E. I. Radzievskaya and G. V. Radzievskii, “On one extremal problem for numerical series”, Ukr. Mat. Zhurn., vol. 57, no. 10, pp. 1430–1434, Oct. 2005, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3698