Burskii, V. P., and A. S. Zhedanov. 2006. “On Dirichlet Problem for String Equation, Poncelet Problem, Pell-Abel Equation, and Some Other Related Problems”. Ukrains’kyi Matematychnyi Zhurnal 58 (4), 435–450. https://umj.imath.kiev.ua/index.php/umj/article/view/3466.