1. Bethuel, F. (1988), “The approximation problem for Sobolev maps between two manifolds”, preprint, and Approximation dans des espaces de Sobolev entre deux variétés et groupes d’homotopie, C.R. Acad. Sci. Paris, t. 307, 293–296.
2. Bethuel, F. (1990), “A characterization of maps in H 1(B 3, S 2)which can be approximated by smooth maps”, to appear in Ann. of IHP, vol. 4.
3. Bethuel, F. and Zheng X. (1988), “Density of smooth functions between two manifolds in Sobolev spaces”, J. Funct. Anal., 60–75.
4. Demengel, F. (1990), “Une caractérisation des applications de W 1,P(B N, S 1) qui peuvent être approchées par des fonctions C ℞”, C.R. Acad. Sci. Paris, t. 310, 553–557.
5. Fédérer, H. (1969), Geometric measure theory, Springer Verlag.