Differentiable Function Theory on Hypersurfaces in R^n (Without Bounds on Their Smoothness) Stephen Semmes Indiana University Mathematics Journal 1990 text pdf 10.1512/iumj.1990.39.39047 10.1512/iumj.1990.39.39047 en Indiana Univ. Math. J. 39 (1990) 985 - 1004 state-of-the-art mathematics http://iumj.org/access/