Lower bounds for incidences with hypersurfaces

Abstract

We present a technique for deriving lower bounds for incidences with hypersurfaces in ${\mathbb R}^d$ with $d\ge 4$. These bounds apply to a large variety of hypersurfaces, such as hyperplanes, hyperspheres, paraboloids, and hypersurfaces of any degree. Beyond being the first non-trivial lower bounds for various incidence problems, our bounds show that some of the known upper bounds for incidence problems in ${\mathbb R}^d$ are tight up to an extra $\varepsilon$ in the exponent. Specifically, for every $m$, $d\ge 4$, and $\varepsilon>0$ there exist $m$ points and $n$ hypersurfaces in ${\mathbb R}^d$ (where $n$ depends on $m$) with no $K_{2,\frac{d-1}{\varepsilon}}$ in the incidence graph and $Ω\left(m^{(2d-2)/(2d-1)}n^{d/(2d-1)-\varepsilon} \right)$ incidences. Moreover, we provide improved lower bounds for the case of no $K_{s,s}$ in the incidence graph, for large constants $s$. Our analysis builds upon ideas from a recent work of Bourgain and Demeter on discrete Fourier restriction to the four- and five-dimensional spheres. Specifically, it is based on studying the additive energy of the integer points in a truncated paraboloid.