Submitted on: 10 January 2015.
Revised on: 23 February 2015.
Accepted on: 28 February 2015.
Rivista di Matematica della Universita di Parma
Submitted on: 09 June 2010.
Revised on: 09 July 2010.
Accepted on: 02 August 2010.
Rivista di Matematica della Universita di Parma
http://reply.papertrans.cn/3/273/27254/27254-13.png
Submitted on: 03 June 2002.
Revised on: 02 July 2002.
Accepted on: 27 July 2002.
Rivista di Matematica della Universita di Parma
http://reply.papertrans.cn/3/273/27254/27254-15.png
Submitted on: 20 June 2016.
Revised on: 29 June 2016.
Accepted on: 22 July 2016.
Rivista di Matematica della Universita di Parma
http://reply.papertrans.cn/3/273/27254/27254-17.png
Submitted on: 29 December 2003.
Revised on: 03 February 2004.
Accepted on: 10 February 2004.
Rivista di Matematica della Universita di Parma
http://reply.papertrans.cn/3/273/27254/27254-19.png
Submitted on: 03 March 2019.
Revised on: 23 March 2019.
Accepted on: 14 April 2019.
Rivista di Matematica della Universita di Parma