(iii)
∪ Aj = X and Ai ∩ Aj = φ, i ≠ j The subsets Ai are called equivalence classes The interesting part of the situation
is that we can go reverse also For example, consider a subdivision of the set Z given
by three mutually disjoint subsets A1, A2 and A3 whose union is Z with
A1 = {x ∈ Z : x is a multiple of 3} = {