The functional Loomis-Whitney type inequality in the Heisenberg groups and Projection theorems over finite fields

Abstract: We develop a functional Loomis--Whitney framework on the finite Heisenberg groups $\mathbb{H}^n(\mathbb{F}_q)$ and discover connections to the boundedness and orthogonal projection problems. For $n=1$ we determine the sharp region of exponents $(u_1,u_2)$ for which the associated bilinear projection form is bounded 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 $L^{3/2}\times L^{3/2}\to L^1$. For general $n$ we prove a multilinear estimate at the critical exponent [ u=\frac{n(2n+1)}{n+1}, ] via an induction on $n$ that exploits the group's fiber structure together with multilinear interpolation. Specializing to indicators yields a sharp Loomis--Whitney type set inequality that controls $|K|$ for every finite $K\subset \mathbb{H}^n(\mathbb{F}_q)$ by the sizes of its $2n$ Heisenberg projections ${\pi_j(K)}$, forcing a large projection in every configuration. A straightening map then converts these bounds into covering statements by additive cosets and provides a new approach to the orthogonal projection problem onto the vertical hyperplanes ${x_j=0}$, which presents an interesting link between commutative and non-commutative settings. The obtained results are optimal up to absolute constants, and in the planar case $n=1$, when the size of the set is not too small, our bound can be sharpened further using a point--line incidence estimate.