Fain B.L., “The Extension of Functions From Sobolev Spaces for Irregular Domains with Preservation of Smoothness Index”, Doklady Akademii Nauk Sssr, 285:2 (1985), 296–301
Maziia V.G., Poborchii S.V., “Extension of Functions From Sobolev,S.l. Spaces Into the Exterior of a Domain with a Cusp on the Boundary”, Doklady Akademii Nauk Sssr, 275:5 (1984), 1066–1069
С. К. Водопьянов, В. М. Гольдштейн, Ю. Г. Решетняк, “О геометрических свойствах функций с первыми обобщенными производными”, УМН, 34:1(205) (1979), 17–65 ; S. K. Vodop'yanov, V. M. Gol'dstein, Yu. G. Reshetnyak, “On geometric properties of functions with generalized first derivatives”, Russian Math. Surveys, 34:1 (1979), 19–74