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Nonlinear Control Sy.. - Free

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118 CHAPTER 4. LYAPUNOV STABILITY IL NONAUTONOMOUS SYSTEMS<br />

Thus, we have<br />

0 < W(x(to),to) - Ta3(82)<br />

0 < a2(81) - al(R) by (4.23)<br />

which contradicts (4.21). It then follows that our conjecture is indeed correct. Now assume<br />

that t > t'. We have<br />

ai(IIx(t)II) < W(x(t),t) oo as IIxII -4 oo.<br />

Thus for any a > 0 there exists b > 0 such that<br />

If IIxoII < a, then<br />

al(b) > a2(a).<br />

ai(b) > a2(a) ? W(xo,to) >_ W(x(t),t) >- al(IIxII)<br />

Thus, IIxII < a = IIxII < b, and we have that all motions are uniformly bounded. Consider<br />

El, 8, and T as follows:<br />

a2 (b) < al (c) T = al (b)<br />

a3 (a)<br />

and using an argument similar to the one used in the proof of Theorem 4.2, we can show<br />

that for t > to + T<br />

11X(011 < -E<br />

provided that IIxoII < a. This proves that all motions converge uniformly to the origin.<br />

Thus, the origin is asymptotically stable in the large.

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