Wróbel, I., A. M. Gomilko, and J. Zemanek. 2007. “On a Criterion for the Uniform Boundedness of a <i>C</i><sub>0</Sub&gt;-Semigroup of Operators in a Hilbert Space”. Ukrains’kyi Matematychnyi Zhurnal 59 (6), 853-58. https://umj.imath.kiev.ua/index.php/umj/article/view/3350.