Spectral Representation of Certain Semi-Groups of Operators Cassius Tulcea Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58006 10.1512/iumj.1959.8.58006 en Indiana Univ. Math. J. 8 (1959) 95 - 109 state-of-the-art mathematics http://iumj.org/access/