Spectral Theory - A Definition of Spectrum

A Definition of Spectrum

Consider a bounded linear transformation T defined everywhere over a general Banach space. We form the transformation:

Here I is the identity operator and ζ is a complex number. The inverse of an operator T, that is T −1, is defined by:

If the inverse exists, T is called regular. If it does not exist, T is called singular.

With these definitions, the resolvent set of T is the set of all complex numbers ζ such that R_ζ exists and is bounded. This set often is denoted as ρ(T). The spectrum of T is the set of all complex numbers ζ such that R_ζ fails to exist or is unbounded. Often the spectrum of T is denoted by σ(T). The function R_ζ for all ζ in ρ(T) (that is, wherever R_ζ exists) is called the resolvent of T. The spectrum of T is therefore the complement of the resolvent set of T in the complex plane. Every eigenvalue of T belongs to σ(T), but σ(T) may contain non-eigenvalues.

This definition applies to a Banach space, but of course other types of space exist as well, for example, topological vector spaces include Banach spaces, but can be more general. On the other hand, Banach spaces include Hilbert spaces, and it is these spaces that find the greatest application and the richest theoretical results. With suitable restrictions, much can be said about the structure of the spectra of transformations in a Hilbert space. In particular, for self-adjoint operators, the spectrum lies on the real line and (in general) is a spectral combination of a point spectrum of discrete eigenvalues and a continuous spectrum.