On a Formula of Kac and Achiezer I. Hirschman, Jr. Indiana University Mathematics Journal 1967 text pdf 10.1512/iumj.1967.16.16012 10.1512/iumj.1967.16.16012 en Indiana Univ. Math. J. 16 (1967) 167 - 196 state-of-the-art mathematics http://iumj.org/access/