On the Essential Spectrum of a Class of Operators in Hilbert Space
Manfred Möller · Mathematische Nachrichten · 1998
Abstract Sufficient conditions are given such that the product T1T2 of two unbounded operators in Hilbert spaces is essentially selfadjoint and that the nonzero numbers in the essential spectrum of the closure of T1T2 coincide with the nonzero numbers in the essential spectrum of T2T1. If the essential spectrum of the closure of T2T1 only consists of zero and M is a bounded operator, then several formulas for the essential spectrum of the closure of T2T1 + M are given.