2 citations to https://www.mathnet.ru/eng/dan40103

1. I. M. Pupyshev, “The Extension of Functions of Sobolev Classes Beyond the Boundary of the Domain on Carnot Groups”, Siberian Adv. Math., 18:2 (2008), 124–141          
2. S. K. Vodop'yanov, A. D.-O. Ukhlov, “Set Functions and Their Applications in the Theory of Lebesgue and Sobolev Spaces. II”, Siberian Adv. Math., 15:1 (2005), 91–125