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Submitted on: 03 October 1999.
Revised on: 06 November 1999.
Accepted on: 18 November 1999.
___________________IEEE-CAA Journal of Automatica Sinica
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Submitted on: 04 November 2001.
Revised on: 31 December 2001.
Accepted on: 23 February 2002.
___________________IEEE-CAA Journal of Automatica Sinica
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