The Spectrum of a Normal Operator and the Problem of Filling in Holes Robert OlinJames Thomson Indiana University Mathematics Journal 1977 text pdf 10.1512/iumj.1977.26.26041 10.1512/iumj.1977.26.26041 en Indiana Univ. Math. J. 26 (1977) 541 - 544 state-of-the-art mathematics http://iumj.org/access/