Submitted on: 15 July 2008.
Revised on: 23 July 2008.
Accepted on: 03 August 2008.
Open Mathematics
http://reply.papertrans.cn/3/237/23654/23654-32.png
http://reply.papertrans.cn/3/237/23654/23654-33.png
Submitted on: 07 July 1999.
Revised on: 08 August 1999.
Accepted on: 24 August 1999.
Open Mathematics
http://reply.papertrans.cn/3/237/23654/23654-35.png
Submitted on: 03 January 2001.
Revised on: 17 January 2001.
Accepted on: 10 February 2001.
Open Mathematics