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Nonlinear Control Sy.. - Free

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82 CHAPTER 3. LYAPUNOV STABILITY I. AUTONOMOUS SYSTEMS<br />

3.6.1 Exponential Stability<br />

As mentioned earlier, exponential stability is the strongest form of stability seen so far. The<br />

advantage of this notion is that it makes precise the rate at which trajectories converge<br />

to the equilibrium point. Our next theorem gives a sufficient condition for exponential<br />

stability.<br />

Theorem 3.4 Suppose that all the conditions of Theorem 3.2 are satisfied, and in addition<br />

assume that there exist positive constants K1, K2, K3 and p such that<br />

Klllxllp < V(x) < K21IxljP<br />

V(x) -K3IIxIlP.<br />

Then the origin is exponentially stable. Moreover, if the conditions hold globally, the x = 0<br />

is globally exponentially stable.<br />

Proof: According to the assumptions of Theorem 3.4, the function V (x) satisfies Lemma<br />

3.1 with al and a2(), satisfying somewhat strong conditions. Indeed, by assumption<br />

or<br />

Klllxllp < V(x)

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