00HW Proposition 2.25. An element fโ๐f\in\mathcal{A} is invertible if and only if fโก(z)โ 0f(z)\neq 0 for any zโ๐โก(๐)z\in\mathfrak{M}(\mathcal{A}). ([Ber, Corollary 1.2.4])