gives an incidence bound from a unique-line-through-two-points condition.<On a combinatorial problem by Nicolaas Govert de Bruijn and Paul Erdős>[Original byline and Theorem 1]{ref}
For a finite nontrivial incidence system, the number of lines is at least the number of points. Equality forces a near-pencil or a finite projective plane. The theorem illustrates an extremal approach: determine a sharp numerical bound, then understand the configurations attaining it.<On a combinatorial problem by Nicolaas Govert de Bruijn and Paul Erdős>[Theorem 1]{ref}
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