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

Created on 2022-08-16.
Last modified on 2023-06-28.

module group-theory.subsemigroups where
open import foundation.dependent-pair-types
open import foundation.equivalences
open import foundation.function-types
open import foundation.identity-types
open import foundation.propositions
open import foundation.sets
open import foundation.subtype-identity-principle
open import foundation.subtypes
open import foundation.universe-levels

open import group-theory.homomorphisms-semigroups
open import group-theory.semigroups


A subsemigroup of a semigroup G is a subtype of G closed under multiplication.


Subsets of semigroups

subset-Semigroup :
  {l1 : Level} (l2 : Level) (G : Semigroup l1)  UU (l1  lsuc l2)
subset-Semigroup l2 G = subtype l2 (type-Semigroup G)

module _
  {l1 l2 : Level} (G : Semigroup l1) (P : subset-Semigroup l2 G)

  is-in-subset-Semigroup : type-Semigroup G  UU l2
  is-in-subset-Semigroup = is-in-subtype P

  is-prop-is-in-subset-Semigroup :
    (x : type-Semigroup G)  is-prop (is-in-subset-Semigroup x)
  is-prop-is-in-subset-Semigroup = is-prop-is-in-subtype P

  type-subset-Semigroup : UU (l1  l2)
  type-subset-Semigroup = type-subtype P

  is-set-type-subset-Semigroup : is-set type-subset-Semigroup
  is-set-type-subset-Semigroup =
    is-set-type-subtype P (is-set-type-Semigroup G)

  set-subset-Semigroup : Set (l1  l2)
  set-subset-Semigroup = set-subset (set-Semigroup G) P

  inclusion-subset-Semigroup : type-subset-Semigroup  type-Semigroup G
  inclusion-subset-Semigroup = inclusion-subtype P

  ap-inclusion-subset-Semigroup :
    (x y : type-subset-Semigroup) 
    x  y  (inclusion-subset-Semigroup x  inclusion-subset-Semigroup y)
  ap-inclusion-subset-Semigroup = ap-inclusion-subtype P

  is-in-subset-inclusion-subset-Semigroup :
    (x : type-subset-Semigroup) 
    is-in-subset-Semigroup (inclusion-subset-Semigroup x)
  is-in-subset-inclusion-subset-Semigroup =
    is-in-subtype-inclusion-subtype P


is-subsemigroup-subset-Semigroup-Prop :
  {l1 l2 : Level} (G : Semigroup l1) (P : subset-Semigroup l2 G) 
  Prop (l1  l2)
is-subsemigroup-subset-Semigroup-Prop G P =
    ( type-Semigroup G)
    ( λ x 
        ( type-Semigroup G)
        ( λ y  hom-Prop (P x) (hom-Prop (P y) (P (mul-Semigroup G x y)))))

is-subsemigroup-subset-Semigroup :
  {l1 l2 : Level} (G : Semigroup l1) (P : subset-Semigroup l2 G)  UU (l1  l2)
is-subsemigroup-subset-Semigroup G P =
  type-Prop (is-subsemigroup-subset-Semigroup-Prop G P)

Subsemigroup :
  {l1 : Level} (l2 : Level) (G : Semigroup l1)  UU (l1  lsuc l2)
Subsemigroup l2 G =
  type-subtype (is-subsemigroup-subset-Semigroup-Prop {l2 = l2} G)

module _
  {l1 l2 : Level} (G : Semigroup l1) (P : Subsemigroup l2 G)

  subset-Subsemigroup : subtype l2 (type-Semigroup G)
  subset-Subsemigroup =
    inclusion-subtype (is-subsemigroup-subset-Semigroup-Prop G) P

  is-subsemigroup-Subsemigroup :
    is-subsemigroup-subset-Semigroup G subset-Subsemigroup
  is-subsemigroup-Subsemigroup = pr2 P

  is-in-Subsemigroup : type-Semigroup G  UU l2
  is-in-Subsemigroup = is-in-subtype subset-Subsemigroup

  is-prop-is-in-Subsemigroup :
    (x : type-Semigroup G)  is-prop (is-in-Subsemigroup x)
  is-prop-is-in-Subsemigroup =
    is-prop-is-in-subtype subset-Subsemigroup

  type-Subsemigroup : UU (l1  l2)
  type-Subsemigroup = type-subtype subset-Subsemigroup

  is-set-type-Subsemigroup : is-set type-Subsemigroup
  is-set-type-Subsemigroup =
    is-set-type-subset-Semigroup G subset-Subsemigroup

  set-Subsemigroup : Set (l1  l2)
  set-Subsemigroup =
    set-subset-Semigroup G subset-Subsemigroup

  inclusion-Subsemigroup :
    type-Subsemigroup  type-Semigroup G
  inclusion-Subsemigroup =
    inclusion-subtype subset-Subsemigroup

  ap-inclusion-Subsemigroup :
    (x y : type-Subsemigroup)  x  y 
    inclusion-Subsemigroup x  inclusion-Subsemigroup y
  ap-inclusion-Subsemigroup =
    ap-inclusion-subtype subset-Subsemigroup

  is-in-subsemigroup-inclusion-Subsemigroup :
    (x : type-Subsemigroup) 
    is-in-Subsemigroup (inclusion-Subsemigroup x)
  is-in-subsemigroup-inclusion-Subsemigroup =
    is-in-subtype-inclusion-subtype subset-Subsemigroup

  is-closed-under-multiplication-Subsemigroup :
    {x y : type-Semigroup G} 
    is-in-Subsemigroup x  is-in-Subsemigroup y 
    is-in-Subsemigroup (mul-Semigroup G x y)
  is-closed-under-multiplication-Subsemigroup {x} {y} = pr2 P x y

  mul-Subsemigroup :
    (x y : type-Subsemigroup)  type-Subsemigroup
  pr1 (mul-Subsemigroup x y) =
    mul-Semigroup G
      ( inclusion-Subsemigroup x)
      ( inclusion-Subsemigroup y)
  pr2 (mul-Subsemigroup x y) =
      ( is-in-subsemigroup-inclusion-Subsemigroup x)
      ( is-in-subsemigroup-inclusion-Subsemigroup y)

  associative-mul-Subsemigroup :
    (x y z : type-Subsemigroup) 
    ( mul-Subsemigroup (mul-Subsemigroup x y) z) 
    ( mul-Subsemigroup x (mul-Subsemigroup y z))
  associative-mul-Subsemigroup x y z =
      ( subset-Subsemigroup)
      ( associative-mul-Semigroup G
        ( inclusion-Subsemigroup x)
        ( inclusion-Subsemigroup y)
        ( inclusion-Subsemigroup z))

  semigroup-Subsemigroup : Semigroup (l1  l2)
  pr1 semigroup-Subsemigroup = set-Subsemigroup
  pr1 (pr2 semigroup-Subsemigroup) = mul-Subsemigroup
  pr2 (pr2 semigroup-Subsemigroup) = associative-mul-Subsemigroup

  preserves-mul-inclusion-Subsemigroup :
    (x y : type-Subsemigroup) 
    inclusion-Subsemigroup (mul-Subsemigroup x y) 
    mul-Semigroup G (inclusion-Subsemigroup x) (inclusion-Subsemigroup y)
  preserves-mul-inclusion-Subsemigroup x y = refl

  hom-inclusion-Subsemigroup :
    type-hom-Semigroup semigroup-Subsemigroup G
  pr1 hom-inclusion-Subsemigroup = inclusion-Subsemigroup
  pr2 hom-inclusion-Subsemigroup = preserves-mul-inclusion-Subsemigroup


Extensionality of the type of all subsemigroups

module _
  {l1 l2 : Level} (G : Semigroup l1) (H : Subsemigroup l2 G)

  has-same-elements-Subsemigroup : Subsemigroup l2 G  UU (l1  l2)
  has-same-elements-Subsemigroup K =
      ( subset-Subsemigroup G H)
      ( subset-Subsemigroup G K)

  extensionality-Subsemigroup :
    (K : Subsemigroup l2 G)  (H  K)  has-same-elements-Subsemigroup K
  extensionality-Subsemigroup =
      ( is-subsemigroup-subset-Semigroup-Prop G)
      ( is-subsemigroup-Subsemigroup G H)
      ( λ x  pair id id)
      ( extensionality-subtype (subset-Subsemigroup G H))

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