HochbergI. T., and ZambitskyM. K. “On the Theory of Linear Operators in Spaces With Two Norms ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 18, no. 1, Jan. 1966, pp. 11-23, https://umj.imath.kiev.ua/index.php/umj/article/view/8649.