Solution
= Solution
A set is <countable set>[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 $\mathbb N$.
= Solution
A set is <countable set>[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 $\mathbb N$.