For all humankind
Academicsubsite
ZixuanZhang
ZixuanZhang
Ponder...

Orbit-Stabiliser Theorem

The map is a well-defined bijection .

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