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arXiv · 2009.07647

Related by Similarity II: Poncelet 3-Periodics in the Homothetic Pair and the Brocard Porism

Dan Reznik·Ronaldo Garcia
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Abstract

Previously we showed the family of 3-periodics in the elliptic billiard (confocal pair) is the image under a variable similarity transform of poristic triangles (those with non-concentric, fixed incircle and circumcircle). Both families conserve the ratio of inradius to circumradius and therefore also the sum of cosines. This is consisten with the fact that a similarity preserves angles. Here we study two new Poncelet 3-periodic families also tied to each other via a variable similarity: (i) a first one interscribed in a pair of concentric, homothetic ellipses, and (ii) a second non-concentric one known as the Brocard porism: fixed circumcircle and Brocard inellipse. The Brocard points of this family are stationary at the foci of the inellipse. A key common invariant is the Brocard angle, and therefore the sum of cotangents. This raises an interesting question: given a non-concentric Poncelet family (limited or not to the outer conic being a circle), can a similar doppelgänger always be found interscribed in a concentric, axis-aligned ellipse and/or conic pair?

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Dan Reznik, Ronaldo Garcia. 2020-09-16. Related by Similarity II: Poncelet 3-Periodics in the Homothetic Pair and the Brocard Porism. https://arxiv.org/abs/2009.07647

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