Chudinovich, I. Y. “Properties of Nonstationary Boundary Operators in a Full Scale of Sobolev-Type Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 46, no. 7, July 1994, pp. 962–965, https://umj.imath.kiev.ua/index.php/umj/article/view/5691.