If A is a unital commutative Banach algebra such that | | a 2 | | = | | a | | 2 {\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.
(Gamelin 2005, p. 25) - Gamelin, Theodore W. (2005). Uniform Algebras. American Mathematical Soc. ISBN 978-0-8218-4049-8. ↩