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Submitted on: 12 October 2004.
Revised on: 18 November 2004.
Accepted on: 16 December 2004.
Periodica Mathematica Hungarica
Submitted on: 14 July 2009.
Revised on: 17 August 2009.
Accepted on: 27 August 2009.
Periodica Mathematica Hungarica