Atanasova, P., A. Georgieva, and L. Trenkova. “EXISTENCE OF CONTINUOUS SOLUTIONS OF A PERTURBED LINEAR VOLTERRA INTEGRAL EQUATIONS”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 103, Dec. 2016, pp. 79-88, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/56.