Completeness

Z = { . . . , - 2 , - 1 , 0 , 1 , 2 , . . . } = { 0 , 1 , - 1 , 2 , - 2 , . . . } .

2.1.4. Example. N can be written as the union of infinitely many disjoint infinite sets, each of which is enumerable.