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Billiards in ellipses revisited

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TLDR
In this article, the authors proved that the sum of cosines of the angles of a periodic billiard polygon remains constant in the one-parameter family of such polygons (that exist due to the Poncelet porism).
Abstract
We prove some recent experimental observations of D. Reznik concerning periodic billiard orbits in ellipses. For example, the sum of cosines of the angles of a periodic billiard polygon remains constant in the one-parameter family of such polygons (that exist due to the Poncelet porism). In our proofs, we use geometric and complex analytic methods.

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Citations
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Journal ArticleDOI

Dan Reznik’s identities and more

TL;DR: In this paper, a non-standard generating function for the billiard ball map is used to obtain some conserved quantities associated with periodic billiard trajectories in ellipses.
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Can the Elliptic Billiard Still Surprise Us

TL;DR: It is shown that Monge's Orthoptic Circle's close relation to 4-periodic Billiard trajectories is well-known and its geometry provided clues with which to generalize 3- periodic invariants to trajectories of an arbitrary number of edges.
Journal ArticleDOI

Eighty New Invariants of N-Periodics in the Elliptic Billiard

TL;DR: 50+ new invariants manifested by the dynamic geometry of N -periodics in the Elliptic Billiard, detected with an experimental/interactive toolbox are introduced.
Posted Content

New Properties of Triangular Orbits in Elliptic Billiards

TL;DR: Some of the proofs omitted from the introduction of new invariants in the one-dimensional family of 3-periodic orbits in the elliptic billiard are presented as well as a few new related facts.
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Related by Similarity II: Poncelet 3-Periodics in the Homothetic Pair and the Brocard Porism.

TL;DR: Two new Poncelet 3-periodic families are studied: a first one interscribed in a pair of concentric, homothetic ellipses, and a second non-concentric one known as the Brocard porism: fixed circumcircle and Brocard inellipse.
References
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I and J

Book

Geometry and billiards

TL;DR: In this article, the authors discuss the existence and non-existence of caustics and periodic trajectories of billiards inside conics and quadrics, as well as in polygons.
Book

Poncelet Porisms and Beyond: Integrable Billiards, Hyperelliptic Jacobians and Pencils of Quadrics

TL;DR: In this paper, Poncelet-Darboux curves and Cayley's condition are used to define the Jacobians of Hyper-Elliptic curves and their Jacobians.
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