Bozhok, R. V. “On the Defect of Nondenseness of Continuous Imbeddings in the Scale of Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, no. 5, May 2008, pp. 704–708, https://umj.imath.kiev.ua/index.php/umj/article/view/3188.