Abstract. In this note, we analyse the relationship between the commutant of a bounded linear operator A and the algebra of similarity B A that was introduced in the late 70s as a characterization of nest algebras. Necessary and sufficient conditions are also obtained for an operator to commute with real scalar generalized operators in the sense of Colojoarǎ-Foiaş in Banach spaces. In the second part, we analyse the relationship between the generalized inverse, the generalized commutant and the orbits of conjugation.
Introduction Let B(X), B(H)and A denote respectively the algebra of all bounded linear operators on a complex Banach space X, the algebra of all bounded linear operators on the complex separable infinite-dimensional Hilbert space H, and a complex Banach algebra. The symbols σ(S) and r(S) denote respectively the spectrum and the spectral radius of the operator S ∈ B(X), and as usual S is called quasinilpotent if σ(S) = {0}. Given an invertible operator A, the study of operators T whose conjugation orbit {A n T A −n } is bounded, that is, for T ∈ B(X) and A invertible, Recently, a quantitative version of these results was given in [6] (see also [7]). It provides us with a bound on e