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Nonlinear Control Sy.. - Free

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8.4. STABILITY OF FEEDBACK INTERCONNECTIONS 213<br />

Figure 8.6: The Feedback <strong>Sy</strong>stem S.<br />

Proof: We prove the theorem assuming that H1 is passive and H2 is strictly passive with<br />

finite gain. The opposite case (i.e., H2 is passive and H1 is strictly passive with finite gain)<br />

is entirely similar. Proceeding as in Theorem 8.4 (equation (8.20), we obtain<br />

Iy1TIIX < b-1IIU1TIIX (8.21)<br />

so that y1 E X whenever u1 E X. Also Y2 = H2y1. Then IIy2TIIX = II(H2y1)TIIx, and since<br />

H2 has finite gain, we obtain<br />

Ily2zTlix < ry(H2)IIy1TIIx

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