MurachA. A. “Elliptic Boundary-Value Problems in Complete Scales of Nikol’skii-Type Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 12, Dec. 1994, pp. 1647–1654, https://umj.imath.kiev.ua/index.php/umj/article/view/5575.