http://reply.papertrans.cn/1/4/369/369-21.png
http://reply.papertrans.cn/1/4/369/369-22.png
http://reply.papertrans.cn/1/4/369/369-23.png
Submitted on: 07 May 2013.
Revised on: 29 May 2013.
Accepted on: 16 June 2013.
___________________Acta Mathematica Vietnamica---SPRINGER SINGAPORE PTE LTD
http://reply.papertrans.cn/1/4/369/369-25.png
Submitted on: 29 August 2015.
Revised on: 22 December 2015.
Accepted on: 15 February 2016.
___________________Acta Mathematica Vietnamica---SPRINGER SINGAPORE PTE LTD
Submitted on: 14 November 2022.
Revised on: 06 December 2022.
Accepted on: 23 December 2022.
___________________Acta Mathematica Vietnamica---SPRINGER SINGAPORE PTE LTD
Submitted on: 03 January 1999.
Revised on: 06 April 1999.
Accepted on: 06 May 1999.
___________________Acta Mathematica Vietnamica---SPRINGER SINGAPORE PTE LTD
Submitted on: 17 March 2008.
Revised on: 02 July 2008.
Accepted on: 30 August 2008.
___________________Acta Mathematica Vietnamica---SPRINGER SINGAPORE PTE LTD
http://reply.papertrans.cn/1/4/369/369-30.png