# Definition:Sign of Permutation

## Definition

Let $n \in \N$ be a natural number.

Let $\N_n$ denote the set of natural numbers $\set {1, 2, \ldots, n}$.

Let $\tuple {x_1, x_2, \ldots, x_n}$ be an ordered $n$-tuple of real numbers.

Let $\pi$ be a permutation of $\N_n$.

Let $\map {\Delta_n} {x_1, x_2, \ldots, x_n}$ be the product of differences of $\tuple {x_1, x_2, \ldots, x_n}$.

Let $\pi \cdot \map {\Delta_n} {x_1, x_2, \ldots, x_n}$ be defined as:

- $\pi \cdot \map {\Delta_n} {x_1, x_2, \ldots, x_n} := \map {\Delta_n} {x_{\map \pi 1}, x_{\map \pi 2}, \ldots, x_{\map \pi n} }$

The **sign of $\pi \in S_n$** is defined as:

- $\map \sgn \pi = \begin{cases} \dfrac {\Delta_n} {\pi \cdot \Delta_n} & : \Delta_n \ne 0 \\ 0 & : \Delta_n = 0 \end{cases}$

## Also denoted as

Some sources use $\map \epsilon \pi$ for $\map \sgn \pi$.

In physics and applied mathematics, the symbol $e_{i j k}$ can often be found for this concept, referred to as the **alternating symbol**, defined as:

- $e_{i j k} = \begin{cases} 1 & : \text {if $\tuple {i, j, k}$ is an even permutation of $\tuple {1, 2, 3}$} \\ -1 & : \text {if $\tuple {i, j, k}$ is an odd permutation of $\tuple {1, 2, 3}$} \\ 0 & : \text {if any two of $\set {i, j, k}$ are equal} \end{cases}$

## Also known as

The **sign** of a permutation is also known as its **signature** or **signum**.

However, on $\mathsf{Pr} \infty \mathsf{fWiki}$ **signum** is not recommended, in order to keep this concept separate from the **signum function** on a set of numbers.

## Sources

- 1965: J.A. Green:
*Sets and Groups*... (previous) ... (next): $\S 7.4$. Kernel and image: Example $142$ - 1996: John F. Humphreys:
*A Course in Group Theory*... (previous) ... (next): Chapter $9$: Permutations: Definition $9.15$ - 1980: A.J.M. Spencer:
*Continuum Mechanics*... (previous) ... (next): $2.1$: Matrices: $(2.10)$