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Submitted on: 14 January 2014.
Revised on: 03 May 2014.
Accepted on: 19 May 2014.
___________________Frontiers of Mathematics in China
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Submitted on: 25 February 2001.
Revised on: 22 April 2001.
Accepted on: 07 June 2001.
___________________Frontiers of Mathematics in China
Submitted on: 18 May 2015.
Revised on: 21 June 2015.
Accepted on: 21 July 2015.
___________________Frontiers of Mathematics in China
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