Kashpirovskii, A. I., and S. M. Torba. 2008. “Characterization of the Rate of Convergence of One Approximate Method for the Solution of an Abstract Cauchy Problem”. Ukrains’kyi Matematychnyi Zhurnal 60 (4), 557–563. https://umj.imath.kiev.ua/index.php/umj/article/view/3175.