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Erdős and Pach celebrating another November day many years ago. The Wolf disguised as Little Red Riding Hood. Pach disguised as another Pach.

This post is authored by János Pach

### A Festive Day: November 19

Today is a festive day. It was on this day, November 19, 1863, that Abraham Lincoln presented his famous Gettysburg Address. Seventy nine years later, on the same day (on his own birthday!), Georgy Zhukov, Marshal of the Soviet Union, launched Operation Uranus, turning the tide of the battle of Stalingrad and of World War II. Now sixty eight years later, here we stand (or sit) and experience my very first attempt to contribute to a blog, as Gil has suggested so many times during the past couple of years. But above all, this is a festive day, because earlier today **Larry Guth **and **Nets Hawk Katz **posted on arXiv

(http://arxiv.org/PS_cache/arxiv/pdf/1011/1011.4105v1.pdf) an almost complete solution of Erdös’s Distinct Distances Problem. The story started with Erdős’s 1946 paper published in the American Mathematical Monthly. In this paper, he posed two general questions about the distribution of distances determined by a finite set of points in a metric space.

1. Unit Distance Problem: At most how many times can the same distance (say, distance 1) occur among a set of n points?

2. Distinct Distances Problem: What is the minimum number of distinct distances determined by a set of n points?

Because of the many failed attempts to give reasonable bounds on these functions even in the plane, one had to realize that these questions are not merely “gems” in recreational mathematics. They have raised deep problems, some of which can be solved using graph theoretic and combinatorial ideas. In fact, the discovery of many important combinatorial techniques and results were motivated by their expected geometric consequences. (For more about the history of this problem, read my book with Pankaj Agarwal: Combinatorial Geometry, and for many related open problems, my book with Peter Brass and Willy Moser: Research Problems in Discrete Geometry.)

Erdős conjectured that in the plane the number of unit distances determined by n points is at most , for a positive constant c, but the best known upper bound, due to Spencer, Szemeredi, and Trotter is only . As for the Distinct Distances Problem, the order of magnitude of the conjectured minimum is , while the best lower bound was , thanks to combined efforts by J. Solymosi – C.D. Toth (2001) and N.H. Katz – G. Tardos (2004).

This was the situation until today! The sensational new paper of Guth and Katz presents a proof of an almost tight lower bound of the order of n/log n. Let us celebrate this fantastic development! In this area of research, it is already considered a great achievement if by introducing an ingenious new idea one is able to improve a bound by a factor of for some positive δ. Continue reading →