Density of orbits of horocycle flows at sub-quadratic polynomial times

TL;DR

This paper investigates the density and equidistribution properties of horocycle flow orbits at sub-quadratic polynomial times and primes, revealing new density results and assuming conjectures for prime-related orbits.

Contribution

It establishes density of orbits at sub-quadratic polynomial times, and under the Hardy-Littlewood conjecture, shows density of prime times orbits, along with equidistribution results for quadratic times.

Findings

Orbits at times n^{2-δ} are dense for non-periodic points.

Prime time orbits are dense assuming Hardy-Littlewood conjecture.

Quadratic time orbits equidistribute along primes congruent to 1 mod 4.

Abstract

Let $\Gamma\subset PSL(2,\mathbb{R})$ be such that the space $X=\Gamma\backslash PSL(2,\mathbb{R})$ is not compact. Let $(h_t)$ be the horocycle flow acting on $X$. We show that for every $x\in X$ that is not periodic for $(h_t)$ and for every $\delta\in (0,1)$ the orbit $\{h_{n^{2-\delta}}x\}_{n\in \mathbb{N}}$ is dense in $X$. Assuming additionally the Hardy-Littlewood conjecture we show that for every non-periodic $x\in X$, $\{h_{p}x\}_{p- \text{prime}}$ is dense in $X$. Finally we show that for $\Gamma=PSL(2,\mathbb{Z})$, $\{h_{n^2}y_q\}_{n<q}$ equidistribute, as $q\to \infty$ along primes congruent to $1 \pmod{4}$, towards Haar measure, where $\{y_q\}$ is a sequence of periodic points of period $q$.