Effective counting for discrete lattice orbits in the plane via Eisenstein series

Abstract

In 1989 Veech showed that for the flat surface formed by gluing opposite sides of two regular n-gons, the set Y⊂R2 of saddle connection holonomy vectors satisfies a quadratic growth estimate |{y∈Y:∥y∥≤R}|∼cYR2, and computed the constant cY. In 1992 he recorded an observation of Sarnak that gives an error estimate |{y∈Y:∥y∥≤R}|=cYR2+O(R43) in the asymptotics. Both Veech's proof of quadratic growth, and Sarnak's error estimate, rely on the theory of Eisenstein series, and are valid in the wider context of counting points in discrete orbits for the linear action of a lattice in SL(R) on the plane. In this paper we expose this technique and use it to obtain the following results. For lattices Γ with trivial residual spectrum, we recover the error estimate O(R43), with a simpler proof. Extending this argument to more general shapes, and using twisted Eisenstein series, for sectors Sα,β={reiθ:r>0,α≤θ≤α+β} we prove an error estimate ∣∣{y∈Y:y∈Sα,β,∥y∥≤R}∣∣=cYβ2πR2+Oε(R85). For dilations of smooth star bodies R⋅Bψ={reiθ:0≤r≤Rψ(θ)}, where R>0 and ψ is smooth, we prove an estimate |{y∈Y:y∈R⋅Bψ}|=cY,ψR2+Oψ,ε(R127).