Ivany, S., and R. Jalilian. “Trigonometric Spline Method for the Numerical Solutions of Singularly Perturbed Volterra Integro-Differential Equations”. Thai Journal of Mathematics, vol. 21, no. 2, July 2023, pp. 299–314, https://thaijmath2.in.cmu.ac.th/index.php/thaijmath/article/view/1460.