A note on the positivity of inverse operators acting on $C^{\ast }$-algebras

Abstract

For a positive and invertible linear operator $T$ acting on a $C^{\ast }$-algebra, we give necessary and sufficient criteria for the inverse operator $T^{-1}$ to be positive, too. Moreover, a simple counterexample shows that $T^{-1}$ need not be positive even if $T$ is unital and its spectrum is contained in the unit circle.

16 pages