BilichenkoR. O., and BabenkoV. F. 2009. “Refinement of a Hardy–Littlewood–Pólya-Type Inequality for Powers of Self-Adjoint Operators in a Hilbert Space”. Ukrains’kyi Matematychnyi Zhurnal 61 (10), 1299-1305. https://umj.imath.kiev.ua/index.php/umj/article/view/3101.