Dzhumabaev, D. S., and S. M. Temesheva. “Criteria for the Existence of an Isolated solution of a Nonlinear Boundary-Value Problem”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 3, Mar. 2018, pp. 356-65, https://umj.imath.kiev.ua/index.php/umj/article/view/1561.