Spectral Conditions and Reducibility of Operator Semigroups M. LambrouW. LongstaffHeydar Radjavi Indiana University Mathematics Journal 1992 text pdf 10.1512/iumj.1992.41.41026 10.1512/iumj.1992.41.41026 en Indiana Univ. Math. J. 41 (1992) 449 - 464 state-of-the-art mathematics http://iumj.org/access/