A countable set A is usually written as A = \ {a_1, a_2, \dots\} which indicates the one-to-one correspondence of A with the set of natural numbers \Nat. This notation is also known as the enumeration ...

Remember that in :numref:`Chapter %s <combinatorics>` we defined, for each natural number n, the set [n] = \{0, 1, \ldots, n-1\}. We then said that a set A is finite if there is a bijection between A ...