In the book "Nonlinear systems" (Khalil, 3rd edition, page 150), an equilibrium is said to be globally uniformly asymptotically stable if
- it is uniformly stable,
- $\delta(\epsilon)$ can be chosen to satisfy $\lim_{\epsilon\to\infty}\delta(\epsilon)=\infty$,
- for each pair of positive numbers $\eta$ and c, there is $T(\eta,c)>0$ such that $$||x||<\eta, \forall t\geq t_0+T(\eta,c), \forall ||x(t_0)||<c.$$
I have a question about the second condition. Why shall we need it? Is it necessary?