Dementiev , S. N., and L. P. Yanovsky. “Solubility of the Nonlinear Valle Pussen Problems ”. Ukrains’kyi Matematychnyi Zhurnal 44, no. 5 (May 26, 1992): 700–703. Accessed April 7, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/7968.