#### Vol. 15, No. 9, 2021

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On the distribution of lattice points on hyperbolic circles

### Dimitrios Chatzakos, Pär Kurlberg, Stephen Lester and Igor Wigman

Vol. 15 (2021), No. 9, 2357–2380
##### Abstract

We study the fine distribution of lattice points lying on expanding circles in the hyperbolic plane $ℍ$. The angles of lattice points arising from the orbit of the modular group ${PSL}_{2}\left(ℤ\right)$, and lying on hyperbolic circles, are shown to be equidistributed for generic radii. However, the angles fail to equidistribute on a thin set of exceptional radii, even in the presence of growing multiplicity. Surprisingly, the distribution of angles on hyperbolic circles turns out to be related to the angular distribution of ${ℤ}^{2}$-lattice points (with certain parity conditions) lying on circles in ${ℝ}^{2}$, along a thin subsequence of radii. A notable difference is that measures in the hyperbolic setting can break symmetry; on very thin subsequences they are not invariant under rotation by $\pi ∕2$, unlike in the Euclidean setting where all measures have this invariance property.

##### Keywords
hyperbolic plane, hyperbolic circle, lattice points, equidistribution
##### Mathematical Subject Classification
Primary: 11E25, 11H06, 11H56, 11N36, 11N37
Secondary: 11N13, 11N35
##### Milestones
Revised: 28 January 2021
Accepted: 28 February 2021
Published: 23 December 2021
##### Authors
 Dimitrios Chatzakos Institut de Mathématiques de Bordeaux Université de Bordeaux France Pär Kurlberg Department of Mathematics KTH Stockholm Sweden Stephen Lester Department of Mathematics King’s College London United Kingdom Igor Wigman Department of Mathematics King’s College London United Kingdom