# Orbit Stabilizer Theorem: Statement, Proof

The orbit-stabilizer theorem of groups says that the size of a finite group G is the multiplication of the size of the orbit of an element a (in A on which G acts) with that of the stabilizer of a. In this article, we will learn about what are orbits and stabilizers. We will also explain the orbit-stabilizer theorem in detail with proof.

## What are Orbits and Stabilizers

Let G be a group and let A be a finite set on which G acts. That is, we have a map

G×A → A

defined by (g, a) $\mapsto$ g⋅a for all g∈G and a∈A.

Definition of Orbits: The orbit of an element a∈A is denoted by G⋅a and defined by

G⋅a = {g⋅a: g ∈ G}.

Definition of Stabilizers: The stabilizer of an element a ∈ A, denoted by Ga, is defined as follows

Ga = {g ∈ G: g⋅a=a}.

## Orbit Stabilizer Theorem

Statement: If G is a finite group acting on a finite set A, then |G| = |G⋅a| × |Ga| for a∈A. That is,

$|G \cdot a|=\dfrac{|G|}{G_a}$.

## Orbit Stabilizer Theorem Proof

We define a mapping φ: G → G⋅a by

φ(g) = g⋅a ∀ g∈G.

Now for g, h ∈ G, we have

φ(g) = φ(h) ⇔ g⋅a = h⋅a ⇔ g-1h⋅a=a ⇔ g-1h∈Ga ⇔ h∈gGa

This shows that both g and h lie in the same coset for the subgroup Ga and hence φ induces a bijective mapping

$G/G_a \xrightarrow{\sim} G\cdot a$.

As G is a finite group, applying Lagrange’s theorem we complete the proof of the orbit-stabilizer theorem which is

$\dfrac{|G|}{|G_a|} = |G\cdot a|$.

Related Topics:

Group Theory: Definition, Examples, Properties

Abelian Group: Definition, Properties, Examples

Cyclic Group: Definition, Orders, Properties, Examples

Normal Subgroup

Quotient Group

Kernel of a Group Homomorphism

First Isomorphism Theorem

Center of a Group

## FAQs on Orbit Stabilizer Theorem

Q1: What is the orbit-stabilizer theorem?

Answer: Let G be a finite group acting on a finite set X. Let x∈X. Then the orbit-stabilizer theorem says that |Orb(x)| = |G|/|Stab(x)| where Orb(x)= {g⋅x: g ∈ G} and Stab(x)= {g ∈ G: g⋅x=x}.

Q2: What is the orbit of a group?

Answer: Let G be a group that acts on a set X. Then for x ∈X, the set {g⋅x: g ∈ G} is called the orbit of x, denoted by G⋅x or Orb(x).

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