Submitted on: 26 June 2000.
Revised on: 05 October 2000.
Accepted on: 18 October 2000.
___________________ESAIM-Mathematical Modelling and Numerical Analysis
Submitted on: 31 May 2024.
Revised on: 20 September 2024.
Accepted on: 14 October 2024.
___________________ESAIM-Mathematical Modelling and Numerical Analysis
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