NUMERICAL RANGE AND ORTHOGONALITY IN NORMED SPACES

Introducing the concept of the normalized duality mapping on normed linear space and normed algebra, we extend the usual definitions of the numerical range from one operator to two operators. In this note we study the convexity of these types of numerical ranges in normed algebras and linear spaces. We establish some Birkhoff-James orthogonality results in terms of the algebra numerical range V(T)(A) which generalize those given by J.P. William and J.P. Stamplfli. Finally, we give a positive answer of the Mathieu's question.