The Spectra of Elementary Operators R. Curto Indiana University Mathematics Journal 1983 text pdf 10.1512/iumj.1983.32.32017 10.1512/iumj.1983.32.32017 en Indiana Univ. Math. J. 32 (1983) 193 - 197 state-of-the-art mathematics http://iumj.org/access/