PlotnikovA. V., and TumbrukakiA. V. “Integro-Differential Equations With Multivalued Solutions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 52, no. 3, Mar. 2000, pp. 359-67, https://umj.imath.kiev.ua/index.php/umj/article/view/4426.