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} = { , – 6, – 3, 0, 3, 6, }
A2 = {x ∈ Z : x – 1 is a multiple of 3} = {