Sokil, B. I. 1997. “On Asymptotic Approximation of a Solution of a Boundary-Value Problem for a Nonlinear Nonautonomous Equation”. Ukrains’kyi Matematychnyi Zhurnal 49 (11), 1580–1583. https://umj.imath.kiev.ua/index.php/umj/article/view/5160.