Uniform algebra

In functional analysis, a uniform algebra A on a compact Hausdorff topological space X is a closed (with respect to the uniform norm) subalgebra of the C*-algebra C(X) (the continuous complex-valued functions on X) with the following properties:^[1]

the constant functions are contained in A

for every x, y ${\displaystyle \in }$ X there is f${\displaystyle \in }$A with f(x)${\displaystyle \neq }$f(y). This is called separating the points of X.

As a closed subalgebra of the commutative Banach algebra C(X) a uniform algebra is itself a unital commutative Banach algebra (when equipped with the uniform norm). Hence, it is, (by definition) a Banach function algebra.

A uniform algebra A on X is said to be natural if the maximal ideals of A are precisely the ideals ${\displaystyle M_{x}}$ of functions vanishing at a point x in X.

If A is a unital commutative Banach algebra such that ${\displaystyle ||a^{2}||=||a||^{2}}$ for all a in A, then there is a compact Hausdorff X such that A is isomorphic as a Banach algebra to a uniform algebra on X. This result follows from the spectral radius formula and the Gelfand representation.

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