SOME PROPERTIES OF AN ALGEBRA OF ALL SETS OF NATURALS E-REDUCIBLE TO A FIXED SET. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 101, p. 81–93, 2013. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/231. Acesso em: 15 dec. 2025.