TheoremBase

Orbits and Stabilizers

For an action of a group on a set, the orbit of a point is the set of its images under the group, and its stabilizer is the set of group elements fixing it.

Statement

In the setting of Routine Facts, Continued: Typed Arithmetic, Maps and Predicativity, let ⋅\cdot be an action of a group GG on a set XX, and let x∈Xx\in X.

Orbit. The orbit of xx is

Gx={g⋅x:g∈G}Gx=\{g\cdot x:g\in G\}

(a set by Subclasses of Sets Are Sets, the Union and Power Set of a Set Exist Uniquely, Binary Unions of Sets Are Sets, and the Universal Class Is Proper (i)).

Stabilizer. The stabilizer of xx is

Gx={g∈G:g⋅x=x}G_{x}=\{g\in G:g\cdot x=x\}

(a set by Subclasses of Sets Are Sets, the Union and Power Set of a Set Exist Uniquely, Binary Unions of Sets Are Sets, and the Universal Class Is Proper (i)).

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