Element of Unital Banach Algebra Close to Identity is Invertible
Theorem
Let $\Bbb F \in \set {\R, \C}$.
Let $\struct {A, \norm \cdot}$ be a unital Banach algebra over $\Bbb F$ with identity element $\mathbf 1_A$.
Let $a \in A$ be such that:
- $\norm {\mathbf 1_A - a} < 1$
Then $a$ is invertible with inverse element $a^{-1}$ satisfying:
- $\ds \norm {a^{-1} } \le \frac 1 {1 - \norm {\mathbf 1_A - a} }$
Proof
Let:
- $x = \mathbf 1_A - a$
From Bound on Norm of Power of Element in Normed Algebra, we have:
- $\norm {x^n} \le \norm x^n$
for each $n \in \Z_{\ge 0}$.
Then we have:
- $\ds \sum_{n \mathop = 0}^\infty \norm {x^n} \le \sum_{n \mathop = 0}^\infty \norm x^n$
Since $\norm x < 1$, we have:
- $\ds \sum_{n \mathop = 0}^\infty \norm x^n$ converges
from Sum of Infinite Geometric Sequence.
So:
- $\ds \sum_{n \mathop = 0}^\infty \norm {x^n}$ converges.
Since $A$ is a Banach space, we have:
- $\ds \sum_{n \mathop = 0}^\infty x^n$ converges.
by Absolutely Convergent Series in Normed Vector Space is Convergent iff Space is Banach.
Let:
- $\ds b = \sum_{n \mathop = 0}^\infty x^n$
We show that:
- $\ds a b = \mathbf 1_A$
and:
- $\ds b a = \mathbf 1_A$
Note that:
- $a = \mathbf 1_A - x$
Note that for each $N \in \N$, we have:
\(\ds \paren {\mathbf 1_A - x} \sum_{n \mathop = 0}^N x^n\) | \(=\) | \(\ds \sum_{n \mathop = 0}^N \paren {\mathbf 1_A - x} x^n\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{n \mathop = 0}^N \paren {x^n - x^{n + 1} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \mathbf 1_A - x^{N + 1}\) |
and similarly:
\(\ds \paren {\sum_{n \mathop = 0}^N x^n} \paren {\mathbf 1_A - x}\) | \(=\) | \(\ds \sum_{n \mathop = 0}^N x^n \paren {\mathbf 1_A - x}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{n \mathop = 0}^N \paren {x^n - x^{n + 1} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \mathbf 1_A - x^{N + 1}\) |
We have:
\(\ds \norm {\mathbf 1_A - x^{N + 1} - \mathbf 1_A}\) | \(=\) | \(\ds \norm {x^{N + 1} }\) | ||||||||||||
\(\ds \) | \(\le\) | \(\ds \norm x^{N + 1}\) | ||||||||||||
\(\ds \) | \(\to\) | \(\ds 0\) | since $\norm x < 1$ |
so that:
- $\mathbf 1_A - x^{N + 1} \to \mathbf 1_A$
So taking $N \to \infty$ in:
- $\ds a \sum_{n \mathop = 0}^N x^n = \mathbf 1_A - x^{N + 1}$
we obtain:
- $a b = \mathbf 1_A$
from Product Rule for Sequence in Normed Algebra.
Similarly taking $N \to \infty$ in:
- $\ds \paren {\sum_{n \mathop = 0}^N x^n} a = \mathbf 1_A - x^{N + 1}$
we obtain:
- $b a = \mathbf 1_A$
So $a$ is invertible with inverse element $b$.
It remains to show that:
- $\ds \norm b \le \frac 1 {1 - \norm x}$
For each $N \in \N$, we have that:
\(\ds \norm {\sum_{n \mathop = 0}^N x^n}\) | \(\le\) | \(\ds \sum_{n \mathop = 0}^N \norm {x^n}\) | Norm Axiom $\text N 3$: Triangle Inequality | |||||||||||
\(\ds \) | \(\le\) | \(\ds \sum_{n \mathop = 0}^N \norm x^n\) | Bound on Norm of Power of Element in Normed Algebra | |||||||||||
\(\ds \) | \(\le\) | \(\ds \sum_{n \mathop = 0}^\infty \norm x^n\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {1 - \norm x}\) | Sum of Infinite Geometric Sequence |
Taking $N \to \infty$, we have:
- $\ds \norm b \le \frac 1 {1 - \norm x} = \frac 1 {1 - \norm {\mathbf 1_A - a} }$
from Modulus of Limit: Normed Vector Space.
$\blacksquare$
Sources
- 2011: Graham R. Allan and H. Garth Dales: Introduction to Banach Spaces and Algebras ... (previous) ... (next): $4.4$: The group of units