Submitted on: 22 April 1999.
Revised on: 12 July 1999.
Accepted on: 06 August 1999.
___________________Asian-European Journal of Mathematics
Submitted on: 11 August 2011.
Revised on: 23 October 2011.
Accepted on: 04 November 2011.
___________________Asian-European Journal of Mathematics
Submitted on: 07 July 2007.
Revised on: 11 October 2007.
Accepted on: 29 November 2007.
___________________Asian-European Journal of Mathematics
Submitted on: 19 March 1998.
Revised on: 29 May 1998.
Accepted on: 16 July 1998.
___________________Asian-European Journal of Mathematics
http://reply.papertrans.cn/1/29/2854/2854-35.png
Submitted on: 13 March 2004.
Revised on: 24 April 2004.
Accepted on: 10 May 2004.
___________________Asian-European Journal of Mathematics