Cioban, M., Cozma, D. and Dohotaru, L. 2021. International Symposium "Actual Problems of Mathematics and Informatics" dedicated to the 90th birthday of professor Ion Valutse. Acta et Commentationes Exact and Natural Sciences. 10, 2 (Jan. 2021), 117–124. DOI:https://doi.org/10.36120/2587-3644.v10i2.117–124.