Examples of projective billiards with open sets of periodic orbits
C. Fiorebe, Discrete and Continuous Dynamical Systems- Series A 44 (2024) 3287–3301.

Open problems on billiards and geometric optics
M. Bialy, C. Fiorebe, A. Glutsyuk, M. Levi, A. Plakhov, S. Tabachnikov, Arnold Mathematical Journal 8 (2022) 411–422.