Wróbel, I., A. M. Gomilko, and J. Zemanek. “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, Vol. 59, no. 6, June 2007, pp. 853-8, https://umj.imath.kiev.ua/index.php/umj/article/view/3350.