Positive decompositions of selfadjoint operators

Abstract

Given a linear bounded selfadjoint operator a on a complex separable Hilbert space H, we study the decompositions of a as a difference of two positive operators whose ranges satisfy an angle condition. These decompositions are related to the canonical decompositions of the indefinite metric space (H, 〈, 〉a), associated to a. As an application, we characterize the orbit of congruence of a in terms of its positive decompositions.