Sylvester–Gallai theorem

The Sylvester–Gallai theorem in geometry states that, given a finite number of points in the Euclidean plane, either

It is named after James Joseph Sylvester, who posed it as a problem in 1893, and Tibor Gallai, who published one of the first proofs of this theorem in 1944.

A line that contains exactly two of a set of points is known as an ordinary line. According to a strengthening of the theorem, every point set has at least a linear number of ordinary lines. There is an algorithm that finds an ordinary line in a set of n points in time proportional to n log n in the worst case.

The Sylvester–Gallai theorem was posed as a problem by J. J. Sylvester (1893). Kelly (1986) suggests that Sylvester may have been motivated by a related phenomenon in algebraic geometry, in which the inflection points of a cubic curve in the complex projective plane form a configuration of nine points and twelve lines in which each line determined by two of the points contains a third point. The Sylvester–Gallai theorem implies that it is impossible for all nine of these points to have real coordinates.

Woodall (1893) claimed to have a short proof, but it was already noted to be incomplete at the time of publication. Eberhard Melchior (1941) proved the projective dual of this theorem, (actually, of a slightly stronger result). Unaware of Melchior's proof, Paul Erdős (1943) again stated the conjecture, which was proved first by Tibor Gallai, and soon afterwards by other authors.

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