Countability

• Countability #Card
  • • $\text{card } X = \aleph_0$ if $X$ is equipotent with $\mathbb{N}$. - $X$ is countable if it is the image of $\mathbb{N}$ (i.e., finite or of cardinality $\aleph_0$).

Back

• $\text{card } X = \aleph_0$ if $X$ is equipotent with $\mathbb{N}$.
• $X$ is countable if it is the image of $\mathbb{N}$ (i.e., finite or of cardinality $\aleph_0$).

Lean

Countable X / 本ノート prop_I_5_3