Multijoints and Factorisation

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 $\sum_{p\in π_j} s(p, e(π_j)) =||S||_d$ for every $k_j$-plane $π_j\subset \mathbb{F}^n$, where $e(π_j)\in \mathrm{Gr}(k_j,\mathbb{F}^n)$ denotes the translate of $π_j$ that contains the origin and $\wedge$ denotes the discrete wedge product.

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