[1] Doktor P.: 
On the density of smooth functions in certain subspaces of Sobolev space. Comment. Math. Univ. Carolinae 14, 4 (1973), 609-622. 
MR 0336317 | 
Zbl 0268.46036[2] Massari V., Pepe L.: 
Sulľ approssimazione degli aperti lipschitziani di $R^N$ con varietá differenziabili. Boll. Un. Mat. Ital. (4) I0 (1974), 532-544. 
MR 0365318[3] Nečas J.: 
About domains of the type $\frac N$. (Russian.) Czechoslovak Math. Ј. 12 (87), 1962, 274-287. 
MR 0152734