A Formula for the Numerical Range of Elementary Operators

Abstract

Let BH be the algebra of bounded linear operators on a complex Hilbert space H. For k-tuples of elements of BH, A=A1,…,Ak and B=B1,…,Bk, let RA,B denote the elementary operator on BH defined by RA,BX=∑i=1kAiXBi. In this paper, we prove the following formula for the numerical range of RA,B: VRA,B,BBH=[∪U∈UHW(∑i=1kUAiU*Bi)-]-, where UH is the set of unitary operators.