# Existence and Uniqueness of Cycle Decomposition

## Theorem

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

Every element of $S_n$ may be uniquely expressed as a cycle decomposition, up to the order of factors.

## Proof

By definition, a cycle decomposition of an element of $S_n$ is a product of disjoint cycles.

### Construction of Disjoint Permutations

Let $\sigma \in S_n$ be a permutation on $S_n$.

Let $\RR_\sigma$ be the equivalence defined in Permutation Induces Equivalence Relation.

Let $\N_n$ be used to denote the (one-based) initial segment of natural numbers:

$\N_n = \closedint 1 n = \set {1, 2, 3, \ldots, n}$

Let $\N_n / \RR_\sigma = \set {E_1, E_2, \ldots, E_m}$ be the quotient set of $\N_n$ determined by $\RR_\sigma$.

$E \in \N_n / \RR_\sigma \implies E \subseteq \N_n$

For any $E_i \in \N_n / \RR_\sigma$, let $\rho_i: \paren {\N_n \setminus E_i} \to \paren {\N_n \setminus E_i}$ be the identity mapping on $\N_n \setminus E_i$.

By Identity Mapping is Permutation, $\rho_i$ is a permutation.

Also, let $\phi_i = \tuple {E_i, E_i, R}$ be a relation where $R$ is defined as:

$\forall x, y \in E_i: \tuple {x, y} \in R \iff \map \sigma x = y$

It is easily seen that $\phi_i$ is many to one.

For all $x \in E_i$:

 $\ds x$ $\RR_\sigma$ $\ds \map \sigma x$ $\ds \leadsto \ \$ $\ds \map \sigma x$ $\in$ $\ds E_i$ $\ds \leadsto \ \$ $\ds \sigma \sqbrk {E_i}$ $\subseteq$ $\ds E_i$

which shows that $\phi_i$ is left-total.

It then follows from the definition of a mapping that $\phi_i: E_i \to E_i$ is a mapping defined by:

$\map {\phi_i} x = \map \sigma x$

It is seen that $\phi_i$ is an injection because $\sigma$ is an injection.

So by Injection from Finite Set to Itself is Permutation, $\phi_i$ is a permutation on $E_i$.

By Intersection with Relative Complement is Empty, $E_i$ and $\N_n \setminus E_i$ are disjoint.

$E_i \cup \paren {\N_n \setminus E_i} = \N_n$

So by Union of Bijections with Disjoint Domains and Codomains is Bijection, let the permutation $\sigma_i \in S_n$ be defined by:

$\map {\sigma_i} x = \map {\paren {\phi_i \cup \rho_i} } x = \begin{cases} \map \sigma x & : x \in E_i \\ x & : x \notin E_i \end{cases}$

By Equivalence Classes are Disjoint, it follows that each of the $\sigma_i$ are disjoint.

$\Box$

### These Permutations are Cycles

It is now to be shown that all of the $\sigma_i$ are cycles.

From Order of Element Divides Order of Finite Group, there exists $\alpha \in \Z_{\gt 0}$ such that $\sigma_i^\alpha = e$, and so:

$\map {\sigma_i^\alpha} x = \map e x = x$

By Well-Ordering Principle, let $k = \min \set {\alpha \in \N_{\gt 0}: \map {\sigma_i^\alpha} x = x}$

Because $\sigma_i$ fixes each $y \notin E_i$, it suffices to show that:

$E_i = \set {x, \map {\sigma_i} x, \ldots, \map {\sigma_i^{k - 1} } x}$

for some $x \in E_i$.

If $x \in E_i$, then for all $t \in \Z$:

$x \mathrel {\RR_\sigma} \map {\sigma_i^t} x \implies \map {\sigma_i^t} x \in E_i$

It has been shown that:

$(1) \quad \set {x, \map {\sigma_i} x, \ldots, \map {\sigma_i^{k - 1} } x} \subseteq E_i$

Let $x, y \in E_i$.

Then:

 $\ds x$ $\RR_\sigma$ $\ds y$ $\ds \leadsto \ \$ $\ds \map {\sigma_i^t} x$ $=$ $\ds y$ for some $t \in \Z$, by Permutation Induces Equivalence Relation $\ds \leadsto \ \$ $\ds \map {\sigma_i^{k q + r} } x$ $=$ $\ds y$ for some $q \in \Z$, and $0 \le r \lt k$ by Division Theorem $\ds \leadsto \ \$ $\ds \map {\sigma_i^r \sigma_i^{k q} } x$ $=$ $\ds y$ $\ds \leadsto \ \$ $\ds \map {\sigma_i^r} x$ $=$ $\ds y$ Fixed Point of Permutation is Fixed Point of Power

It has been shown that:

$(2) \quad E_i \subseteq \set {x, \map {\sigma_i} x, \ldots, \map {\sigma_i^{k - 1} } x}$

Combining $(1)$ and $(2)$ yields:

$E_i = \set {x, \map {\sigma_i} x, \ldots, \map {\sigma_i^{k - 1} } x}$

$\Box$

### The Product of These Cycles form the Permutation

Finally, it is now to be shown that $\sigma = \sigma_1 \sigma_2 \cdots \sigma_m$.

$x \in \N_n \implies x \in E_j$

for some $j \in \set {1, 2, \ldots, m}$.

Therefore:

 $\ds \map {\sigma_1 \sigma_2 \cdots \sigma_m} x$ $=$ $\ds \map {\sigma_1 \sigma_2 \cdots \sigma_j} x$ $\ds$ $=$ $\ds \map {\sigma_j} x$ because $\sigma_j \sqbrk {E_j} = E_j$ $\ds$ $=$ $\ds \map \sigma x$ Definition of $\sigma_i$

and so existence of a cycle decomposition has been shown.

$\Box$

### Uniqueness of Cycle Decomposition

Take the cycle decomposition of $\sigma$, which is $\sigma_1 \sigma_2 \cdots \sigma_m$.

Let $\tau_1 \tau_2 \cdots \tau_s$ be some product of disjoint cycles such that $\sigma = \tau_1 \tau_2 \cdots \tau_s$.

It is assume that this product describes $\sigma$ completely and does not contain any duplicate $1$-cycles.

Let $x$ be a moved element of $\sigma$.

Then there exists a $j \in \set {1, 2, \ldots, s}$ such that $\map {\tau_j} x \ne x$.

And so:

 $\ds \map \sigma x$ $=$ $\ds \map {\tau_1 \tau_2 \cdots \tau_j} x$ $\ds$ $=$ $\ds \map {\tau_j} x$ Power of Moved Element is Moved

It has already been shown that $x \in E_i$ for some $i \in \set {1, 2, \ldots, m}$.

Therefore:

 $\ds \map {\sigma_i} x$ $=$ $\ds \map {\tau_j} x$ $\ds \map {\sigma_i^2} x$ $=$ $\ds \map {\tau_{j \prime} \tau_j} x$ by Power of Moved Element is Moved $\ds$ $=$ $\ds \map {\tau_j^2} x$ because $\map {\sigma_i^2} x \ne \map {\sigma_i} x$ and this product is disjoint $\ds \vdots$ $\vdots$ $\ds \vdots$ $\ds \map {\sigma_i^{k - 1} } x$ $=$ $\ds \map {\tau_j^{k - 1} } x$

This effectively shows that $\sigma_i = \tau_j$.

Doing this for every $E_i$ implies that $m = s$ and that there exists a $\rho \in S_m$ such that:

$\sigma_{\map \rho i} = \tau_i$

In other words, $\tau_1 \tau_2 \cdots \tau_m$ is just a reordering of $\sigma_1 \sigma_2 \cdots \sigma_m$.

$\blacksquare$