ShmardaZ. “Existence and Uniqueness of Solution of the Cauchy Problem for Singular Systems of Integro-Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 45, no. 12, Dec. 1993, pp. 1716–1720, https://umj.imath.kiev.ua/index.php/umj/article/view/5983.