Submitted on: 30 November 2003.
Revised on: 18 March 2004.
Accepted on: 25 April 2004.
___________________Demonstratio Mathematica---DE GRUYTER POLAND SP Z O O
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Submitted on: 30 August 2008.
Revised on: 21 October 2008.
Accepted on: 14 November 2008.
___________________Demonstratio Mathematica---DE GRUYTER POLAND SP Z O O
Submitted on: 01 April 2019.
Revised on: 23 July 2019.
Accepted on: 20 September 2019.
___________________Demonstratio Mathematica---DE GRUYTER POLAND SP Z O O
Submitted on: 28 November 1999.
Revised on: 16 January 2000.
Accepted on: 04 March 2000.
___________________Demonstratio Mathematica---DE GRUYTER POLAND SP Z O O
Submitted on: 30 March 2009.
Revised on: 07 July 2009.
Accepted on: 17 July 2009.
___________________Demonstratio Mathematica---DE GRUYTER POLAND SP Z O O
Submitted on: 21 February 2010.
Revised on: 20 March 2010.
Accepted on: 30 April 2010.
___________________Demonstratio Mathematica---DE GRUYTER POLAND SP Z O O
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