Banach algebra

In functional analysis, a Banach algebra is an associative algebra over the real or complex numbers which at the same time is also a Banach space. The algebra multiplication and the Banach space norm are required to be related by the following inequality:

||xy|| ≤ ||x|| ||y||     for all x and y

(i.e., the norm of the product is less than or equal to the product of the norms.) This ensures that the multiplication operation is continuous.

A Banach algebra is called "unitary" if it has an identity element for the multiplication and "commutative" if its multiplication is commutative.

Banach algebras can also be defined over fields of p-adic numbers. This is part of p-adic analysis.

Examples

• The set of real (or complex) numbers is a Banach algebra with norm given by the absolute value.
• The set of all real or complex n-by-n matrices becomes a Banach algebra if we equip it with a sub-multiplicative matrix norm.
• Take the Banach space Rⁿ (or Cⁿ) with norm ||x|| = max |x_i| and define multiplication componentwise: (x₁,...,x_n)(y₁,...,y_n) = (x₁y₁,...,x_ny_n).
• The quaternions form a 4-dimensional real Banach algebra, with the norm being given by the absolute value of quaternions.
• The algebra of bounded real- or complex-valued functions defined on some set (with pointwise multiplication and the supremum norm) is a Banach algebra.
• The algebra of continuous real- or complex-valued functions on some compact space (again with pointwise operations and supremum norm) is a Banach algebra.
• The algebra of all continuous linear operators on a Banach space (with functional composition as multiplication and the operator norm as norm) is a Banach algeba.
• The continuous linear operators on a Hilbert space form a C-star-algebra and therefore a Banach algebra.
• If G is a locally compact Hausdorff topological group and μ its Haar measure, then the Banach space L¹(G) of all μ-integrable functions on G becomes a Banach algebra under the convolution xy(g) = ∫ x(h) y(h^-1g) dμ(h) for x, y in L¹(G).

Properties

Several elementary functions which are defined via power series may be defined in any unitary Banach algebra; examples include the exponential function and the trigonometric functions. The formula for the geometric series and the binomial theorem also remain valid in general unitary Banach algebras.

The set of invertible elements in any unitary Banach algebra is an open set, and the inversion operation on this set is continuous, so that it forms a topological group under multiplication.

Unitary Banach algebras provide a natural setting to study general spectral theory. The spectrum of an element x consists of all those scalars λ such that x -λ1 is not invertible. (In the Banach algebra of all n-by-n matrices mentioned above, the spectrum of a matrix coincides with the set of all its eigenvalues.) The spectrum of any element is compact. If the base field is the field of complex numbers, then the spectrum of any element is non-empty.

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:

• Every real Banach algebra which is a division algebra is isomorphic to the reals, the complexes, or the quaternions.
• Every unitary real Banach algebra with no zero divisors, and in which every principal ideal is closed, is isomorphic to the reals, the complexes, or the quaternions.
• Every commutative real unitary noetherian Banach algebra with no zero divisors is isomorphic to the real or complex numbers.
• Every commutative real unitary noetherian Banach algebra (possibly having zero divisors) is finite-dimensional.