Homomorphisms of semirings

Content created by Egbert Rijke and Fredrik Bakke.

Created on 2023-05-06.
Last modified on 2023-11-27.

module ring-theory.homomorphisms-semirings where
Imports
open import foundation.dependent-pair-types
open import foundation.equivalences
open import foundation.fundamental-theorem-of-identity-types
open import foundation.homotopies
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.torsorial-type-families
open import foundation.universe-levels

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

open import ring-theory.semirings

Idea

Homomorphisms of semirings are homomorphisms of their underlying additive commutative monoids that preserve multiplication and the multiplicative unit.

Definitions

module _
  {l1 l2 : Level} (R : Semiring l1) (S : Semiring l2)
  where

  is-homomorphism-semiring-prop-hom-Commutative-Monoid :
    ( hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring S))  Prop (l1  l2)
  is-homomorphism-semiring-prop-hom-Commutative-Monoid f =
    Σ-Prop
      ( preserves-mul-prop-Semigroup
        ( multiplicative-semigroup-Semiring R)
        ( multiplicative-semigroup-Semiring S)
        ( map-hom-Commutative-Monoid
          ( additive-commutative-monoid-Semiring R)
          ( additive-commutative-monoid-Semiring S)
          ( f)))
      ( λ H 
        preserves-unit-prop-hom-Semigroup
          ( multiplicative-monoid-Semiring R)
          ( multiplicative-monoid-Semiring S)
          ( ( map-hom-Commutative-Monoid
              ( additive-commutative-monoid-Semiring R)
              ( additive-commutative-monoid-Semiring S)
              ( f)) ,
            ( H)))

  is-homomorphism-semiring-hom-Commutative-Monoid :
    ( hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring S))  UU (l1  l2)
  is-homomorphism-semiring-hom-Commutative-Monoid f =
    type-Prop (is-homomorphism-semiring-prop-hom-Commutative-Monoid f)

  is-prop-is-homomorphism-semiring-hom-Commutative-Monoid :
    ( f :
      hom-Commutative-Monoid
        ( additive-commutative-monoid-Semiring R)
        ( additive-commutative-monoid-Semiring S)) 
    is-prop (is-homomorphism-semiring-hom-Commutative-Monoid f)
  is-prop-is-homomorphism-semiring-hom-Commutative-Monoid f =
    is-prop-type-Prop (is-homomorphism-semiring-prop-hom-Commutative-Monoid f)

  hom-set-Semiring : Set (l1  l2)
  hom-set-Semiring =
    set-subset
      ( hom-set-Commutative-Monoid
        ( additive-commutative-monoid-Semiring R)
        ( additive-commutative-monoid-Semiring S))
      ( is-homomorphism-semiring-prop-hom-Commutative-Monoid)

  hom-Semiring : UU (l1  l2)
  hom-Semiring = type-Set hom-set-Semiring

  is-set-hom-Semiring : is-set hom-Semiring
  is-set-hom-Semiring = is-set-type-Set hom-set-Semiring

  module _
    (f : hom-Semiring)
    where

    hom-additive-commutative-monoid-hom-Semiring :
      hom-Commutative-Monoid
        ( additive-commutative-monoid-Semiring R)
        ( additive-commutative-monoid-Semiring S)
    hom-additive-commutative-monoid-hom-Semiring = pr1 f

    map-hom-Semiring : type-Semiring R  type-Semiring S
    map-hom-Semiring =
      map-hom-Commutative-Monoid
        ( additive-commutative-monoid-Semiring R)
        ( additive-commutative-monoid-Semiring S)
        ( hom-additive-commutative-monoid-hom-Semiring)

    preserves-addition-hom-Semiring :
      {x y : type-Semiring R} 
      map-hom-Semiring (add-Semiring R x y) 
      add-Semiring S (map-hom-Semiring x) (map-hom-Semiring y)
    preserves-addition-hom-Semiring =
      preserves-mul-hom-Commutative-Monoid
        ( additive-commutative-monoid-Semiring R)
        ( additive-commutative-monoid-Semiring S)
        ( hom-additive-commutative-monoid-hom-Semiring)

    preserves-zero-hom-Semiring :
      map-hom-Semiring (zero-Semiring R)  zero-Semiring S
    preserves-zero-hom-Semiring =
      preserves-unit-hom-Commutative-Monoid
        ( additive-commutative-monoid-Semiring R)
        ( additive-commutative-monoid-Semiring S)
        ( hom-additive-commutative-monoid-hom-Semiring)

    preserves-mul-hom-Semiring :
      {x y : type-Semiring R} 
      map-hom-Semiring (mul-Semiring R x y) 
      mul-Semiring S (map-hom-Semiring x) (map-hom-Semiring y)
    preserves-mul-hom-Semiring = pr1 (pr2 f)

    preserves-unit-hom-Semiring :
      map-hom-Semiring (one-Semiring R)  one-Semiring S
    preserves-unit-hom-Semiring = pr2 (pr2 f)

    is-homomorphism-semiring-hom-Semiring :
      is-homomorphism-semiring-hom-Commutative-Monoid
        ( hom-additive-commutative-monoid-hom-Semiring)
    pr1 is-homomorphism-semiring-hom-Semiring = preserves-mul-hom-Semiring
    pr2 is-homomorphism-semiring-hom-Semiring = preserves-unit-hom-Semiring

    hom-multiplicative-monoid-hom-Semiring :
      hom-Monoid
        ( multiplicative-monoid-Semiring R)
        ( multiplicative-monoid-Semiring S)
    pr1 (pr1 hom-multiplicative-monoid-hom-Semiring) =
      map-hom-Semiring
    pr2 (pr1 hom-multiplicative-monoid-hom-Semiring) =
      preserves-mul-hom-Semiring
    pr2 hom-multiplicative-monoid-hom-Semiring =
      preserves-unit-hom-Semiring

The identity homomorphism of semirings

module _
  {l : Level} (R : Semiring l)
  where

  hom-additive-commutative-monoid-id-hom-Semiring :
    hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring R)
  hom-additive-commutative-monoid-id-hom-Semiring =
    id-hom-Commutative-Monoid (additive-commutative-monoid-Semiring R)

  preserves-mul-id-hom-Semiring :
    {x y : type-Semiring R}  mul-Semiring R x y  mul-Semiring R x y
  preserves-mul-id-hom-Semiring = refl

  preserves-unit-id-hom-Semiring :
    one-Semiring R  one-Semiring R
  preserves-unit-id-hom-Semiring = refl

  id-hom-Semiring : hom-Semiring R R
  pr1 id-hom-Semiring = hom-additive-commutative-monoid-id-hom-Semiring
  pr1 (pr2 id-hom-Semiring) = preserves-mul-id-hom-Semiring
  pr2 (pr2 id-hom-Semiring) = preserves-unit-id-hom-Semiring

Composition of homomorphisms of semirings

module _
  {l1 l2 l3 : Level}
  (R : Semiring l1) (S : Semiring l2) (T : Semiring l3)
  (g : hom-Semiring S T) (f : hom-Semiring R S)
  where

  hom-additive-commutative-monoid-comp-hom-Semiring :
    hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring T)
  hom-additive-commutative-monoid-comp-hom-Semiring =
    comp-hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring S)
      ( additive-commutative-monoid-Semiring T)
      ( hom-additive-commutative-monoid-hom-Semiring S T g)
      ( hom-additive-commutative-monoid-hom-Semiring R S f)

  hom-multiplicative-monoid-comp-hom-Semiring :
    hom-Monoid
      ( multiplicative-monoid-Semiring R)
      ( multiplicative-monoid-Semiring T)
  hom-multiplicative-monoid-comp-hom-Semiring =
    comp-hom-Monoid
      ( multiplicative-monoid-Semiring R)
      ( multiplicative-monoid-Semiring S)
      ( multiplicative-monoid-Semiring T)
      ( hom-multiplicative-monoid-hom-Semiring S T g)
      ( hom-multiplicative-monoid-hom-Semiring R S f)

  map-comp-hom-Semiring :
    type-Semiring R  type-Semiring T
  map-comp-hom-Semiring =
    map-hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring T)
      ( hom-additive-commutative-monoid-comp-hom-Semiring)

  preserves-mul-comp-hom-Semiring :
    {x y : type-Semiring R} 
    map-comp-hom-Semiring (mul-Semiring R x y) 
    mul-Semiring T (map-comp-hom-Semiring x) (map-comp-hom-Semiring y)
  preserves-mul-comp-hom-Semiring =
    preserves-mul-hom-Monoid
      ( multiplicative-monoid-Semiring R)
      ( multiplicative-monoid-Semiring T)
      ( hom-multiplicative-monoid-comp-hom-Semiring)

  preserves-unit-comp-hom-Semiring :
    map-comp-hom-Semiring (one-Semiring R)  one-Semiring T
  preserves-unit-comp-hom-Semiring =
    preserves-unit-hom-Monoid
      ( multiplicative-monoid-Semiring R)
      ( multiplicative-monoid-Semiring T)
      ( hom-multiplicative-monoid-comp-hom-Semiring)

  comp-hom-Semiring : hom-Semiring R T
  pr1 comp-hom-Semiring = hom-additive-commutative-monoid-comp-hom-Semiring
  pr1 (pr2 comp-hom-Semiring) = preserves-mul-comp-hom-Semiring
  pr2 (pr2 comp-hom-Semiring) = preserves-unit-comp-hom-Semiring

Homotopies of homomorphisms of semirings

module _
  {l1 l2 : Level} (R : Semiring l1) (S : Semiring l2)
  where

  htpy-hom-Semiring : (f g : hom-Semiring R S)  UU (l1  l2)
  htpy-hom-Semiring f g =
    htpy-hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring S)
      ( hom-additive-commutative-monoid-hom-Semiring R S f)
      ( hom-additive-commutative-monoid-hom-Semiring R S g)

  refl-htpy-hom-Semiring : (f : hom-Semiring R S)  htpy-hom-Semiring f f
  refl-htpy-hom-Semiring f =
    refl-htpy-hom-Commutative-Monoid
      ( additive-commutative-monoid-Semiring R)
      ( additive-commutative-monoid-Semiring S)
      ( hom-additive-commutative-monoid-hom-Semiring R S f)

Properties

Homotopies characterize identifications of homomorphisms of semirings

module _
  {l1 l2 : Level} (R : Semiring l1) (S : Semiring l2)
  (f : hom-Semiring R S)
  where

  is-torsorial-htpy-hom-Semiring :
    is-torsorial (htpy-hom-Semiring R S f)
  is-torsorial-htpy-hom-Semiring =
    is-torsorial-Eq-subtype
      ( is-torsorial-htpy-hom-Commutative-Monoid
        ( additive-commutative-monoid-Semiring R)
        ( additive-commutative-monoid-Semiring S)
        ( hom-additive-commutative-monoid-hom-Semiring R S f))
      ( is-prop-is-homomorphism-semiring-hom-Commutative-Monoid R S)
      ( hom-additive-commutative-monoid-hom-Semiring R S f)
      ( refl-htpy-hom-Semiring R S f)
      ( is-homomorphism-semiring-hom-Semiring R S f)

  htpy-eq-hom-Semiring :
    (g : hom-Semiring R S)  (f  g)  htpy-hom-Semiring R S f g
  htpy-eq-hom-Semiring .f refl = refl-htpy-hom-Semiring R S f

  is-equiv-htpy-eq-hom-Semiring :
    (g : hom-Semiring R S)  is-equiv (htpy-eq-hom-Semiring g)
  is-equiv-htpy-eq-hom-Semiring =
    fundamental-theorem-id
      is-torsorial-htpy-hom-Semiring
      htpy-eq-hom-Semiring

  extensionality-hom-Semiring :
    (g : hom-Semiring R S)  (f  g)  htpy-hom-Semiring R S f g
  pr1 (extensionality-hom-Semiring g) = htpy-eq-hom-Semiring g
  pr2 (extensionality-hom-Semiring g) = is-equiv-htpy-eq-hom-Semiring g

  eq-htpy-hom-Semiring :
    (g : hom-Semiring R S)  htpy-hom-Semiring R S f g  f  g
  eq-htpy-hom-Semiring g = map-inv-equiv (extensionality-hom-Semiring g)

Associativity of composition of homomorphisms of semirings

module _
  {l1 l2 l3 l4 : Level}
  (R : Semiring l1) (S : Semiring l2) (T : Semiring l3) (U : Semiring l4)
  (h : hom-Semiring T U)
  (g : hom-Semiring S T)
  (f : hom-Semiring R S)
  where

  associative-comp-hom-Semiring :
    comp-hom-Semiring R S U (comp-hom-Semiring S T U h g) f 
    comp-hom-Semiring R T U h (comp-hom-Semiring R S T g f)
  associative-comp-hom-Semiring =
    eq-htpy-hom-Semiring R U
      ( comp-hom-Semiring R S U (comp-hom-Semiring S T U h g) f)
      ( comp-hom-Semiring R T U h (comp-hom-Semiring R S T g f))
      ( refl-htpy)

  inv-associative-comp-hom-Semiring :
    comp-hom-Semiring R T U h (comp-hom-Semiring R S T g f) 
    comp-hom-Semiring R S U (comp-hom-Semiring S T U h g) f
  inv-associative-comp-hom-Semiring =
    eq-htpy-hom-Semiring R U
      ( comp-hom-Semiring R T U h (comp-hom-Semiring R S T g f))
      ( comp-hom-Semiring R S U (comp-hom-Semiring S T U h g) f)
      ( refl-htpy)

Unit laws for composition of homomorphisms of semirings

module _
  {l1 l2 : Level} (R : Semiring l1) (S : Semiring l2)
  (f : hom-Semiring R S)
  where

  left-unit-law-comp-hom-Semiring :
    comp-hom-Semiring R S S (id-hom-Semiring S) f  f
  left-unit-law-comp-hom-Semiring =
    eq-htpy-hom-Semiring R S
      ( comp-hom-Semiring R S S (id-hom-Semiring S) f)
      ( f)
      ( refl-htpy)

  right-unit-law-comp-hom-Semiring :
    comp-hom-Semiring R R S f (id-hom-Semiring R)  f
  right-unit-law-comp-hom-Semiring =
    eq-htpy-hom-Semiring R S
      ( comp-hom-Semiring R R S f (id-hom-Semiring R))
      ( f)
      ( refl-htpy)

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