Kostyshyn, L. P., and B. A. Shuvar. “Aggregate-Iterative Methods for the Approximation of Solutions of Boundary-Value Problems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 55, no. 10, Oct. 2003, pp. 1425-31, https://umj.imath.kiev.ua/index.php/umj/article/view/4011.