Pentagram in the trajectory of bar billiards around a triangle

Takeo Noda, Shin-ichi Yasutomi

Published 2023-04-17Version 1

We consider a bar billiards problem for a triangle in the unit circle. For the point on the unit circle, we construct a line from it in a counterclockwise direction tangent to the triangle, and examine a map corresponding to the point of intersection with the circle. We provide sufficient conditions for the rotation number $\rho$ of this map to be $\frac{2}{5}$, as well as necessary conditions for $\rho=\frac{2}{5}$.