Banach algebra


In mathematics, especially functional analysis, a Banach algebra, named after Stefan Banach, is an associative algebra A over the real or complex numbers that at the same time is also a Banach space, that is, a normed space that is complete in the metric induced by the norm. The norm is required to satisfy
This ensures that the multiplication operation is continuous.
A Banach algebra is called unital if it has an identity element for the multiplication whose norm is 1, and commutative if its multiplication is commutative.
Any Banach algebra can be embedded isometrically into a unital Banach algebra so as to form a closed ideal of. Often one assumes a priori that the algebra under consideration is unital: for one can develop much of the theory by considering and then applying the outcome in the original algebra. However, this is not the case all the time. For example, one cannot define all the trigonometric functions in a Banach algebra without identity.
The theory of real Banach algebras can be very different from the theory of complex Banach algebras. For example, the spectrum of an element of a nontrivial complex Banach algebra can never be empty, whereas in a real Banach algebra it could be empty for some elements.
Banach algebras can also be defined over fields of p-adic numbers. This is part of p-adic analysis.

Examples

The prototypical example of a Banach algebra is, the space of continuous functions on a locally compact space that vanish at infinity. is unital if and only if X is compact. The complex conjugation being an involution, is in fact a C*-algebra. More generally, every C*-algebra is a Banach algebra.
The algebra of the quaternions is not a complex Banach algebra, for if is a complex Banach algebra that is also a division algebra, then , since if is a point in the non-empty spectrum of, is not invertible, hence since is a division algebra, whence .

Properties

Several elementary functions that are defined via power series may be defined in any unital Banach algebra; examples include the exponential function and the trigonometric functions, and more generally any entire function. The formula for the geometric series remains valid in general unital Banach algebras. The binomial theorem also holds for two commuting elements of a Banach algebra.
The set of invertible elements in any unital Banach algebra is an open set, and the inversion operation on this set is continuous, so that it forms a topological group under multiplication.
If a Banach algebra has unit 1, then 1 cannot be a commutator; i.e.,   for any x, y ∈ A. This is because xy and yx have the same spectrum except possibly 0.
The various algebras of functions given in the examples above have very different properties from standard examples of algebras such as the reals. For example:
Unital Banach algebras over the complex field provide a general setting to develop spectral theory. The spectrum of an element x ∈ A, denoted by, consists of all those complex scalars λ such that x − λ1 is not invertible in A. The spectrum of any element x is a closed subset of the closed disc in C with radius ||x|| and center 0, and thus is compact. Moreover, the spectrum of an element x is non-empty and satisfies the spectral radius formula:
Given x ∈ A, the holomorphic functional calculus allows to define ƒ ∈ A for any function ƒ holomorphic in a neighborhood of Furthermore, the spectral mapping theorem holds:
When the Banach algebra A is the algebra L of bounded linear operators on a complex Banach space X , the notion of the spectrum in A coincides with the usual one in operator theory. For ƒ ∈ C, one sees that:
The norm of a normal element x of a C*-algebra coincides with its spectral radius. This generalizes an analogous fact for normal operators.
Let A  be a complex unital Banach algebra in which every non-zero element x is invertible. For every a ∈ A, there is λ ∈ C such that
a − λ1 is not invertible hence a = λ1 : this algebra A is naturally isomorphic to C.

Ideals and characters

Let A  be a unital commutative Banach algebra over C. Since A is then a commutative ring with unit, every non-invertible element of A belongs to some maximal ideal of A. Since a maximal ideal in A is closed, is a Banach algebra that is a field, and it follows from the Gelfand–Mazur theorem that there is a bijection between the set of all maximal ideals of A and the set Δ of all nonzero homomorphisms from A  to C. The set Δ is called the "structure space" or "character space" of A, and its members "characters."
A character χ is a linear functional on A that is at the same time multiplicative, χ = χ χ, and satisfies χ = 1. Every character is automatically continuous from A  to C, since the kernel of a character is a maximal ideal, which is closed. Moreover, the norm of a character is one. Equipped with the topology of pointwise convergence on A, the character space, Δ, is a Hausdorff compact space.
For any x ∈ A,
where is the Gelfand representation of x defined as follows: is the continuous function from Δ to C given by   The spectrum of in the formula above, is the spectrum as element of the algebra C of complex continuous functions on the compact space Δ. Explicitly,
As an algebra, a unital commutative Banach algebra is semisimple if and only if its Gelfand representation has trivial kernel. An important example of such an algebra is a commutative C*-algebra. In fact, when A is a commutative unital C*-algebra, the Gelfand representation is then an isometric *-isomorphism between A and C.

Banach *-algebras

A Banach *-algebra A is a Banach algebra over the field of complex numbers, together with a map * : A → A that has the following properties:
  1. * = x for all x in A.
  2. * = x* + y* for all x, y in A.
  3. for every λ in C and every x in A; here, denotes the complex conjugate of λ.
  4. * = y* x* for all x, y in A.
In other words, a Banach *-algebra is a Banach algebra over which is also a *-algebra.
In most natural examples, one also has that the involution is isometric, that is,
Some authors include this isometric property in the definition of a Banach *-algebra.