Few Distinct Distances Implies Many Points on a Line

In this post I will show that if a planar n  point set spans a small number of distinct distances, there must exist a line containing \Omega(\log n)  points of the set. This result originated in a conversation between Shakhar Smorodinsky, Frank de Zeeuw, and myself, a few months ago in IPAM. I understand that Orit Raz and Micha Sharir independently obtained similar observations at about the same time.

Shakhar Smorodinsky and Frank de Zeeuw.

Guth and Katz proved that every set of n  points in {\mathbb R}^2  determines \Omega(n/ \log n)  distinct distances. This almost completely settled a conjecture of Erdős , who observed that the \sqrt{n}\times \sqrt{n}  integer lattice determines \Theta(n/\sqrt{\log n})  distinct distances, and conjectured that every set of n  points determines at least this number of distinct distances. Beyond the remaining \sqrt{\log n}  gap, a major open problem is characterizing the point sets that determine a small number of distinct distances.

Given a set of n  points in {\mathbb R}^2  , we say that it is near-optimal if it determines O(n/\sqrt{\log n})  distinct distances. Erdős asked whether every near-optimal set “has lattice structure”. Since this seems to be a very challenging problem, Erdős wrote “The first step would be to decide if there always is a line which contains cn^{1/2}  of the x_i  [i.e., of the points of the set], (and in fact n^{\varepsilon}  would already be interesting).”

Embarrassingly, almost three decades later the n^{\varepsilon}  bound seems as distant as it ever was. Up to now, the best known bound was a variant of a proof of Szemerédi, which was communicated by Erdős.

Theorem 1 (Szemerédi). Let \cal P  be a near-optimal set of n  points. Then there exists a line containing \Omega(\sqrt{\log n})  points of \cal P  .

Proof.     Let x  denote the maximum number of points of \cal P  that are on a common line. Let T = \{(a,p,q)\in {\cal P}^3 \mid |ap|=|aq|\}  , where a,p,q  are three distinct points and where (a,p,q)  and (a,q,p)  are counted as the same triple. The proof is based on double counting |T|  , and we begin by deriving an upper bound for it. Given a pair of points p,q\in{\cal P}  , the triplet (a,p,q)  is in T  if and only if a  is on the perpendicular bisector of the segment pq  . By the assumption, each such perpendicular bisector contains at most x  points of {\cal P}  , which implies

|T| \le x \binom{n}{2} = \frac{x(n^2-n)}{2} \qquad \qquad (1).

For the lower bound, notice that for every point p\in {\cal P}  , the points of {\cal P}\setminus\{p\}  are contained in at most d = O(n/\sqrt{\log n})  concentric circles around p  . We denote these circles as C_{p,1},\ldots,C_{p,d}  and set n_{p,i} = |C_{p,i} \cap {\cal P}|  . Notice that \sum_{i=1}^dn_{p,i} = n-1  for every p\in{\cal P}  . By the Cauchy-Schwarz inequality, we have \sum_{i=1}^d n_{p,i}^2 \ge \frac{1}{d}(n-1)^2  . This in turn implies

|T| = \sum_{p\in{\cal P}}\sum_{i=1}^d \binom{n_{p,i}}{2} = \frac{1}{2}\sum_{p\in{\cal P}}\sum_{i=1}^d (n_{p,i}^2-n_{p,i})
\ge \frac{1}{2}\sum_{p\in{\cal P}} \left( \frac{1}{d}(n-1)^2 - (n-1)\right) = \frac{n(n-1)(n-1-d)}{2d}. \qquad (2)

Combining (1)  and (2)  immediately implies

x \ge \frac{n(n-1)(n-1-d)}{2d} \cdot \frac{2}{(n^2-n)} =  \Omega\left(\sqrt{\log n}\right). \qquad \qquad \qquad \qquad \Box

Szemerédi’s proof is almost identical to the proof of the Kővári–Sós–Turán theorem. We now prove a variant of this theorem.

Theorem 2 (Kővári–Sós–Turán variant). Let \cal C  be a set of n  circles and let \cal P  be a set of m  points, both in {\mathbb R}^2  , such that the incidence graph of {\cal P}\times{\cal C}  contains no copy of K_{2,s}  (where s  may depend on m,n  ). Then

I({\cal P},{\cal C}) = O(mn^{1/2}s^{1/2}+n).

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