A set \(D\) is called
Branches: 1
Corollaries: 2
Definitions: 3 4 5 6 7 8 9 10 11
Explanations: 12 13
Parts: 14
Proofs: 15 16 17 18 19
Propositions: 20 21 22
This is equivalent with saying that there is a surjective function function \(f:\mathbb N\mapsto D.\) Some books define countability by requiring a bijective function between $D$ and $\mathbb N,$ but the above definition has the advantage that it is also applicable for a finite set $D.$ Thus, all finite sets are countable. ↩