MikhailetsV. A., and MurachA. A. “Elliptic Operators in a Refined Scale of Functional Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 5, May 2005, pp. 689–696, https://umj.imath.kiev.ua/index.php/umj/article/view/3635.