Weakly differentiable functions on varifolds Ulrich Menne Indiana University Mathematics Journal 2016 text pdf 10.1512/iumj.2016.65.5829 10.1512/iumj.2016.65.5829 en Indiana Univ. Math. J. 65 (2016) 977 - 1088 state-of-the-art mathematics http://iumj.org/access/