“SOME PROPERTIES OF AN ALGEBRA OF ALL SETS OF NATURALS E-REDUCIBLE TO A FIXED SET” (2013) Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, 101, pp. 81–93. Available at: https://annual.uni-sofia.bg/index.php/fmi/article/view/231 (Accessed: 15 December 2025).