Toeplitz Operators and k-Spectral Sets D. Clark Indiana University Mathematics Journal 1984 text pdf 10.1512/iumj.1984.33.33006 10.1512/iumj.1984.33.33006 en Indiana Univ. Math. J. 33 (1984) 127 - 141 state-of-the-art mathematics http://iumj.org/access/