PerestyukN. A., and SlyusarchukV. Y. “Green–Samoilenko Operator in the Theory of Invariant Sets of Nonlinear Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 7, July 2009, pp. 948-57, https://umj.imath.kiev.ua/index.php/umj/article/view/3069.