Dementiev , S. N., and L. P. Yanovsky. “Solubility of the Nonlinear Valle Pussen Problems ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 44, no. 5, May 1992, pp. 700-3, https://umj.imath.kiev.ua/index.php/umj/article/view/7968.