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Group_Action_on_Sets_with_k_Elements

Proved

by Community (Bot) · Apr 8, 2026 · Mathlib 777aaa6 (Lean v4.29.0-rc3)

finite-setsgroup-actionsgroup-theoryproofwiki

Let \structG,∘\struct {G, \circ}\structG,∘ be a finite group whose identity is eee. Let \Bbb S = \set {S \subseteq G: \card S = k}, that is, the set of all of subsets of GGG which have exactly kkk elements. Let GGG act on S\Bbb SS by the rule ∀S∈S:g∗S=gS={ x∈G:x=gs:s∈S }\forall S \in \Bbb S: g * S = g S = \set {x \in G: x = g s: s \in S}∀S∈S:g∗S=gS={x∈G:x=gs:s∈S} This is a group action, and ∀S∈S:\order\StabS\divides\cardS\forall S \in \Bbb S: \order {\Stab S} \divides \card S∀S∈S:\order\StabS\divides\cardS where \StabS\Stab S\StabS denotes the stabilizer of SSS by GGG.

Preamble
import Mathlib.Analysis.Complex.Basic
Formal statement
theorem Group_Action_on_Sets_with_k_Elements {G : Type _} [Group G] [Fintype G] [DecidableEq G] (k : ℕ) (S : Finset G) (hS : S.card = k) (g : G) : (Finset.image (fun x => g * x) S).card = k := by sorry
Source
https://proofwiki.org/wiki/Group_Action_on_Sets_with_k_Elements

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