1. Numerical Ranges of Operators on Normed Spaces and Elements of Normed Algebras;Bonsall,1971
2. Numerical Ranges II;Bonsall,1973
3. Numerical Ranges. Studies in Functional Analysis;Bonsall,1980
4. Numerical ranges of the powers of an operator;Choi;J. Math. Anal. Appl.,2010
5. Numerical Range;Gustafson,1997