1. F.J. Almgren, W. Browder and E.H. Lieb,Co-area, liquid crystals, and minimal surfaces, inPartial Differential Equations,Lecture Notes in Math.1306. Springer (1988) 1–22.
2. Bethuel F., The approximation problem for Sobolev maps between manifolds.Acta Math.167(1992) 153–206.
3. Brezis H., Coron J.M. and Lieb E.H., Harmonic maps with defects.Comm. Math. Phys.107(1986) 649–705.
4. H. Federer,Geometric measure theory,Grundlehren Math. Wissen.153. Springer, New York (1969).
5. Federer H. and Fleming W., Normal and integral currents.Annals of Math.72(1960) 458–520.