Multijoints and Factorisation

TL;DR

This paper addresses the dual multijoint problem, establishing the existence of factorisations for multijoints of planes over arbitrary fields, and derives a universal discrete wedge product property related to Bourgain and Guth's theorem.

Contribution

It proves the existence of factorisations for multijoints of planes over any field and introduces a universal property of the discrete wedge product applicable to arbitrary functions.

Findings

Established a universal factorisation property for multijoints.

Proved a discrete analogue of Bourgain and Guth's theorem.

Provided bounds involving discrete wedge products and factorising functions.

Abstract

We solve the dual multijoint problem and prove the existence of so-called "factorisations" for arbitrary fields and multijoints of $k_j$-planes. More generally, we deduce a discrete analogue of a theorem due in essence to Bourgain and Guth. Our result is a universal statement which describes a property of the discrete wedge product without any explicit reference to multijoints and is stated as follows: Suppose that $k_1 + \ldots + k_d = n$. There is a constant $C=C(n)$ so that for any field $\mathbb{F}$ and for any finitely supported function $S : \mathbb{F}^n \rightarrow \mathbb{R}_{\geq 0}$, there are factorising functions $s_{k_j} : \mathbb{F}^n\times \mathrm{Gr}(k_j, \mathbb{F}^n)\rightarrow \mathbb{R}_{\geq 0}$ such that $$(V_1 \wedge\cdots\wedge V_d)S(p)^d \leq C\prod_{j=1}^d s_{k_j}(p, V_j),$$ for every $p\in \mathbb{F}^n$ and every tuple of planes $V_j\in \mathrm{Gr}(k_j, \mathbb{F}^n)$, and $$\sum_{p\in \pi_j} s(p, e(\pi_j)) =||S||_d$$ for every $k_j$-plane $\pi_j\subset \mathbb{F}^n$, where $e(\pi_j)\in \mathrm{Gr}(k_j,\mathbb{F}^n)$ denotes the translate of $\pi_j$ that contains the origin and $\wedge$ denotes the discrete wedge product.