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Nonlinear Control Sy.. - Free

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4.3. STABILITY THEOREMS 113<br />

Thus, W5(x, t) > ki V5 (x) for some constant k1, which implies that is positive definite.<br />

It is decrescent since<br />

IW5(x,t)I R such<br />

that<br />

(i) W (x, t) is (a) positive definite, and (b) decrescent, and<br />

(ii) The derivative of W (x, t) is negative definite in D, then<br />

the equilibrium state is uniformly asymptotically stable.<br />

Remarks: There is an interesting difference between Theorems 4.1 and 4.2. Namely, in<br />

Theorem 4.1 the assumption of W(., ) being decrescent is optional. Indeed, if W(., ) is

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