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Vasile Preda
”Gheorghe Mihoc-Caius Iacob” Institute of Mathematical Statistics and Applied Mathematics
”Costin C. Kiriţescu” National Institute of Economic Research
Faculty of Mathematics and Computer Science,
University of BucharestBucharest,Bucharest
Irina Băncescu
”Costin C. Kiriţescu” National Institute of Economic Research
Bucharest, Romania