Banach function algebra

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In functional analysis a Banach function algebra on a compact Hausdorff space X is unital subalgebra, A of the commutative C*-algebra C(X) of all continuous, complex valued functions from X, together with a norm on A which makes it a Banach algebra.

For every $x\in X$ define $\varepsilon_x(f)=f(x)\ (f\in A)$ . Then $\varepsilon_x$ is a non-zero homomorphism (character) on $A$ .

Theorem: A Banach function algebra is semisimple (that is its Jacobson radical is equal to zero) and each commutative unital, semisimple Banach algebra is isomorphic (via the Gelfand transform) to a Banach function algebra on its character space (the space of algebra homomorphisms from A into the complex numbers given the relative weak* topology).

If the norm on $A$ is the uniform norm (or sup-norm) on $X$ , then $A$ is called a uniform algebra. Uniform algebras are an important special case of Banach function algebras.