An equivalence relation (reflexive + symmetric + transitive) partitions the set into disjoint equivalence classes. Every element belongs to exactly one class. The equivalence class of a = {x in A : xRa}. The number of equivalence relations on n elements equals the Bell number B(n): B(1)=1, B(2)=2, B(3)=5, B(4)=15, B(5)=52.