Roitberg, Ya. A., and A. V. Sklyarets. “Sobolev Problem in the Complete Scale of Banach Spaces”. Ukrains’kyi Matematychnyi Zhurnal, vol. 48, no. 11, Nov. 1996, pp. 1555-63, https://umj.imath.kiev.ua/index.php/umj/article/view/5209.