Stepanets, O. I. and Shydlich, A. L. (2005) “On one extremal problem for positive series”, Ukrains’kyi Matematychnyi Zhurnal, 57(12), pp. 1677–1683. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/3718 (Accessed: 18April2024).