# Cartesian Product of Countable Sets is Countable

## Theorem

The cartesian product of two countable sets is countable.

### Corollary

Let $k$ be an integer such that $k > 1$.

Then the cartesian product of $k$ countable sets is countable.

## Informal Proof

Let $S = \set {s_0, s_1, s_2, \dotsc}$ and $T = \set {t_0, t_1, t_2, \dotsc}$ be countable sets.

If both $S$ and $T$ are finite, the result follows immediately.

Suppose either of $S$ or $T$ (or both) is countably infinite.

We can write the elements of $S \times T$ in the form of an infinite table:

$\begin{array} {*{4}c} \tuple {s_0, t_0} & \tuple {s_0, t_1} & \tuple {s_0, t_2} & \cdots \\ \tuple {s_1, t_0} & \tuple {s_1, t_1} & \tuple {s_1, t_2} & \cdots \\ \tuple {s_2, t_0} & \tuple {s_2, t_1} & \tuple {s_2, t_2} & \cdots \\ \vdots & \vdots & \vdots & \ddots \\ \end{array}$

This table clearly contains all the elements of $S \times T$.

Now we can count the elements of $S \times T$ by processing the table diagonally.

First we pick $\tuple {s_0, t_0}$.

Then we pick $\tuple {s_0, t_1}, \tuple {s_1, t_0}$. Then we pick $\tuple {s_0, t_2}, \tuple {s_1, t_1}, \tuple {s_2, t_0}$.

We can see that all the elements of $S \times T$ will (eventually) be listed, and there is a specific number (element of $\N$) to index each of its elements with.

Thus we have the required one-to-one correspondence between $S \times T$ and $\N$, and our assertion is proved.

$\blacksquare$

## Formal Proof 1

Let $S, T$ be countable sets.

From the definition of countable, there exists a injection from $S$ to $\N$, and similarly one from $T$ to $\N$.

Hence there exists an injection $g$ from $S \times T$ to $\N^2$.

Now let us investigate the cardinality of $\N^2$.

From the Fundamental Theorem of Arithmetic, every natural number greater than $1$ has a unique prime decomposition.

Thus, if a number can be written as $2^n 3^m$, it can be done thus in only one way.

So, consider the function $f: \N^2 \to \N$ defined by:

$\map f {n, m} = 2^n 3^m$.

Now suppose $\exists m, n, r, s \in \N$ such that $\map f {n, m} = \map f {r, s}$.

Then $2^n 3^m = 2^r 3^s$ so that $n = r$ and $m = s$.

Thus $f$ is an injection, whence $\N^2$ is countably infinite.

Now we see that as $g$ and $f$ are injective, it follows from Composite of Injections is Injection that $f \circ g: S \times T \to \N$ is also injective.

Hence the result.

$\blacksquare$

## Formal Proof 2

Let $S, T$ be countable sets.

Let $S = \set {s_1, s_2, s_3, \dotsc}$ and $T = \set {t_1, t_2, t_3, \dotsc}$ be enumerations of $S$ and $T$ respectively.

Let $f: S \times T: \N$ be the mapping defined as:

$\forall \tuple {s_k, t_l} \in S \times T: \map f {s_k, t_l} = \dfrac {\paren {k + l - 1} \paren {k + l - 2} } 2 + \dfrac {l + \paren {-1}^{k + 1} } 2 k + \dfrac {1 + \paren {-1}^{k + l - 1} } 2 l$

Then $f$ gives an enumeration of $S \times T$.

This enumeration can be depicted schematically as:

$\begin {array} {} \tuple {s_1, t_1} & & \tuple {s_1, t_2} & \to & \tuple {s_1, t_3} & & \tuple {s_1, t_4} & \to & \dotsc \\ \downarrow & \nearrow & & \swarrow & & \nearrow & \dotsc \\ \tuple {s_2, t_1} & & \tuple {s_2, t_2} & & \tuple {s_2, t_3} & \dotsc \\ & \swarrow & & \nearrow & \dotsc \\ \tuple {s_3, t_1} & & \tuple {s_2, t_3} & \dotsc \\ \downarrow & \nearrow & \dotsc \\ \tuple {s_3, t_1} & \dotsc \\ \vdots \\ \end{array}$

Hence the result.

$\blacksquare$