Part I Preliminaries
Chapter 1 Banach Algebras
1.1 Jacobson radical and derivation
In this book, we assume that every vector space is over the complex field C. A complex associative algebra is a vector space A over the complex field C, with a multiplication satisfying the following properties:
x(yz) = (xy)z, x(y + z) = xy + xz, (y + z)x = yx + zx, λ(xy) = (λx)y = x(λy),
for all x, y, z 2 A and λ 2 C.
If moreover A is a normed space for a norm ||.|| and satisfies the norm inequality for all x, y∈A, we say that A is a normed algebra. Furthermore, If A is a Banach space, we say that A is a Banach algebra.
If there is an element in A, denoted by 1, with 1x = x1 = x, for every x ∈A. Then A is called unital, and 1 is called the unit. If a normed algebra A is not unital, it is always possible to imbed it isometrically in the normed algebra with unit as in [1, Chapter III, Section 1]. In the following of this section, let A be a unital Banach algebra. For some x 2 A, if there is y∈ A, such that xy = yx = 1, then we call x is invertible in A. The set of all the invertible elements in A is denoted by G(A). Then we can define the spectrum of x in A, denoted by σA(x) (or σ(x) for brief) as follows.
It is well known that σ(x) is nonempty and compact by [1, Theorem 3.2.8]. The spectral radius of x in A, denoted by ρA(x) (or ρ(x) for brief) is defined by . Then by Gelfand’s Theorem [1, Theorem 3.2.8]. If ρ(x) = 0, then x is called quasinilpotent. We also need the holomorphic functional calculus, which is also called Riesz functional calculus. One can find the information in [1, Chapter III, Section 3].
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