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Submitted on: 23 September 2014.
Revised on: 12 October 2014.
Accepted on: 03 November 2014.
University Politehnica of Bucharest Scientific Bulletin-Series A-Applied Mathematics and Physics
Submitted on: 02 August 2002.
Revised on: 17 August 2002.
Accepted on: 13 September 2002.
University Politehnica of Bucharest Scientific Bulletin-Series A-Applied Mathematics and Physics
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Submitted on: 30 March 2007.
Revised on: 20 April 2007.
Accepted on: 28 April 2007.
University Politehnica of Bucharest Scientific Bulletin-Series A-Applied Mathematics and Physics