Geometric and Topological Properties of the Numerical Range Jim Agler Indiana University Mathematics Journal 1982 text pdf 10.1512/iumj.1982.31.31053 10.1512/iumj.1982.31.31053 en Indiana Univ. Math. J. 31 (1982) 767 - 777 state-of-the-art mathematics http://iumj.org/access/