Boichuk О. A., V. F. Zhuravlev, and A. M. Samoilenko. 1997. “Weakly Nonlinear Boundary-Value Problems for Operator Equations With Pulse Influence”.
Ukrains’kyi Matematychnyi Zhurnal 49 (2): 272–288.
https://umj.imath.kiev.ua/index.php/umj/article/view/5004.