http://reply.papertrans.cn/3/265/26417/26417-31.png
http://reply.papertrans.cn/3/265/26417/26417-32.png
http://reply.papertrans.cn/3/265/26417/26417-33.png
http://reply.papertrans.cn/3/265/26417/26417-34.png
http://reply.papertrans.cn/3/265/26417/26417-35.png
Submitted on: 24 February 2000.
Revised on: 03 April 2000.
Accepted on: 19 April 2000.
Research in Mathematics---TAYLOR & FRANCIS LTD