Math 2.0

Mathematical logic and foundations Definition

Countable set

Builds on Injective function

Definition

A set AA is countable when there is an injective function f ⁣:A→ℕf \colon A \to \mathbb{N}.

Remarks

Equivalently, AA is finite or countably infinite. A nonempty set is countable exactly when there is a surjection from ℕ\mathbb{N} to it. Some authors use "countable" only for countably infinite sets, and "at most countable" for this notion. Subsets of countable sets are countable.

Reading path

To understand this page from the ground up, read these 15 pages in order. The foundations come first.

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Each page uses the pages above it. The colour of the bar on a box shows how strong the evidence is, and the box also gives it as text. The list has the same pages.
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  1. Element definition
  2. Proposition concept
  3. Set definition
  4. Implication definition
  5. Predicate definition
  6. Ordered pair definition
  7. Existential quantifier definition
  8. Universal quantifier definition
  9. Cartesian product definition
  10. Subset definition
  11. Uniqueness quantifier definition
  12. Binary relation definition
  13. Function definition
  14. Injective function definition
  15. Countable set definition

Where this leads

232 pages build on this one.

Further reading

  1. Web https://en.wikipedia.org/wiki/Countable_set, lead section
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DefinesCountable set
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  • edge instance_of in: one example (0 of 1)
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