Gabrel', O. M., & Mulyavka, O. H. (2003). Approximate Solution of the de la Vallée Poussin Problem for a Differential Equation of Neutral Type by the Projection-Iterative Method. Ukrains’kyi Matematychnyi Zhurnal, 55(2), 278-283. https://umj.imath.kiev.ua/index.php/umj/article/view/3905