The BV-energy of maps into a manifold:relaxation and density results

Let Y be a smooth compact oriented Riemannian manifold without boundary, and assume that its 1-homology group has no torsion. Weak limits of graphs of smooth maps u_k:B^n\to Y with equibounded total variation give rise to equivalence classes of Cartesian currents in cart^{1,1}(B^n\times Y) for which we introduce a natural BV-energy. Assume moreover that the first homotopy group of lY is commutative. In any dimension n we prove that every element T in cart^{1,1}(B^n\times Y can be approximated weakly in the sense of currents by a sequence of graphs of smooth maps u_k:B^n\to Y with total variation converging to the BV-energy of T. As a consequence, we characterize the lower semicontinuous envelope of functions of bounded variations from B^n into Y.