Resolvent Mapping is Analytic/Banach Algebra
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Theorem
Let $\struct {A, \norm {\, \cdot \,} }$ be a unital Banach algebra over $\C$.
Let ${\mathbf 1}_A$ be the identity element of $A$.
Let $x \in A$.
Let $\map {\rho_A} x$ be the resolvent set of $x$ in $A$.
Define $R : \map {\rho_A} x \to A$ by:
- $\map R \lambda = \paren {\lambda {\mathbf 1}_A - x}^{-1}$
Then $R$ is analytic with derivative:
- $\map {R'} \lambda = -\paren {\lambda {\mathbf 1}_A - x}^{-2}$
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Proof
Let $\lambda, \mu \in \map {\rho_A} x$ be such that $\lambda \ne \mu$.
Then, we have:
\(\ds \frac {\paren {\mu {\mathbf 1}_A - x}^{-1} - \paren {\lambda {\mathbf 1}_A - x}^{-1} } {\mu - \lambda}\) | \(=\) | \(\ds \frac {\paren {\mu {\mathbf 1}_A - x}^{-1} \paren { {\mathbf 1}_A - \paren {\mu {\mathbf 1}_A - x} \paren {\lambda {\mathbf 1}_A - x}^{-1} } } {\mu - \lambda}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\paren {\mu {\mathbf 1}_A - x}^{-1} \paren { \paren {\lambda {\mathbf 1}_A - x} - \paren {\mu {\mathbf 1}_A - x} } \paren {\lambda {\mathbf 1}_A - x}^{-1} } {\mu - \lambda}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\paren {\mu {\mathbf 1}_A - x}^{-1} \paren {\lambda - \mu} \paren {\lambda {\mathbf 1}_A - x}^{-1} } {\mu - \lambda}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds -\paren {\mu {\mathbf 1}_A - x}^{-1} \paren {\lambda {\mathbf 1}_A - x}^{-1}\) |
From Resolvent Mapping is Continuous: Banach Algebra, we have:
- $\paren {\mu {\mathbf 1}_A - x}^{-1} \to \paren {\lambda {\mathbf 1}_A - x}^{-1}$ as $\mu \to \lambda$ in $\map {\rho_A} x$.
So, from Product Rule for Sequence in Normed Algebra, we have:
- $-\paren {\mu {\mathbf 1}_A - x}^{-1} \paren {\lambda {\mathbf 1}_A - x}^{-1} \to -\paren {\lambda {\mathbf 1}_A - x}^{-2}$ as $\mu \to \lambda$.
So, we have that:
- $\ds \lim_{\mu \mathop \to \lambda} \frac {\paren {\mu {\mathbf 1}_A - x}^{-1} - \paren {\lambda {\mathbf 1}_A - x}^{-1} } {\mu - \lambda}$ exists
and:
- $\ds \lim_{\mu \mathop \to \lambda} \frac {\map R \mu - \map R \lambda} {\mu - \lambda} = \lim_{\mu \mathop \to \lambda} \frac {\paren {\mu {\mathbf 1}_A - x}^{-1} - \paren {\lambda {\mathbf 1}_A - x}^{-1} } {\mu - \lambda} = -\paren {\lambda {\mathbf 1}_A - x}^{-2}$
So, we obtain:
- $\map {R'} \lambda = -\paren {\lambda {\mathbf 1}_A - x}^{-2}$
$\blacksquare$