Disk algebra

In functional and complex analysis, the disk algebra A(D) (also spelled disc algebra) is the set of holomorphic functions

f : D → C,

where D is the open unit disk in the complex plane C, f extends to a continuous function on the closure of D. That is,

$A(\mathbf {D} )=H^{\infty }(\mathbf {D} )\cap C({\overline {\mathbf {D} }}),$

where H^∞(D) denotes the Banach space of bounded analytic functions on the unit disc D (i.e. a Hardy space). When endowed with the pointwise addition (f+g)(z)=f(z)+g(z), and pointwise multiplication (fg)(z)=f(z)g(z), this set becomes an algebra over C, since if ƒ and g belong to the disk algebra then so do ƒ + g and ƒg.

Given the uniform norm,

$\|f\|=\sup\{|f(z)|\mid z\in \mathbf {D} \}=\max\{|f(z)|\mid z\in {\overline {\mathbf {D} }}\},$

by construction it becomes a uniform algebra and a commutative Banach algebra.

By construction the disc algebra is a closed subalgebra of the Hardy space H^∞. In contrast to the stronger requirement that a continuous extension to the circle exists, it is a lemma of Fatou that a general element of H^∞ can be radially extended to the circle almost everywhere.