On the lattice property of the plane and some problems of Dirac, Motzkin and Erdős in combinatorial geometry
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LetS be a set ofn non-collinear points in the Euclidean plane. It will be shown here that for some point ofS the number ofconnecting lines through it exceedsc · n. This gives a partial solution to an old problem of Dirac and Motzkin. We also prove the following conjecture of Erdős: If any straight line contains at mostn−x points ofS, then the number of connecting lines determined byS is greater thanc · x · n.
AMS subject classification (1980)51 M 05 05 C 35
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