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14.1: Groups Acting on Sets

( \newcommand{\kernel}{\mathrm{null}\,}\)

Let X be a set and G be a group. A (left) action of G on X is a map G \times X \rightarrow X given by (g,x) \mapsto gx\text{,} where

  1. ex = x for all x \in X\text{;}
  2. (g_1 g_2)x = g_1(g_2 x) for all x \in X and all g_1, g_2 \in G\text{.}

Under these considerations X is called a G-set. Notice that we are not requiring X to be related to G in any way. It is true that every group G acts on every set X by the trivial action (g,x) \mapsto x\text{;} however, group actions are more interesting if the set X is somehow related to the group G\text{.}

Example 14.1

Let G = GL_2( {\mathbb R} ) and X = {\mathbb R}^2\text{.}

Solution

Then G acts on X by left multiplication. If v \in {\mathbb R}^2 and I is the identity matrix, then Iv = v\text{.} If A and B are 2 \times 2 invertible matrices, then (AB)v = A(Bv) since matrix multiplication is associative.

Example 14.2

Let G = D_4 be the symmetry group of a square. If X = \{ 1, 2, 3, 4 \} is the set of vertices of the square, then we can consider D_4 to consist of the following permutations:

\{ (1), (1 \, 3), (2 \, 4), (1 \, 4 \, 3 \, 2), (1 \, 2 \, 3 \, 4), (1 \, 2)(3 \, 4), (1 \, 4)(2 \, 3), (1 \, 3)(2 \, 4) \}\text{.} \nonumber

Solution

The elements of D_4 act on X as functions. The permutation (1 \, 3)(2 \, 4) acts on vertex 1 by sending it to vertex 3\text{,} on vertex 2 by sending it to vertex 4\text{,} and so on. It is easy to see that the axioms of a group action are satisfied.

In general, if X is any set and G is a subgroup of S_X\text{,} the group of all permutations acting on X\text{,} then X is a G-set under the group action

(\sigma, x) \mapsto \sigma(x) \nonumber

for \sigma \in G and x \in X\text{.}

Example 14.3

If we let X = G\text{,} then every group G acts on itself by the left regular representation; that is, (g,x) \mapsto \lambda_g(x) = gx\text{,} where \lambda_g is left multiplication:

\begin{gather*} e \cdot x = \lambda_e x = ex = x\\ (gh) \cdot x = \lambda_{gh}x = \lambda_g \lambda_h x = \lambda_g(hx) = g \cdot ( h \cdot x)\text{.} \end{gather*}

Solution

If H is a subgroup of G\text{,} then G is an H-set under left multiplication by elements of H\text{.}

Example 14.4

Let G be a group and suppose that X=G\text{.} If H is a subgroup of G\text{,} then G is an H-set under conjugation; that is, we can define an action of H on G\text{,}

H \times G \rightarrow G\text{,} \nonumber

via

(h,g) \mapsto hgh^{-1} \nonumber for h \in H and g \in G\text{.}

Solution

Clearly, the first axiom for a group action holds. Observing that

\begin{align*} (h_1 h_2, g) & = h_1 h_2 g (h_1 h_2 )^{-1}\\ & = h_1( h_2 g h_2^{-1}) h_1^{-1}\\ & = (h_1, (h_2, g) )\text{,} \end{align*}

we see that the second condition is also satisfied.

Example 14.5

Let H be a subgroup of G and {\mathcal L}_H the set of left cosets of H\text{.} The set {\mathcal L}_H is a G-set under the action

(g, xH) \mapsto gxH\text{.} \nonumber

Solution

Again, it is easy to see that the first axiom is true. Since (g g')xH = g( g'x H)\text{,} the second axiom is also true.

If G acts on a set X and x, y \in X\text{,} then x is said to be G-equivalent to y if there exists a g \in G such that gx =y\text{.} We write x \sim_G y or x \sim y if two elements are G-equivalent.

Proposition 14.6

Let X be a G-set. Then G-equivalence is an equivalence relation on X\text{.}

Proof

The relation \sim is reflexive since ex = x\text{.} Suppose that x \sim y for x, y \in X\text{.} Then there exists a g such that gx = y\text{.} In this case g^{-1}y=x\text{;} hence, y \sim x\text{.} To show that the relation is transitive, suppose that x \sim y and y \sim z\text{.} Then there must exist group elements g and h such that gx = y and hy= z\text{.} So z = hy = (hg)x\text{,} and x is equivalent to z\text{.}

If X is a G-set, then each partition of X associated with G-equivalence is called an orbit of X under G\text{.} We will denote the orbit that contains an element x of X by {\mathcal O}_x\text{.}

Example 14.7

Let G be the permutation group defined by

G =\{(1), (1 \, 2 \, 3), (1 \, 3 \, 2), (4 \, 5), (1 \, 2 \, 3)(4 \, 5), (1 \, 3 \, 2)(4 \, 5) \} \nonumber

and X = \{ 1, 2, 3, 4, 5\}\text{.}

Solution

Then X is a G-set. The orbits are {\mathcal O}_1 = {\mathcal O}_2 = {\mathcal O}_3 =\{1, 2, 3\} and {\mathcal O}_4 = {\mathcal O}_5 = \{4, 5\}\text{.}

Now suppose that G is a group acting on a set X and let g be an element of G\text{.} The fixed point set of g in X\text{,} denoted by X_g\text{,} is the set of all x \in X such that gx = x\text{.} We can also study the group elements g that fix a given x \in X\text{.} This set is more than a subset of G\text{,} it is a subgroup. This subgroup is called the stabilizer subgroup or isotropy subgroup of x\text{.} We will denote the stabilizer subgroup of x by G_x\text{.}

Remark 14.8

The ideal gas law is easy to remember and apply in solving problems, as long as you get the proper values a

Example 14.9

Let X = \{1, 2, 3, 4, 5, 6\} and suppose that G is the permutation group given by the permutations

\{ (1), (1 \, 2)(3 \, 4 \, 5 \, 6), (3 \, 5)(4 \, 6), (1 \, 2)( 3 \, 6 \, 5 \, 4) \}\text{.} \nonumber

Solution

Then the fixed point sets of X under the action of G are

\begin{gather*} X_{(1)} = X,\\ X_{(3 \, 5)(4 \, 6)} = \{1,2\},\\ X_{(1 \, 2)(3 \, 4 \, 5 \, 6)} = X_{(1 \, 2)(3 \, 6 \,5 \, 4)} = \emptyset\text{,} \end{gather*}

and the stabilizer subgroups are

\begin{gather*} G_1 = G_2 = \{(1), (3 \, 5)(4 \, 6) \},\\ G_3 = G_4 = G_5 = G_6 = \{(1)\}\text{.} \end{gather*}

It is easily seen that G_x is a subgroup of G for each x \in X\text{.}

Proposition 14.10

Let G be a group acting on a set X and x \in X\text{.} The stabilizer group of x\text{,} G_x\text{,} is a subgroup of G\text{.}

Proof

Clearly, e \in G_x since the identity fixes every element in the set X\text{.} Let g, h \in G_x\text{.} Then gx = x and hx = x\text{.} So (gh)x = g(hx) = gx = x\text{;} hence, the product of two elements in G_x is also in G_x\text{.} Finally, if g \in G_x\text{,} then x = ex = (g^{-1}g)x = (g^{-1})gx = g^{-1} x\text{.} So g^{-1} is in G_x\text{.}

We will denote the number of elements in the fixed point set of an element g \in G by |X_g| and denote the number of elements in the orbit of x \in X by |{\mathcal O}_x|\text{.} The next theorem demonstrates the relationship between orbits of an element x \in X and the left cosets of G_x in G\text{.}

Theorem 14.11

Let G be a finite group and X a finite G-set. If x \in X\text{,} then |{\mathcal O}_x| = [G:G_x]\text{.}

Proof

We know that |G|/|G_x| is the number of left cosets of G_x in G by Lagrange's Theorem (Theorem 6.10). We will define a bijective map \phi between the orbit {\mathcal O}_x of X and the set of left cosets {\mathcal L}_{G_x} of G_x in G\text{.} Let y \in {\mathcal O}_x\text{.} Then there exists a g in G such that g x = y\text{.} Define \phi by \phi( y ) = g G_x\text{.} To show that \phi is one-to-one, assume that \phi(y_1) = \phi(y_2)\text{.} Then

\phi(y_1) = g_1 G_x = g_2 G_x = \phi(y_2)\text{,} \nonumber

where g_1 x = y_1 and g_2 x = y_2\text{.} Since g_1 G_x = g_2 G_x\text{,} there exists a g \in G_x such that g_2 = g_1 g\text{,}

y_2 = g_2 x = g_1 g x = g_1 x = y_1; \nonumber

consequently, the map \phi is one-to-one. Finally, we must show that the map \phi is onto. Let g G_x be a left coset. If g x = y\text{,} then \phi(y) = g G_x\text{.}

 


This page titled 14.1: Groups Acting on Sets is shared under a GNU Free Documentation License 1.3 license and was authored, remixed, and/or curated by Thomas W. Judson (Abstract Algebra: Theory and Applications) via source content that was edited to the style and standards of the LibreTexts platform.

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