Cores of monoids

Content created by Fredrik Bakke, Egbert Rijke and Gregor Perčič.

Created on 2023-09-15.
Last modified on 2024-03-11.

module group-theory.cores-monoids where
open import category-theory.functors-large-precategories

open import foundation.dependent-pair-types
open import foundation.identity-types
open import foundation.propositions
open import foundation.subtypes
open import foundation.universe-levels

open import group-theory.groups
open import group-theory.homomorphisms-groups
open import group-theory.homomorphisms-monoids
open import group-theory.invertible-elements-monoids
open import group-theory.monoids
open import group-theory.precategory-of-groups
open import group-theory.precategory-of-monoids
open import group-theory.semigroups
open import group-theory.submonoids


The core of a monoid M is the maximal group contained in M, and consists of all the elements that are invertible.


module _
  {l : Level} (M : Monoid l)

  subtype-core-Monoid : type-Monoid M  Prop l
  subtype-core-Monoid = is-invertible-element-prop-Monoid M

  submonoid-core-Monoid : Submonoid l M
  pr1 submonoid-core-Monoid = subtype-core-Monoid
  pr1 (pr2 submonoid-core-Monoid) = is-invertible-element-unit-Monoid M
  pr2 (pr2 submonoid-core-Monoid) = is-invertible-element-mul-Monoid M

  monoid-core-Monoid : Monoid l
  monoid-core-Monoid = monoid-Submonoid M submonoid-core-Monoid

  semigroup-core-Monoid : Semigroup l
  semigroup-core-Monoid = semigroup-Submonoid M submonoid-core-Monoid

  type-core-Monoid : UU l
  type-core-Monoid = type-Submonoid M submonoid-core-Monoid

  mul-core-Monoid : type-core-Monoid  type-core-Monoid  type-core-Monoid
  mul-core-Monoid = mul-Semigroup semigroup-core-Monoid

  associative-mul-core-Monoid :
    (x y z : type-core-Monoid) 
    mul-core-Monoid (mul-core-Monoid x y) z 
    mul-core-Monoid x (mul-core-Monoid y z)
  associative-mul-core-Monoid =
    associative-mul-Semigroup semigroup-core-Monoid

  unit-core-Monoid : type-core-Monoid
  pr1 unit-core-Monoid = unit-Monoid M
  pr2 unit-core-Monoid = is-invertible-element-unit-Monoid M

  left-unit-law-mul-core-Monoid :
    (x : type-core-Monoid) 
    mul-core-Monoid unit-core-Monoid x  x
  left-unit-law-mul-core-Monoid x =
    eq-type-subtype subtype-core-Monoid (left-unit-law-mul-Monoid M (pr1 x))

  right-unit-law-mul-core-Monoid :
    (x : type-core-Monoid) 
    mul-core-Monoid x unit-core-Monoid  x
  right-unit-law-mul-core-Monoid x =
    eq-type-subtype subtype-core-Monoid (right-unit-law-mul-Monoid M (pr1 x))

  is-unital-core-Monoid : is-unital-Semigroup semigroup-core-Monoid
  pr1 is-unital-core-Monoid = unit-core-Monoid
  pr1 (pr2 is-unital-core-Monoid) = left-unit-law-mul-core-Monoid
  pr2 (pr2 is-unital-core-Monoid) = right-unit-law-mul-core-Monoid

  inv-core-Monoid : type-core-Monoid  type-core-Monoid
  pr1 (inv-core-Monoid x) =
    inv-is-invertible-element-Monoid M (pr2 x)
  pr2 (inv-core-Monoid x) =
    is-invertible-element-inv-is-invertible-element-Monoid M (pr2 x)

  left-inverse-law-mul-core-Monoid :
    (x : type-core-Monoid) 
    mul-core-Monoid (inv-core-Monoid x) x  unit-core-Monoid
  left-inverse-law-mul-core-Monoid x =
      ( subtype-core-Monoid)
      ( is-left-inverse-inv-is-invertible-element-Monoid M (pr2 x))

  right-inverse-law-mul-core-Monoid :
    (x : type-core-Monoid) 
    mul-core-Monoid x (inv-core-Monoid x)  unit-core-Monoid
  right-inverse-law-mul-core-Monoid x =
      ( subtype-core-Monoid)
      ( is-right-inverse-inv-is-invertible-element-Monoid M (pr2 x))

  is-group-core-Monoid' :
    is-group-is-unital-Semigroup semigroup-core-Monoid is-unital-core-Monoid
  pr1 is-group-core-Monoid' = inv-core-Monoid
  pr1 (pr2 is-group-core-Monoid') = left-inverse-law-mul-core-Monoid
  pr2 (pr2 is-group-core-Monoid') = right-inverse-law-mul-core-Monoid

  is-group-core-Monoid : is-group-Semigroup semigroup-core-Monoid
  pr1 is-group-core-Monoid = is-unital-core-Monoid
  pr2 is-group-core-Monoid = is-group-core-Monoid'

  core-Monoid : Group l
  pr1 core-Monoid = semigroup-core-Monoid
  pr2 core-Monoid = is-group-core-Monoid

  inclusion-core-Monoid :
    type-core-Monoid  type-Monoid M
  inclusion-core-Monoid =
    inclusion-Submonoid M submonoid-core-Monoid

  preserves-mul-inclusion-core-Monoid :
    {x y : type-core-Monoid} 
    inclusion-core-Monoid (mul-core-Monoid x y) 
    mul-Monoid M
      ( inclusion-core-Monoid x)
      ( inclusion-core-Monoid y)
  preserves-mul-inclusion-core-Monoid {x} {y} =
    preserves-mul-inclusion-Submonoid M submonoid-core-Monoid {x} {y}

  hom-inclusion-core-Monoid :
    hom-Monoid monoid-core-Monoid M
  hom-inclusion-core-Monoid =
    hom-inclusion-Submonoid M submonoid-core-Monoid


The core of a monoid is a functor from monoids to groups

The functorial action of core-Monoid

module _
  {l1 l2 : Level} (M : Monoid l1) (N : Monoid l2) (f : hom-Monoid M N)

  map-core-hom-Monoid : type-core-Monoid M  type-core-Monoid N
  pr1 (map-core-hom-Monoid x) = map-hom-Monoid M N f (pr1 x)
  pr2 (map-core-hom-Monoid x) =
    preserves-invertible-elements-hom-Monoid M N f (pr2 x)

  preserves-mul-hom-core-hom-Monoid :
    {x y : type-core-Monoid M} 
    map-core-hom-Monoid (mul-core-Monoid M x y) 
    mul-core-Monoid N (map-core-hom-Monoid x) (map-core-hom-Monoid y)
  preserves-mul-hom-core-hom-Monoid =
      ( subtype-core-Monoid N)
      ( preserves-mul-hom-Monoid M N f)

  hom-core-hom-Monoid : hom-Group (core-Monoid M) (core-Monoid N)
  pr1 hom-core-hom-Monoid = map-core-hom-Monoid
  pr2 hom-core-hom-Monoid = preserves-mul-hom-core-hom-Monoid

  preserves-unit-hom-core-hom-Monoid :
    map-core-hom-Monoid (unit-core-Monoid M)  unit-core-Monoid N
  preserves-unit-hom-core-hom-Monoid =
    preserves-unit-hom-Group (core-Monoid M) (core-Monoid N) hom-core-hom-Monoid

  preserves-inv-hom-core-hom-Monoid :
    {x : type-core-Monoid M} 
    map-core-hom-Monoid (inv-core-Monoid M x) 
    inv-core-Monoid N (map-core-hom-Monoid x)
  preserves-inv-hom-core-hom-Monoid =
    preserves-inv-hom-Group (core-Monoid M) (core-Monoid N) hom-core-hom-Monoid

The functorial laws of core-Monoid

module _
  {l : Level} (M : Monoid l)

  preserves-id-hom-core-hom-Monoid :
    hom-core-hom-Monoid M M (id-hom-Monoid M)  id-hom-Group (core-Monoid M)
  preserves-id-hom-core-hom-Monoid =
      ( core-Monoid M)
      ( core-Monoid M)
      ( λ _  eq-type-subtype (subtype-core-Monoid M) refl)

module _
  {l1 l2 l3 : Level} (M : Monoid l1) (N : Monoid l2) (K : Monoid l3)

  preserves-comp-hom-core-hom-Monoid :
    (g : hom-Monoid N K) (f : hom-Monoid M N) 
    hom-core-hom-Monoid M K (comp-hom-Monoid M N K g f) 
      ( core-Monoid M)
      ( core-Monoid N)
      ( core-Monoid K)
      ( hom-core-hom-Monoid N K g)
      ( hom-core-hom-Monoid M N f)
  preserves-comp-hom-core-hom-Monoid g f =
      ( core-Monoid M)
      ( core-Monoid K)
      ( λ _  eq-type-subtype (subtype-core-Monoid K) refl)

The functor core-Monoid

core-monoid-functor-Large-Precategory :
  functor-Large-Precategory  l  l)
  core-monoid-functor-Large-Precategory =
  core-monoid-functor-Large-Precategory {X = M} {Y = N} =
  hom-core-hom-Monoid M N
  core-monoid-functor-Large-Precategory {X = M} {Y = N} {Z = K} =
  preserves-comp-hom-core-hom-Monoid M N K
  core-monoid-functor-Large-Precategory {X = M} =
  preserves-id-hom-core-hom-Monoid M

The core functor is right adjoint to the forgetful functor from groups to monoids

This remains to be formalized. This forgetful functor also has a left adjoint, corresponding to groupification of the monoid.

Recent changes