Evasive sets, twisted varieties, and container-clique trees

TL;DR

This paper introduces new constructions of evasive sets in finite affine spaces and twisted varieties in projective spaces, providing bounds, enumeration results, and a novel container method with applications to combinatorial problems.

Contribution

It establishes the existence of large evasive sets with smaller parameters, introduces the concept of twisted varieties, and develops a new container method for combinatorial enumeration.

Findings

Existence of large evasive sets with size rom ^{n-k} o \u1d4f(q^{n-k})

Upper bound of 2^{O(q^{n-k})} on the number of evasive sets

A new container method technique applied to combinatorial enumeration

Abstract

In the affine space $\mathbb{F}_q^n$ over the finite field of order $q$, a point set $S$ is said to be $(d,k,r)$-evasive if the intersection between $S$ and any variety, of dimension $k$ and degree at most $d$, has cardinality less than $r$. As $q$ tends to infinity, the size of a $(d,k,r)$-evasive set in $\mathbb{F}_q^n$ is at most $O\left(q^{n-k}\right)$ by a simple averaging argument. We exhibit the existence of such evasive sets of sizes at least $\Omega\left(q^{n-k}\right)$ for much smaller values of $r$ than previously known constructions, and establish an enumerative upper bound $2^{O(q^{n-k})}$ for the total number of such evasive sets. The existence result is based on our study of twisted varieties. In the projective space $\mathbb{P}^n$ over an algebraically closed field, a variety $V$ is said to be $d$-twisted if the intersection between $V$ and any variety, of dimension $n - \dim(V)$ and degree at most $d$, has dimension zero. We prove an upper bound on the smallest possible degree of twisted varieties which is best possible in a mild sense. The enumeration result includes a new technique for the container method which we believe is of independent interest. To illustrate the potential of this technique, we give a simpler proof of a result by Chen--Liu--Nie--Zeng that characterizes the maximum size of a collinear-triple-free subset in a random sampling of $ \mathbb{F}_q^2$ up to polylogarithmic factors.