N-LEVEL AND N-UPPER LEVEL SOFT SETS. FUDMA JOURNAL OF SCIENCES, [S. l.], v. 3, n. 4, p. 285–290, 2023. Disponível em: http://www.fjsadmin.fudutsinma.edu.ng/index.php/fjs/article/view/1649. Acesso em: 12 sep. 2025.