Bound on C0 Semigroup
Theorem
Let $\GF \in \set {\R, \C}$.
Let $X$ be a Banach space over $\GF$.
Let $\family {\map T t}_{t \ge 0}$ be a $C_0$ semigroup.
Let $\struct {\map B X, \norm {\, \cdot \,}_{\map B X} }$ be the space of bounded linear transformations equipped with the canonical norm.
Then there exists $M \ge 1$ and $\omega \ge 0$ such that:
- $\norm {\map T t}_{\map B X} \le M e^{\omega t}$
for each $t \in \hointr 0 \infty$.
Proof
We first show that $\norm {\map T t}_{\map B X}$ is bounded on $\closedint 0 \delta$ for some $\delta > 0$.
Aiming for a contradiction, suppose $\norm {\map T t}_{\map B X}$ is unbounded on $\closedint 0 \delta$ for each $\delta > 0$.
Then for each $n \in \N$ we can pick $t_n \in \closedint 0 {\frac 1 n}$ such that $\norm {\map T {t_n} }_{\map B X} \ge n$.
Then, we have:
- $\ds \sup_{n \mathop \in \N} \norm {\map T {t_n} }_{\map B X} = \infty$
By definition, each $\map T {t_n}$ is a linear operator.
So we can apply Principle of Condensation of Singularities to obtain:
- $\ds \sup_{n \mathop \in \N} \norm {\map T {t_n} x} = \infty$
for some $x \in X$.
But since:
- $\ds \lim_{t \mathop \to 0^+} \map T {t_n} x = x$
and:
- $\ds \lim_{n \mathop \to \infty} t_n = 0$ by the Squeeze Theorem
we must have that:
- $\ds \sup_{n \mathop \in \N} \norm {\map T {t_n} x} < \infty$
by Convergent Sequence in Normed Vector Space is Bounded.
So we have a contradiction, and there exists $\delta > 0$ such that $\norm {\map T t}_{\map B X}$ is bounded on $\closedint 0 \delta$.
Suppose that:
- $\norm {\map T t}_{\map B X} \le M$
for $t \in \closedint 0 \delta$.
Since $\map T 0 = I$, we have $M \ge 1$.
Now for general $t \ge 0$, write:
- $t = N \delta + r$ for $N \in \Z_{\ge 0}$ and $r \in \hointr 0 \delta$.
Then we have:
\(\ds \norm {\map T t}_{\map B X}\) | \(=\) | \(\ds \norm {\map T {N \delta + r} }_{\map B X}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \norm {\map T {N \delta} \map T r}_{\map B X}\) | Definition of Semigroup of Bounded Linear Operators | |||||||||||
\(\ds \) | \(=\) | \(\ds \norm {\paren {\map T \delta}^N \map T r}_{\map B X}\) | ||||||||||||
\(\ds \) | \(\le\) | \(\ds \norm {\paren {\map T \delta}^N}_{\map B X} \norm {\map T r}_{\map B X}\) | Norm on Bounded Linear Transformation is Submultiplicative | |||||||||||
\(\ds \) | \(\le\) | \(\ds \norm {\map T \delta}_{\map B X}^N \norm {\map T r}_{\map B X}\) | Bound on Norm of Power of Element in Normed Algebra | |||||||||||
\(\ds \) | \(=\) | \(\ds M^{N + 1}\) | since $r \in \closedint 0 \delta$ |
Since $t = N \delta + r$, we have:
- $\ds \frac t \delta = N + \frac r \delta \ge N$
so we have:
\(\ds M^{N + 1}\) | \(=\) | \(\ds M M^N\) | ||||||||||||
\(\ds \) | \(\le\) | \(\ds M M^{\frac t \delta}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds M e^{\frac t \delta \log M}\) |
Taking:
- $\ds \omega = \frac {\log M} \delta$
we have:
- $\ds \norm {\map T t}_{\map B X} \le M e^{\omega t}$
for each $t \in \hointr 0 \infty$.
$\blacksquare$
Sources
- 1983: Amnon Pazy: Semigroups of Linear Operators and Applications to Partial Differential Equations ... (previous) ... (next): $1.2$: Strongly Continuous Semigroups of Bounded Linear Operators