Kaloshin, Vadim, and Alfonso Sorrentino. “On the Integrability of Birkhoff Billiards.”https://doi.org/10.1098/rsta.2017.0419__Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. The Royal Society, 2018.https://doi.org/10.1098/rsta.2017.0419 .