[1] P. Doktor: On the density of smooth functions in certain subspaces of Sobolev space. CMUC 14, 4 (1973), 609-622. MR 0336317 | Zbl 0268.46036

[2] S. Fučík J. Nečas V. Souček: Ûvod do variacniho poctu. Lecture notes of Prague university, 1972.

[3] M. M. Вайнбгрг: Вариационный метод и метод монотонных операторов. Наука, Москва 1972. Zbl 1170.01322

[4] F. Stummel: Perturbation Theory for Sobolev Spaces. To appear in the Proceedings of the Royal Society of Edinburgh. MR 0410433 | Zbl 0358.46027