Transitive higher group actions

Content created by Egbert Rijke and Fredrik Bakke.

Created on 2023-09-30.
Last modified on 2023-10-09.

module higher-group-theory.transitive-higher-group-actions where
open import foundation.0-connected-types
open import foundation.dependent-pair-types
open import foundation.identity-types
open import foundation.inhabited-types
open import foundation.propositional-truncations
open import foundation.propositions
open import foundation.regensburg-extension-fundamental-theorem-of-identity-types
open import foundation.surjective-maps
open import foundation.transport-along-identifications
open import foundation.universe-levels

open import higher-group-theory.higher-group-actions
open import higher-group-theory.higher-groups
open import higher-group-theory.orbits-higher-group-actions


Consider an ∞-group G and an ∞-group action of G on X. We say that X is transitive if its type of orbits is connected.

Equivalently, we say that X is abstractly transitive if the underlying type of X is inhabited and for any element x : X (sh G) of the underlying type of X the action map

  g ↦ mul-action-∞-Group G X g x

is surjective. The equivalence of these two conditions is established via the Regensburg extension of the fundamental theorem of identity types.

Note that it is necessary to include the condition that X is inhabited in the condition that G acts transitively on X. A first reason is that this makes the condition of being abstractly transitive equivalent to the condition of being transitive. A second reason is that this way we will be able to recover the familiar property that a G-action X is a G-torsor if and only if it is both free and transitive.


The predicate of being a transitive higher group action

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

  is-transitive-prop-action-∞-Group : Prop (l1  l2)
  is-transitive-prop-action-∞-Group =
    is-0-connected-Prop (orbit-action-∞-Group G X)

  is-transitive-action-∞-Group : UU (l1  l2)
  is-transitive-action-∞-Group = type-Prop is-transitive-prop-action-∞-Group

  is-prop-is-transitive-action-∞-Group : is-prop is-transitive-action-∞-Group
  is-prop-is-transitive-action-∞-Group =
    is-prop-type-Prop is-transitive-prop-action-∞-Group

The predicate of being an abstractly transitive higher group action

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

  is-abstractly-transitive-prop-action-∞-Group : Prop (l1  l2)
  is-abstractly-transitive-prop-action-∞-Group =
      ( is-inhabited-Prop (type-action-∞-Group G X))
      ( Π-Prop
        ( type-action-∞-Group G X)
        ( λ x  is-surjective-Prop  g  mul-action-∞-Group G X g x)))

  is-abstractly-transitive-action-∞-Group : UU (l1  l2)
  is-abstractly-transitive-action-∞-Group =
    type-Prop is-abstractly-transitive-prop-action-∞-Group

  is-prop-is-abstractly-transitive-action-∞-Group :
    is-prop is-abstractly-transitive-action-∞-Group
  is-prop-is-abstractly-transitive-action-∞-Group =
    is-prop-type-Prop is-abstractly-transitive-prop-action-∞-Group

Transitive higher group actions

transitive-action-∞-Group :
  {l1 : Level} (l2 : Level) (G : ∞-Group l1)  UU (l1  lsuc l2)
transitive-action-∞-Group l2 G =
  Σ (action-∞-Group l2 G) (is-transitive-action-∞-Group G)

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

  action-transitive-action-∞-Group : action-∞-Group l2 G
  action-transitive-action-∞-Group = pr1 X

  is-transitive-transitive-action-∞-Group :
    is-transitive-action-∞-Group G action-transitive-action-∞-Group
  is-transitive-transitive-action-∞-Group = pr2 X

  type-transitive-action-∞-Group : UU l2
  type-transitive-action-∞-Group =
    type-action-∞-Group G action-transitive-action-∞-Group

  mul-transitive-action-∞-Group :
    type-∞-Group G 
    type-transitive-action-∞-Group  type-transitive-action-∞-Group
  mul-transitive-action-∞-Group =
    mul-action-∞-Group G action-transitive-action-∞-Group

  associative-mul-transitive-action-∞-Group :
    (g h : type-∞-Group G) (x : type-transitive-action-∞-Group) 
    mul-transitive-action-∞-Group (mul-∞-Group G g h) x 
    mul-transitive-action-∞-Group h (mul-transitive-action-∞-Group g x)
  associative-mul-transitive-action-∞-Group =
    associative-mul-action-∞-Group G action-transitive-action-∞-Group

  unit-law-mul-transitive-action-∞-Group :
    (x : type-transitive-action-∞-Group) 
    mul-transitive-action-∞-Group (unit-∞-Group G) x  x
  unit-law-mul-transitive-action-∞-Group =
    unit-law-mul-action-∞-Group G action-transitive-action-∞-Group


If an action is abstractly transitive, then transport is surjective

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

    is-surjective-tr-is-abstractly-transitive-action-∞-Group :
      is-abstractly-transitive-action-∞-Group G X 
      (u : classifying-type-∞-Group G)
      (x : X (shape-∞-Group G)) 
      is-surjective  (p : shape-∞-Group G  u)  tr X p x)
    is-surjective-tr-is-abstractly-transitive-action-∞-Group H u x =
        ( mere-eq-classifying-type-∞-Group G (shape-∞-Group G) u)
        ( is-surjective-Prop _)
        ( λ where refl  pr2 H x)

An action is transitive if and only if it is abstractly transitive

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

  is-transitive-is-abstractly-transitive-action-∞-Group :
    is-abstractly-transitive-action-∞-Group G X 
    is-transitive-action-∞-Group G X
  is-transitive-is-abstractly-transitive-action-∞-Group (H , K) =
      ( shape-∞-Group G)
      ( is-0-connected-classifying-type-∞-Group G)
      ( λ f u 
          ( compute-map-out-of-identity-type f u)
          ( is-surjective-tr-is-abstractly-transitive-action-∞-Group G X
            ( H , K)
            ( u)
            ( f (shape-∞-Group G) refl)))
      ( H)

    is-inhabited-is-transitive-action-∞-Group :
      is-transitive-action-∞-Group G X  is-inhabited (type-action-∞-Group G X)
    is-inhabited-is-transitive-action-∞-Group H =
        ( is-inhabited-is-0-connected H)
        ( is-inhabited-Prop _)
        ( λ (u , x) 
            ( mere-eq-classifying-type-∞-Group G (shape-∞-Group G) u)
            ( is-inhabited-Prop _)
            ( λ where refl  unit-trunc-Prop x))

  is-surjective-mul-right-is-transitive-action-∞-Group :
    is-transitive-action-∞-Group G X 
    (x : type-action-∞-Group G X) 
    is-surjective  g  mul-action-∞-Group G X g x)
  is-surjective-mul-right-is-transitive-action-∞-Group H x =
      ( shape-∞-Group G)
      ( H)
      ( λ u p  tr X p x)
      ( shape-∞-Group G)

  is-abstractly-transitive-is-transitive-action-∞-Group :
    is-transitive-action-∞-Group G X 
    is-abstractly-transitive-action-∞-Group G X
  pr1 (is-abstractly-transitive-is-transitive-action-∞-Group H) =
    is-inhabited-is-transitive-action-∞-Group H
  pr2 (is-abstractly-transitive-is-transitive-action-∞-Group H) =
    is-surjective-mul-right-is-transitive-action-∞-Group H

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

  is-inhabited-transitive-action-∞-Group :
    is-inhabited (type-transitive-action-∞-Group G X)
  is-inhabited-transitive-action-∞-Group =
    is-inhabited-is-transitive-action-∞-Group G
      ( action-transitive-action-∞-Group G X)
      ( is-transitive-transitive-action-∞-Group G X)

  inhabited-type-transitive-action-∞-Group :
    Inhabited-Type l2
  pr1 inhabited-type-transitive-action-∞-Group =
    type-transitive-action-∞-Group G X
  pr2 inhabited-type-transitive-action-∞-Group =

  is-abstractly-transitive-transitive-action-∞-Group :
    is-abstractly-transitive-action-∞-Group G
      ( action-transitive-action-∞-Group G X)
  is-abstractly-transitive-transitive-action-∞-Group =
    is-abstractly-transitive-is-transitive-action-∞-Group G
      ( action-transitive-action-∞-Group G X)
      ( is-transitive-transitive-action-∞-Group G X)

  is-surjective-tr-transitive-action-∞-Group :
    (u : classifying-type-∞-Group G) (x : type-transitive-action-∞-Group G X) 
      ( λ (p : shape-∞-Group G  u) 
        tr (action-transitive-action-∞-Group G X) p x)
  is-surjective-tr-transitive-action-∞-Group =
    is-surjective-tr-is-abstractly-transitive-action-∞-Group G
      ( action-transitive-action-∞-Group G X)
      ( is-abstractly-transitive-transitive-action-∞-Group)

  is-surjective-mul-right-transitive-action-∞-Group :
    (x : type-transitive-action-∞-Group G X) 
    is-surjective  g  mul-transitive-action-∞-Group G X g x)
  is-surjective-mul-right-transitive-action-∞-Group =
    is-surjective-tr-transitive-action-∞-Group (shape-∞-Group G)

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