V. Kaloshin and A. Sorrentino, “On the integrability of Birkhoff billiards,” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 376, no. 2131. The Royal Society, 2018.
LibreCat: https://research-explorer.ista.ac.at/record/8419
DOI: 10.1098/rsta.2017.0419