Spectrum of Element in Maximal Commutative Subalgebra of Unital Banach Algebra
Theorem
Let $\struct {A, \norm {\, \cdot \,} }$ be a unital Banach algebra.
Let $C$ be a commutative subalgebra of $A$ that is maximal with respect to set inclusion.
Let $x \in C$.
Let $\map {\sigma_A} x$ and $\map {\sigma_C} x$ be the spectra of $x$ in $A$ and $C$ respectively.
Then:
- $\map {\sigma_C} x = \map {\sigma_A} x$
Proof
Let $\map G A$ and $\map G C$ be the groups of units of $A$ and $C$ respectively.
From Maximal Commutative Subalgebra of Unital Algebra is Unital, $C$ is unital.
From Spectrum of Element in Unital Subalgebra, we have $\map {\sigma_A} x \subseteq \map {\sigma_C} x$.
Now let $\lambda \in \C \setminus \map {\sigma_A} x$.
Then $\lambda {\mathbf 1}_A - x \in \map G A$.
Let $z = \paren {\lambda {\mathbf 1}_A - x}^{-1}$.
We want to show that $z \in C$.
Let $y \in C$.
Since $y$ commutes with $\lambda {\mathbf 1}_A$ and $z$, we have:
- $z y \paren {\lambda {\mathbf 1}_A - x} z = z \paren {\lambda {\mathbf 1}_A - x} y z$
Since:
- $\paren {\lambda {\mathbf 1}_A - x} z = {\mathbf 1}_A$
and:
- $z \paren {\lambda {\mathbf 1}_A - x} = {\mathbf 1}_A$
we have $z y = y z$.
So $C \cup \set z$ is a set of commuting elements.
Let $\widetilde C$ be the subalgebra generated by $C$.
From Subalgebra Generated by Commuting Elements is Commutative, $\widetilde C$ is commutative.
We also have $C \subseteq \widetilde C$.
Since $C$ maximal with respect to set inclusion, we have:
- $\widetilde C = C$
Hence $z \in C$.
So, we have $\lambda {\mathbf 1}_A x \in \map G C$.
Hence, we have $\lambda \in \C \setminus \map {\sigma_C} x$.
So, we have:
- $\C \setminus \map {\sigma_A} x \subseteq \C \setminus \map {\sigma_C} x$
From Set Complement inverts Subsets, we have:
- $\map {\sigma_C} x \subseteq \map {\sigma_A} x$
Hence, we have:
- $\map {\sigma_C} x = \map {\sigma_A} x$
$\blacksquare$