Counting Lattice Points Near Korányi Spheres via Generalized Radon Transforms

Abstract In this note, we study a lattice point counting problem for spheres in Heisenberg groups, incorporating both the non-isotropic dilation structure and the non-commutative group law. More specifically, we establish an upper bound for the average number of lattice points in a $$\delta $$ δ -neighborhood of a Korányi sphere of large radius, where the average is considered over Heisenberg group translations of the sphere. This is in contrast with previous works of Garg–Nevo–Taylor, Gath and Campolongo–Taylor, which either count lattice points on dilates of a fixed sphere or consider averages over Euclidean translations of the sphere. We observe that incorporating the Heisenberg group structure allows us to circumvent the degeneracy arising from the vanishing of the Gaussian curvature at the poles of the Korányi sphere. In fact, in lower dimensions (the first and second Heisenberg group), our method establishes an upper bound for this number which gives a logarithmic improvement over the bound implied by the previously known results. Even for the higher dimensional Heisenberg groups, we recover the bounds implied by the result of Garg–Nevo–Taylor using generalized Radon transforms. Further, we obtain upper bounds for the average number of lattice points near more general spheres described with respect to radial, Heisenberg homogeneous norms.