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Reflection-induced perspectivities among triangles. (English) Zbl 1183.51014

Summary: In the plane of a reference triangle ABC, let DEF be a triangle and P a point. A line U through P is defined as a successful reflector through P if the reflection D\('\)E\('\)F\('\) of DEF in U is perspective to ABC, in the sense that the lines AD\('\), BE\('\), CF\('\) concur. The point of concurrence is the perspector, P(DEF,P,U).
The main theorem is that for given DEF and P, there are either infinitely many successful reflectors through P, or else there are at most four. Examples include choices of DEF and P for which the Steiner axes are the successful reflectors, and also choices for which there are infinitely many successful reflectors, in which case P is called a pivot for DEF. In three of the examples, the locus of P(DEF,P,U) as U rotates about P, called the pivotal curve, is the isogonal conjugate of a circle.

MSC:

51N20 Euclidean analytic geometry
14H50 Plane and space curves
15A15 Determinants, permanents, traces, other special matrix functions