# Incidences Between Points and Lines on Two- and Three-Dimensional Varieties

Micha Sharir, Noam Solomon

Research output: Contribution to journalArticlepeer-review

## Abstract

Let P be a set of m points and L a set of n lines in R4, such that the points of P lie on an algebraic three-dimensional variety of degree D that does not contain hyperplane or quadric components (a quadric is an algebraic variety of degree two), and no 2-flat contains more than s lines of L. We show that the number of incidences between P and L is (Formula Presented.) for some absolute constant of proportionality. This significantly improves the bound of the authors (Sharir, Solomon, Incidences between points and lines in R4. Discrete Comput Geom 57(3), 702–756, 2017), for arbitrary sets of points and lines in R4, when D is not too large. Moreover, when D and s are constant, we get a linear bound. The same bound holds when the three-dimensional surface is embedded in any higher-dimensional space. The bound extends (with a slight deterioration, when D is large) to the complex field too. For a complex three-dimensional variety, of degree D, embedded in C4 (or in any higher-dimensional Cd), under the same assumptions as above, we have (Formula Presented.). For the proof of these bounds, we revisit certain parts of , combined with the following new incidence bound, for which we present a direct and fairly simple proof. Going back to the real case, let P be a set of m points and L a set of n lines in Rd, for d≥ 3 , which lie in a common two-dimensional algebraic surface of degree D that does not contain any 2-flat, so that no 2-flat contains more than s lines of L (here we require that the lines of L also be contained in the surface). Then the number of incidences between P and L is (Formula Presented.) When d= 3 , this improves the bound of Guth and Katz (On the Erdős distinct distances problem in the plane. Ann Math 181(1), 155–190, 2015) for this special case, when D≪ n1 / 2. Moreover, the bound does not involve the term O(nD). This term arises in most standard approaches, and its removal is a significant aspect of our result. Again, the bound is linear when D= O(1). This bound too extends (with a slight deterioration, when D is large) to the complex field. For a complex two-dimensional variety, of degree D, when the ambient space is C3 (or any higher-dimensional Cd), under the same assumptions as above, we have (Formula Presented.). These new incidence bounds are among the very few bounds, known so far, that hold over the complex field. The bound for two-dimensional (resp., three-dimensional) varieties coincides with the bound in the real case when D= O(m1 / 3) (resp., D= O(m1 / 6)).

Original language English 88-130 43 Discrete and Computational Geometry 59 1 https://doi.org/10.1007/s00454-017-9940-5 Published - 1 Jan 2018

## Keywords

• Algebraic techniques for discrete geometry
• Geometric incidences
• Lines on varieties
• Polynomial partitioning
• Ruled surfaces

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