Submitted on: 14 November 2018.
Revised on: 28 January 2019.
Accepted on: 11 February 2019.
___________________Communications in Mathematics and Applications
Submitted on: 07 November 2001.
Revised on: 31 December 2001.
Accepted on: 11 February 2002.
___________________Communications in Mathematics and Applications
Submitted on: 09 January 2006.
Revised on: 15 February 2006.
Accepted on: 15 March 2006.
___________________Communications in Mathematics and Applications
Submitted on: 02 January 2003.
Revised on: 20 April 2003.
Accepted on: 06 May 2003.
___________________Communications in Mathematics and Applications
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Submitted on: 31 August 2004.
Revised on: 11 December 2004.
Accepted on: 17 January 2005.
___________________Communications in Mathematics and Applications