Ditchev, A. (2013) “SOME PROPERTIES OF AN ALGEBRA OF ALL SETS OF NATURALS E-REDUCIBLE TO A FIXED SET”, Ann. Sofia Univ. Fac. Math. and Inf, 101, pp. 81–93. Available at: https://admin.uni-sofia.bg/index.php/fmi/article/view/231 (Accessed: 15 January 2025).