Solving Tur\'an's Tetrahedron Problem for the $\ell_2$-Norm

TL;DR

This paper introduces a new extremality measure for hypergraphs based on the codegree squared sum, determines its asymptotic maximum for $K_4^3$ and $K_5^3$, and relates it to Turán's tetrahedron problem.

Contribution

It defines the codegree squared sum as a new extremal measure and asymptotically determines its maximum for specific hypergraphs, connecting to Turán's conjecture.

Findings

Asymptotic extremal values for $ ext{co}_2(G)$ for $K_4^3$ and $K_5^3$

Stability results showing extremal hypergraphs resemble conjectured structures

Existence of a scaled limit and blow-up invariance for $ ext{exco}_2(n,H)$

Abstract

Tur\'an's famous tetrahedron problem is to compute the Tur\'an density of the tetrahedron $K_4^3$. This is equivalent to determining the maximum $\ell_1$-norm of the codegree vector of a $K_4^3$-free $n$-vertex $3$-uniform hypergraph. We introduce a new way for measuring extremality of hypergraphs and determine asymptotically the extremal function of the tetrahedron in our notion. The codegree squared sum, $\text{co}_2(G)$, of a $3$-uniform hypergraph $G$ is the sum of codegrees squared $d(x,y)^2$ over all pairs of vertices $xy$, or in other words, the square of the $\ell_2$-norm of the codegree vector of the pairs of vertices. We define $\text{exco}_2(n,H)$ to be the maximum $\text{co}_2(G)$ over all $H$-free $n$-vertex $3$-uniform hypergraphs $G$. We use flag algebra computations to determine asymptotically the codegree squared extremal number for $K_4^3$ and $K_5^3$ and additionally prove stability results. In particular, we prove that the extremal $K_4^3$-free hypergraphs in $\ell_2$-norm have approximately the same structure as one of the conjectured extremal hypergraphs for Tur\'an's conjecture. Further, we prove several general properties about $\text{exco}_2(n,H)$ including the existence of a scaled limit, blow-up invariance and a supersaturation result.