Higher Order Differences and Differentials for Functions in Sobolev Spaces Marvin Marcus Indiana University Mathematics Journal 1976 text pdf 10.1512/iumj.1976.25.25028 10.1512/iumj.1976.25.25028 en Indiana Univ. Math. J. 25 (1976) 321 - 326 state-of-the-art mathematics http://iumj.org/access/