The center of a group

Content created by Jonathan Prieto-Cubides, Fredrik Bakke, Egbert Rijke and Maša Žaucer.

Created on 2022-07-15.
Last modified on 2023-03-19.

module group-theory.centers-groups where
open import foundation.dependent-pair-types
open import foundation.identity-types
open import foundation.propositions
open import foundation.universe-levels

open import group-theory.central-elements-groups
open import group-theory.groups
open import group-theory.homomorphisms-groups
open import group-theory.normal-subgroups
open import group-theory.subgroups


The center of a group consists of those elements that are central.


module _
  {l : Level} (G : Group l)

  subtype-center-Group : type-Group G  Prop l
  subtype-center-Group = is-central-element-group-Prop G

  subgroup-center-Group : Subgroup l G
  pr1 subgroup-center-Group = subtype-center-Group
  pr1 (pr2 subgroup-center-Group) = is-central-element-unit-Group G
  pr1 (pr2 (pr2 subgroup-center-Group)) = is-central-element-mul-Group G
  pr2 (pr2 (pr2 subgroup-center-Group)) = is-central-element-inv-Group G

  group-center-Group : Group l
  group-center-Group = group-Subgroup G subgroup-center-Group

  type-center-Group : UU l
  type-center-Group =
    type-Subgroup G subgroup-center-Group

  mul-center-Group :
    (x y : type-center-Group)  type-center-Group
  mul-center-Group = mul-Subgroup G subgroup-center-Group

  associative-mul-center-Group :
    (x y z : type-center-Group) 
    mul-center-Group (mul-center-Group x y) z 
    mul-center-Group x (mul-center-Group y z)
  associative-mul-center-Group =
    associative-mul-Subgroup G subgroup-center-Group

  inclusion-center-Group :
    type-center-Group  type-Group G
  inclusion-center-Group =
    inclusion-Subgroup G subgroup-center-Group

  is-central-element-inclusion-center-Group :
    (x : type-center-Group) 
    is-central-element-Group G (inclusion-center-Group x)
  is-central-element-inclusion-center-Group x =
    is-in-subgroup-inclusion-Subgroup G subgroup-center-Group x

  preserves-mul-inclusion-center-Group :
    (x y : type-center-Group) 
    inclusion-center-Group (mul-center-Group x y) 
    mul-Group G
      ( inclusion-center-Group x)
      ( inclusion-center-Group y)
  preserves-mul-inclusion-center-Group =
    preserves-mul-inclusion-Subgroup G subgroup-center-Group

  hom-inclusion-center-Group :
    type-hom-Group group-center-Group G
  hom-inclusion-center-Group =
    hom-inclusion-Subgroup G subgroup-center-Group

  is-normal-subgroup-center-Group :
    is-normal-Subgroup G subgroup-center-Group
  is-normal-subgroup-center-Group x y =
    is-central-element-conjugation-Group G
      ( inclusion-center-Group y)
      ( x)
      ( is-central-element-inclusion-center-Group y)

  center-Group : Normal-Subgroup l G
  pr1 center-Group = subgroup-center-Group
  pr2 center-Group = is-normal-subgroup-center-Group

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