http://reply.papertrans.cn/1/44/4333/4333-31.png
http://reply.papertrans.cn/1/44/4333/4333-32.png
Submitted on: 20 October 2013.
Revised on: 22 January 2014.
Accepted on: 05 March 2014.
___________________Bulletin of Mathematical Analysis and Applications
Submitted on: 29 March 2009.
Revised on: 04 July 2009.
Accepted on: 21 August 2009.
___________________Bulletin of Mathematical Analysis and Applications
Submitted on: 28 August 1998.
Revised on: 06 October 1998.
Accepted on: 01 November 1998.
___________________Bulletin of Mathematical Analysis and Applications
http://reply.papertrans.cn/1/44/4333/4333-36.png