Joints of varieties

TL;DR

This paper extends the Guth--Katz joints theorem from lines to algebraic varieties, providing tight bounds on the number of joints formed by varieties of arbitrary dimensions and degrees, using a novel polynomial method extension.

Contribution

It introduces a new polynomial method extension to higher-dimensional objects, proving bounds for joints of algebraic varieties and confirming Carbery's conjecture.

Findings

Bound of O(N^{3/2}) joints for N planes in 6D

Generalization to varieties of arbitrary dimensions and degrees

Confirmation of Carbery's conjecture on joints with multiplicities

Abstract

We generalize the Guth--Katz joints theorem from lines to varieties. A special case says that $N$ planes (2-flats) in 6 dimensions (over any field) have $O(N^{3/2})$ joints, where a joint is a point contained in a triple of these planes not all lying in some hyperplane. More generally, we prove the same bound when the set of $N$ planes is replaced by a set of 2-dimensional algebraic varieties of total degree $N$, and a joint is a point that is regular for three varieties whose tangent planes at that point are not all contained in some hyperplane. Our most general result gives upper bounds, tight up to constant factors, for joints with multiplicities for several sets of varieties of arbitrary dimensions (known as Carbery's conjecture). Our main innovation is a new way to extend the polynomial method to higher dimensional objects, relating the degree of a polynomial and its orders of vanishing on a given set of points on a variety.