# Symmetric group

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The group of all permutations (self-bijections) of a set $X$ with the operation of composition (see Permutation group). The symmetric group on a set $X$ is denoted by $S ( X)$. For equipotent $X$ and $X ^ \prime$ the groups $S ( X)$ and $S ( X ^ \prime )$ are isomorphic. The symmetric group of a finite set $X = \{ 1 \dots n \}$ is denoted by $S _ {n}$. Every abstract group is isomorphic to a subgroup of the symmetric group $S ( X)$ of some set $X$( Cayley's theorem).

Let $X$ be a finite set. Every permutation $\pi$ on $X$ can be uniquely described as a product of disjoint cycles (the (disjoint) cycle decomposition of a permutation); the sequence of integers

$$z ( \pi ) = ( a _ {1} \dots a _ {n} ),$$

where $a _ {i}$ is the number of cycles of length $i$ of $\pi$, is called the cycle type (or cycle index) of $\pi$. Two permutations $\pi$ and $\pi ^ \prime$ are conjugate in $S _ {n}$ if and only if they have the same cycle type. Permutations with cycle type

$$\{ n - 2, 1, 0 \dots 0 \}$$

are called transpositions; they form a system of generators for $S _ {n}$. The set of transpositions $\Sigma = \{ ( i, i + 1) : i = 1 \dots n - 1 \}$ is a minimal set of generators for $S _ {n}$. In general, a set $\Delta = \{ ( i _ {k} , j _ {k} ) : i _ {k} \neq j _ {k} \}$ generates $S _ {n}$ if the graph with $X$ as set of vertices and the pairs $( i _ {k} , j _ {k} )$ as edges is a tree . The number of such generating sets is $n ^ {n - 2 }$.

The alternating group $A _ {n}$ is a normal subgroup of $S _ {n}$. If $n \neq 4$, $A _ {n}$ is the unique non-trivial proper normal subgroup, and $S _ {4}$ contains only one other non-trivial normal subgroup — the Klein four-group: $K _ {4} = \{ 1, ( 1 2)( 3, 4), ( 1 3)( 2 4), ( 1 4) ( 2 3) \}$. For $n \leq 4$ the group $S _ {n}$ is solvable, but for $n \geq 5$ it is not solvable and $A _ {n}$ is a simple non-Abelian group. Hölder's theorem: For $n \neq 2, 6$, the group $S _ {n}$ is complete (see Complete group). The group $S _ {2}$ is commutative, and $S _ {6}$ has an outer automorphism of order 2.

All maximal intransitive and imprimitive subgroups in $S _ {n}$ are known. For each partition $n = m _ {1} + m _ {2}$, $m _ {1} \neq m _ {2}$, the only maximal intransitive subgroups in $S _ {n}$ are the subgroups $S _ {m _ {1} } \times S _ {m _ {2} }$. The only transitive imprimitive maximal subgroups in $S _ {n}$ are the wreath products (cf. Wreath product) of $S _ {m _ {1} }$ with $S _ {m _ {2} }$( for every decomposition $n = m _ {1} m _ {2}$). The primitive maximal subgroups in $S _ {n}$ have not yet been described (1992), but certain infinite series of them are known. For example, $S _ {n}$ acts naturally on the set $B _ {n} ^ {m}$ of all subsets of $m$ elements of $\{ 1 \dots n \}$; for $n \neq 2m$ this determines a primitive group of permutations of $B _ {n} ^ {m}$. It is maximal in $S ( B _ {n} ^ {m} )$ if $( m, n) \neq ( 2, 6)$, $( 2, 8)$, $( 3, 10)$, $( 4, 12)$, $( m, 2m)$, $( m, 2m + 1)$( see ). Another series is obtained by considering the group $\Gamma L _ {n} ( q)$ of semi-linear transformations of the $n$- dimensional vector space over the finite field of $q$ elements. This group defines a primitive group of permutations of the Grassmann manifold $G _ {n,m} ( q)$, $1 \leq m \leq n - 1$, which is maximal in $S ( G _ {n,m} ( q))$ for $n \geq 3$.

Let $X$ be an infinite set. The group of all permutations of $X$ that move only a finite number of elements of $X$ is called the finitary, or restricted, symmetric group on $X$, and is denoted by $\mathop{\rm SF} ( X)$; its subgroup of even permutations is called the finite, or restricted, alternating group on $X$, and is denoted by $\mathop{\rm AF} ( X)$. The subgroups $\mathop{\rm SF} ( X)$ and $\mathop{\rm AF} ( X)$ are normal in $S ( X)$. More generally, let $\alpha$ be the cardinality of $X$ and let $\beta \leq \alpha$ be an infinite cardinal; the set of permutations of $X$ which move at most $\beta$ elements of $X$ is a subgroup of $S ( X)$, denoted by $S _ \beta ( X)$. Along with $\mathop{\rm SF} ( X)$ and $\mathop{\rm AF} ( X)$, the groups $S _ \beta ( X)$ are normal subgroups in $S ( X)$ for all $\beta \leq \alpha$, and in fact there are no other non-trivial normal subgroups in $S ( X)$( the Schreier–Ulam–Baer theorem, see ).