A partition of a nonempty set A can be defined in three ways. Definition 1 states that the collection S consists of pairwise disjoint nonempty subsets of A, and every element of A belongs to a subset in S. Definition 2 states that the collection S consists of nonempty subsets of A, and every element of A belongs to exactly one subset in S. Definition 3 states that the collection S consists of subsets of A that satisfy three properties: (1) every subset in S is nonempty, (2) every two subsets of A are equal or disjoint, and (3) the union of all subsets in S is A.
a) any collection S of subsets of A satisfying Definition 1 satisfies Definition 2.
b) any collection S of subsets of A satisfying Definition 2 satisfies Definition 3.
c) any collection S of subsets of A satisfying Definition 3 satisfies Definition 1.