Decompositions of linear maps

Abstract

In the first part we show that the decomposition of a bounded selfadjoint linear map from a ${C^\ast }$-algebra into a given von Neumann algebra as a difference of two bounded positive linear maps is always possible if and only if that range algebra is a “strictly finite” von Neumann algebra of type I. In the second part we define a “polar decomposition” for some bounded linear maps and show that polar decomposition is possible if and only if the map satisfies a certain “norm condition". We combine the concepts of polar and positive decompositions to show that polar decomposition for a selfadjoint map is equivalent to a strict Hahn-Jordan decomposition (see Theorems 2.2.4 and 2.2.8).