Monotone operator functions on $C^*$-algebra

Abstract

The article is devoted to investigation of classes of functions monotone as functions on general $C^*$-algebras that are not necessarily the $C^*$-algebras of all bounded linear operators on a Hilbert space as it is in classical case of matrix and operator monotone functions. We show that for general $C^*$-algebras the classes of monotone functions coincide with the standard classes of matrix and operator monotone functions. For every class we give exact characterization of $C^*$-algebras that have this class of monotone functions, providing at the same time a monotonicity characterization of subhomogeneous $C^*$-algebras. We use this characterization to generalize one function based monotonicity conditions for commutativity of a $C^*$-algebra, to one function based monotonicity conditions for subhomogeneity. As a $C^*$-algebraic counterpart of standard matrix and operator monotone scaling, we investigate, by means of projective $C^*$-algebras and relation lifting, the existence of $C^*$-subalgebras of a given monotonicity class.