BoichukО. A., ZhuravlevV. F., and SamoilenkoA. M. 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.