Torba, S. M. “Direct and Inverse Theorems of Approximate Methods for the Solution of an Abstract Cauchy Problem”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 6 (June 25, 2007): 838–852. Accessed April 25, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3349.