Submitted on: 23 July 2006.
Revised on: 16 October 2006.
Accepted on: 06 November 2006.
___________________Collectanea Mathematica---SPRINGER-VERLAG ITALIA SRL
Submitted on: 16 May 2024.
Revised on: 25 August 2024.
Accepted on: 05 October 2024.
___________________Collectanea Mathematica---SPRINGER-VERLAG ITALIA SRL
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Submitted on: 21 October 2010.
Revised on: 18 February 2011.
Accepted on: 10 March 2011.
___________________Collectanea Mathematica---SPRINGER-VERLAG ITALIA SRL
Submitted on: 27 June 2018.
Revised on: 16 September 2018.
Accepted on: 01 November 2018.
___________________Collectanea Mathematica---SPRINGER-VERLAG ITALIA SRL
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