A Property for Certain Derivatives Clifford Weil Indiana University Mathematics Journal 1974 text pdf 10.1512/iumj.1974.23.23044 10.1512/iumj.1974.23.23044 en Indiana Univ. Math. J. 23 (1974) 527 - 536 state-of-the-art mathematics http://iumj.org/access/