On the Preservation of Classes of Functions A. Zygmund Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58056 10.1512/iumj.1959.8.58056 en Indiana Univ. Math. J. 8 (1959) 889 - 895 state-of-the-art mathematics http://iumj.org/access/