A functional Loomis-Whitney type inequality in the Heisenberg group and projection theorems over finite fields

Abstract

We establish functional Loomis--Whitney type inequalities in the finite Heisenberg group ${\mathbb{H}}^n({\mathbb{F}}_q)$. For $n=1$, we determine the sharp region of exponents $(u_1,u_2)$ for which the Heisenberg Loomis--Whitney inequality \[ \frac{1}{q^3}\sum_{(x,t)\in \mathbb{H}^1(\mathbb{F}_q)} f_1(π_1(x,t))\,f_2(π_2(x,t)) \;\lesssim\; \|f_1\|_{L^{u_1}(\mathbb{F}_q^2,dx)}\|f_2\|_{L^{u_2}(\mathbb{F}_q^2,dx)} \] holds uniformly in $q$, namely \[ \frac{1}{u_1}+\frac{2}{u_2}\le 2 \quad\text{and}\quad \frac{2}{u_1}+\frac{1}{u_2}\le 2, \] which includes the endpoint estimate $L^{\frac{3}{2}}\times L^{\frac{3}{2}}\to L^1$. For general $n$, we prove the symmetric multilinear estimate at the endpoint exponent $u=\frac{n(2n+1)}{n+1},$ using an induction on $n$ that exploits the Heisenberg fiber structure together with a multilinear interpolation scheme. Specializing to indicator functions yields a sharp Loomis--Whitney type set inequality bounding $|K|$ for every finite $K\subset {\mathbb{H}}^n({\mathbb{F}}_q)$ in terms of the sizes of its $2n$ Heisenberg projections $\{{\pi }_j(K){\}}_{j=1}^{2n}$, and in particular, \[ \max_{1\le j\le 2n} |π_j(K)| \;\gtrsim_n\; |K|^{\frac{2n+1}{2(n+1)}}\,q^{-\frac{1}{2(n+1)}}. \] This result is optimal up to absolute constants. Moreover, when $n=1$ and $|K|>q$, we obtain a stronger statement via Vinh's point--line incidence theorem. We also discuss connections to a boundedness problem for multilinear forms/operators over finite fields studied by Bhowmik, Iosevich, Koh, and Pham (2025), and to orthogonal projection/covering questions in ${\mathbb{F}}_q^{2n+1}$ studied by Chen (2018).

V2: typos corrected and main results unchanged