StepanetsO. I., and ShydlichA. L. “On One Extremal Problem for Positive Series”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 12 (December 25, 2005): 1677–1683. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3718.