http://reply.papertrans.cn/3/278/27770/27770-31.png
Submitted on: 16 May 2004.
Revised on: 11 June 2004.
Accepted on: 27 June 2004.
SEMIGROUP FORUM
Submitted on: 18 November 2015.
Revised on: 26 November 2015.
Accepted on: 16 December 2015.
SEMIGROUP FORUM
http://reply.papertrans.cn/3/278/27770/27770-34.png
http://reply.papertrans.cn/3/278/27770/27770-35.png
Submitted on: 07 March 1998.
Revised on: 15 March 1998.
Accepted on: 11 April 1998.
SEMIGROUP FORUM---SPRINGER