We study the fine distribution of lattice points lying on expanding circles in the hyperbolic
plane
.
The angles of lattice points arising from the orbit of the modular group
, 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
-lattice
points (with certain parity conditions) lying on circles in
, 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
,
unlike in the Euclidean setting where all measures have this invariance property.
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