Orbit-Stabiliser Theorem
The map is a well-defined bijection .
Theorem 4.9 (Orbit-Stabiliser Theorem)
Suppose acts on a set . Then for any , the formula
defines a well-defined bijection
Counting form
For finite , the bijection gives , or equivalently .
Proof
Let and define . If , then for some , so ; hence is well-defined. It is surjective by the definition of . If , then , so and is injective.
Examples
For the action on the vertices of a regular -gon, an orbit has size and a vertex stabiliser has size , giving .
For the isometry group of a cube acting on face centres, an orbit has size and a face stabiliser has size , giving .
Related
Stated in
- Theorem 4.9 (Orbit-Stabiliser Theorem)ยง4.2 Orbit-Stabiliser Theorem
