# Definition:Cyclic Permutation

(Redirected from Definition:K-Cycle)

## Definition

Let $S_n$ denote the symmetric group on $n$ letters.

Let $\rho \in S_n$ be a permutation on $S$.

Then $\rho$ is a cyclic permutation of length $k$ if and only if there exists $k \in \Z: k > 0$ and $i \in \Z$ such that:

$(1): \quad k$ is the smallest such that $\map {\rho^k} i = i$
$(2): \quad \rho$ fixes each $j$ not in $\set {i, \map \rho i, \ldots, \map {\rho^{k - 1} } i}$.

$\rho$ is usually denoted using cycle notation as:

$\begin{pmatrix} i & \map \rho i & \ldots & \map {\rho^{k - 1} } i \end{pmatrix}$

but some sources introduce it using two-row notation:

$\begin{pmatrix} a_1 & a_2 & \cdots & a_k & \cdots & i & \cdots \\ a_2 & a_3 & \cdots & a_1 & \cdots & i & \cdots \end{pmatrix}$

## Also known as

A cyclic permutation of length $k$ is also known as:

a cycle of length $k$
a $k$-cycle
generally, just a cycle.

Some sources give it as circular permutation.

## Examples

### Symmetric Group on $3$ Letters

All the non-identity elements of the Symmetric Group on 3 Letters is a cyclic permutation.

Expressed in cycle notation, they are as follows:

 $\displaystyle e$ $:=$ $\displaystyle \text { the identity mapping}$ $\displaystyle p$ $:=$ $\displaystyle \tuple {1 2 3}$ $\displaystyle q$ $:=$ $\displaystyle \tuple {1 3 2}$

 $\displaystyle r$ $:=$ $\displaystyle \tuple {2 3}$ $\displaystyle s$ $:=$ $\displaystyle \tuple {1 3}$ $\displaystyle t$ $:=$ $\displaystyle \tuple {1 2}$

### Non-Cyclic Element of Symmetric Group on $4$ Letters

Not all permutations are cycles.

Here is an example (written in two-row notation) of a permutation of the Symmetric Group on 4 Letters which is not a cycle:

$\begin{pmatrix} 1 & 2 & 3 & 4 \\ 2 & 1 & 4 & 3 \end{pmatrix}$

## Also see

• Results about cyclic permutations can be found here.