Karaev, M. T. “On Estimate for Numerical Radius of Some Contractions”. Ukrains’kyi Matematychnyi Zhurnal 58, no. 10 (October 25, 2006): 1335–1339. Accessed May 3, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3536.