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Submitted on: 20 August 2007.
Revised on: 24 October 2007.
Accepted on: 24 November 2007.
___________________Discrete and Continuous Dynamical Systems-Series S
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Submitted on: 30 April 2008.
Revised on: 06 July 2008.
Accepted on: 07 August 2008.
___________________Discrete and Continuous Dynamical Systems-Series S
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Submitted on: 18 March 2015.
Revised on: 30 May 2015.
Accepted on: 01 July 2015.
___________________Discrete and Continuous Dynamical Systems-Series S
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