Assume $\lambda>0$ and a continous function $x(t), x:[0,\infty) \rightarrow \mathbb{R}^n$. Does it hold that
$$ \lim_{t \to \infty}\frac{1}{t} \log(|x(t)|)<0 \Leftrightarrow \exists C>0: \forall t\geq 0 \quad |x(t)| \leq C\cdot\exp^{-\lambda t} $$
?
From right to left is trivial, for the opposite direction I'm unsure wheter it holds.
This problem comes from different definitions for stability, when the statement holds, then they are equivalent.
2026-03-27 03:49:35.1774583375
Can we derive an exponential bound from a special limit of a function?
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If $$ \lim_{t\to\infty}\frac{1}{t} \log(|x(t)|)<0$$ then there exist positive numbers $T$ and $\lambda$ such that $\frac{1}{t} \log(|x(t)|)<-\lambda $ for $t >T$. So $|x(t)|<e^{-\lambda t}$ for $t>T$. Now use the fact that $e^{\lambda t} x(t)$ is continuous, hence bounded on $[0,T]$.