Kostyshyn, L.P., and Ya.J. Bigun. “Approximate Solution of Linear Differential Equations of Neutral Type by Aggregative-Iterative Method”. Carpathian Mathematical Publications, vol. 3, no. 2, Dec. 2011, pp. 100–107, https://vps.pnu.edu.ua/index.php/cmp/article/view/3071.