On Extremal Properties of k-CNF: Capturing Threshold Functions

TL;DR

This paper investigates the limitations of k-CNF formulas in representing threshold functions, linking the problem to extremal hypergraph theory and providing new bounds and constructions, especially for 2-CNF cases.

Contribution

It formulates the problem as an extremal hypergraph problem, connects it to Turán's problem, and offers optimal constructions for certain parameters, advancing understanding of k-CNF expressiveness.

Findings

For t = n - k, the problem is equivalent to Turán's hypergraph problem.

Constructed optimal solutions for 2-CNF when t = αn.

Potential for improved lower bounds in depth-3 circuits if constructions are extended for k > 2.

Abstract

We consider a basic question on the expressiveness of $k$-CNF formulas: How well can $k$-CNF formulas capture threshold functions? Specifically, what is the largest number of assignments (of Hamming weight $t$) accepted by a $k$-CNF formula that only accepts assignments of weight at least $t$? Among others, we provide the following results: - While an optimal solution is known for $t \leq n/k$, the problem remains open for $t > n/k$. We formulate a (monotone) version of the problem as an extremal hypergraph problem and show that for $t = n-k$, the problem is exactly the Tur\'{a}n problem. - For $t = \alpha n$ with constant $\alpha$, we provide a construction and show its optimality for $2$-CNF. Optimality of the construction for $k>2$ would give improved lower bounds for depth-$3$ circuits.