Definition 28.2.1 (Invertible).label Let $A$ be a unital Banach algebra and $x \in A$, then $x$ is invertible if there exists $x^{-1}\in A$ such that $xx^{-1}= x^{-1}x = 1$. The set $G(A)$ denotes the collection of all invertible elements in $A$.
Definition 28.2.1 (Invertible).label Let $A$ be a unital Banach algebra and $x \in A$, then $x$ is invertible if there exists $x^{-1}\in A$ such that $xx^{-1}= x^{-1}x = 1$. The set $G(A)$ denotes the collection of all invertible elements in $A$.