'Politehnica'' University of Bucharest. Scientific Bulletin. Series A. Applied Mathematics and Physics (Politehn. Univ. Bucharest Sci. Bull. Ser. A Appl. Math. Phys.) (20230101), 85, no.~3, 3-10. ISSN: 1223-7027 (print).eISSN: 2286-3672.
'Politehnica'' University of Bucharest. Scientific Bulletin. Series A. Applied Mathematics and Physics (Politehn. Univ. Bucharest Sci. Bull. Ser. A Appl. Math. Phys.) (20220101), 84, no.~1, 3-12. ISSN: 1223-7027 (print).eISSN: 2286-3672.