G. V. Radzievskii, “On the best approximations and rate of convergence of decompositions in the root vectors of an operator”, Ukr. Mat. Zhurn., vol. 49, no. 6, pp. 754–773, Jun. 1997, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5060