We consider two cases where the symmetric group acts on subsets of the natural set. The first is a special case of the second. Then some other examples are discussed.
The general linear group GLn(R)
acts on the set of vectors in
Rn.
The dihedral group Dn acts on the vertices of the regular
n-gon.
The map
2
is bijective: the inverse of
2 is (
-1)2. Thus, for an arbitrary
element {a, b} from X:
and similarly
Finally, we check that the map S5 ->
Sym(X),
->
2 is a morphism S5 ->
Sym(X), so that we indeed have a permutation representation.
Let
,
be arbitrary
elements of S5. We need to verify that (
)2 =
2
2, that is, that left and right
hand side represent the same bijection. This is simple: for each
{a,b} in X we have
)2({a,b})
= {(
)(a), (
)(b)} = {
(a)),
(b))} =
(a),
(b)}) =
2({a,b})) = (
2)({a,b}).We name the subsets of {1,2,3,4,5} of size 2 by letters as follows:
| a = {1,2} | f = {2,4} | |
| b = {1,3} | g = {2,5} | |
| c = {1,4} | h = {3,4} | |
| d = {1,5} | i = {3,5} | |
| e = {2,3} | j = {4,5} |
Thus, for example:
Of course we can restrict the action on the pairs of {1,2,3,4,5} to any subgroup of S5. In particular to A5.
| |Xk| = |
| |
X}.
This defines a bijection
Sn the element
Of course we can restrict the action on Xk
to any subgroup of Sn,
in particular to An.
/n
induces the n-cycle
on these vertices.
A reflection in the axis through the center of the n-gon and the vertex 1 induces the permutation
in case n is even, and
in case n is odd.
This yields a permutation representation of Dn into Sn.