1. Akopyan, A.: Angles $$\phi =\pi -\theta _i$$ (resp. $$\phi ^{\prime }=\phi -\theta _i^{\prime }$$), so equivalent to invariant sum (resp. product) of cosines. Private Communication (2020)
2. Akopyan, A.: Corollary of Theorem 6 in Akopyan et al., Billiards in Ellipses Revisited (2020). Private Communication (2020)
3. Akopyan, A.: Follows from previous results: the construction is affine and holds for any two concentric conics. Private (2020). Communication
4. Akopyan, A.: Perpendicular feet to N-periodic or its tangential polygon are cyclic. Private (2020). Communication
5. Akopyan, A., Schwartz, R., Tabachnikov, S.: Billiards in ellipses revisited. Eur. J. Math. (2020). https://doi.org/10.1007/s40879-020-00426-9