# Bijection between S x T and T x S

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## Theorem

Let $S$ and $T$ be sets.

Let $S \times T$ be the Cartesian product of $S$ and $T$.

Then there exists a bijection from $S \times T$ to $T \times S$.

## Proof

Let $\phi: S \times T \to T \times S$ be the mapping defined as:

- $\forall \tuple {s, t} \in S \times T: \map \phi {s, t} = \tuple {t, s}$

Then $\phi$ is the bijection required, as follows:

The domain of $\phi$ is $S \times T$.

Let $\tuple {t, s} \in T \times S$.

Then there exists $\tuple {s, t} \in S \times T$ such that $\map \phi {s, t} = \tuple {t, s}$.

Thus $\phi$ is a surjection.

Let $\map \phi {s_1, t_1} = \map \phi {s_2, t_2}$ for some $\tuple {s_1, t_1}$ and $\tuple {s_2, t_2}$ in $S \times T$.

Then:

\(\displaystyle \map \phi {s_1, t_1}\) | \(=\) | \(\displaystyle \map \phi {s_2, t_2}\) | |||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \tuple {t_1, s_1}\) | \(=\) | \(\displaystyle \tuple {t_2, s_2}\) | Definition of $\phi$ | |||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \tuple {s_1, t_1}\) | \(=\) | \(\displaystyle \tuple {s_2, t_2}\) | Definition of Ordered Pair |

and so $\phi$ is an injection.

Hence the result by definition of bijection.

$\blacksquare$

## Sources

- 1971: Allan Clark:
*Elements of Abstract Algebra*... (previous) ... (next): Chapter $1$: Mappings: $\S 13 \beta$