'Politehnica'' University of Bucharest. Scientific Bulletin. Series A. Applied Mathematics and Physics (Politehn. Univ. Bucharest Sci. Bull. Ser. A Appl. Math. Phys.) (20210101), 83, no.~2, 49-58. ISSN: 1223-7027 (print).eISSN: 2286-3672.
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