Kashpirovskii, A. I., and Yu. V. Mytnik. “Approximation of Solutions of Operator-Differential Equations by Operator Polynomials”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 11 (November 25, 1998): 1506–1516. Accessed May 19, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4802.