AsrorovF. A., and PerestyukN. A. 1994. “Green-Samoilenko Function and Existence of Integral Sets of Linear Extensions of Nonautonomous Equations”. Ukrains’kyi Matematychnyi Zhurnal 46 (8), 1067–1071. https://umj.imath.kiev.ua/index.php/umj/article/view/5659.