Past exam of the mathematics course of the University of Cambridge / 2022 / ia / Paper 4 / 8F / a / Solution

A set is countable when it is finite or its elements can be put in one-to-one correspondence with a subset of $\mathbb{N}$. Equivalently, there is an injection from the set into $\mathbb{N}$.

Solved by gpt-5.6-sol high.