1. Adams, R. A.: Sobolev Spaces, Academic Press, New York, 1975.
2. Burenkov, V. I.: On a method of extension of differentiable functions, Trudy Mat. Inst. Steklov. 140 (1976), 27–67 (Russian).
3. Fain, B. L.: On smooth preserving extension of functions in Sobolev spaces for irregular domains, Dokl. Akad. Nauk Sssr 285 (1985), 296–301 (Russian).
4. Gagliardo, E.: Caratterizzazioni delle tracce sulla frontiera relative ad alcune classi di funzioni in più variabili, Rend. Sem. Mat. Univ. Padova 27 (1957), 284–305.
5. Jones, P. W.: Quasiconformal mappings and extendability of functions in Sobolev spaces, Acta Math. 147 (1981), 71–88.