Stabilizers of concrete group actions

Content created by Egbert Rijke, Fredrik Bakke, Jonathan Prieto-Cubides, Julian KG, fernabnor and louismntnu.

Created on 2022-07-09.
Last modified on 2023-11-24.

module group-theory.stabilizer-groups-concrete-group-actions where
open import foundation.0-connected-types
open import foundation.dependent-pair-types
open import foundation.function-types
open import foundation.mere-equality
open import foundation.propositional-truncations
open import foundation.sets
open import foundation.subtypes
open import foundation.type-arithmetic-dependent-pair-types
open import foundation.universe-levels

open import group-theory.concrete-group-actions
open import group-theory.concrete-groups
open import group-theory.subgroups-concrete-groups
open import group-theory.transitive-concrete-group-actions


The stabilizer of an element x : X point of a concrete G-set X : BG → Set is the connected component at the element (point , x) in the type of orbits of X. This type is a indeed concrete group of which the underlying type is the type of elements g : G such that g x = x.


module _
  {l1 l2 : Level} (G : Concrete-Group l1) (X : action-Concrete-Group l2 G)

  action-stabilizer-action-Concrete-Group :
    type-action-Concrete-Group G X  action-Concrete-Group (l1  l2) G
  action-stabilizer-action-Concrete-Group x u =
      ( X u)
      ( λ y  mere-eq-Prop (shape-Concrete-Group G , x) (u , y))

  is-transitive-action-stabilizer-action-Concrete-Group :
    (x : type-action-Concrete-Group G X) 
    is-transitive-action-Concrete-Group G
      ( action-stabilizer-action-Concrete-Group x)
  is-transitive-action-stabilizer-action-Concrete-Group x =
      ( associative-Σ
        ( classifying-type-Concrete-Group G)
        ( type-Set  X)
        ( mere-eq (shape-Concrete-Group G , x)))
      ( is-0-connected-mere-eq
        ( ( shape-Concrete-Group G , x) ,
          ( refl-mere-eq (shape-Concrete-Group G , x)))
        ( λ (uy , p) 
          apply-universal-property-trunc-Prop p
            ( mere-eq-Prop
              ( ( shape-Concrete-Group G , x) ,
                ( refl-mere-eq (shape-Concrete-Group G , x)))
              ( uy , p))
            ( λ q 
                ( eq-type-subtype
                  ( mere-eq-Prop (shape-Concrete-Group G , x))
                  ( q)))))

  subgroup-stabilizer-action-Concrete-Group :
    (x : type-action-Concrete-Group G X)  subgroup-Concrete-Group (l1  l2) G
  pr1 (pr1 (subgroup-stabilizer-action-Concrete-Group x)) =
    action-stabilizer-action-Concrete-Group x
  pr2 (pr1 (subgroup-stabilizer-action-Concrete-Group x)) =
    is-transitive-action-stabilizer-action-Concrete-Group x
  pr1 (pr2 (subgroup-stabilizer-action-Concrete-Group x)) = x
  pr2 (pr2 (subgroup-stabilizer-action-Concrete-Group x)) =
    refl-mere-eq (shape-Concrete-Group G , x)

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