Submitted on: 28 May 1999.
Revised on: 21 July 1999.
Accepted on: 07 August 1999.
___________________CHINESE ANNALS OF MATHEMATICS SERIES B
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Submitted on: 07 September 1999.
Revised on: 10 December 1999.
Accepted on: 23 January 2000.
___________________CHINESE ANNALS OF MATHEMATICS SERIES B
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Submitted on: 18 March 2019.
Revised on: 21 April 2019.
Accepted on: 27 May 2019.
___________________CHINESE ANNALS OF MATHEMATICS SERIES B
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