Burskii, V. P., and A. S. Zhedanov. “On Dirichlet Problem for String Equation, Poncelet Problem, Pell-Abel Equation, and Some Other Related Problems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 4, Apr. 2006, pp. 435–450, https://umj.imath.kiev.ua/index.php/umj/article/view/3466.