Eugen J. Ionaşcu, The ``Circle" of Apollonius in Hyperbolic Geometry,

Forum Geometricorum, 18 (2018) 135—140.

Abstract. In Euclidean geometry the circle of Apollonius is the locus of points in the plane from which two collinear adjacent segments are perceived as having the same length. In Hyperbolic geometry, the analog of this locus is an algebraic curve of degree four which can be bounded or “unbounded”. We study this locus and give a simple description of this curve using the Poincaré half-plane model. In the end, we give the motivation of our investigation and calculate the probability that three collinear adjacent segments can be seen as of the same positive length under some natural assumptions about the setting of the randomness considered.

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