“…Integro-differential equations are widely used in many fields such as control theory, biology, and mechanics, and the qualitative theory of integro-differential equations creates an important branch of nonlinear analysis; see, for instance, the monographs [2,6,7] and the papers [5, 10-12, 16, 24, 26, 31, 33, 34]. For the results of existence of solutions and existence of extremal solutions for such equations under different boundary conditions, we refer the reader to the monographs by Guo et al [13] and Lakshmikantham and Rama Mohana Rao [23], the related literature for integro-differential equations [1,4,8,15,27,30], and for functional integro-differential equations [3,14,17,20,21,28,35,36], and the references cited therein.…”