open import Algebra.Group.Cat.Base
open import Algebra.Group.Subgroup
open import Algebra.Ring.Ideal
open import Algebra.Prelude
open import Algebra.Group
open import Algebra.Ring

open import Data.Power
open import Data.Dec

module Algebra.Ring.Quotient {ℓ} (R : Ring ℓ) where

open Ring-on (R .snd)
private module R = Ring-on (R .snd)

Quotient rings🔗

Let $R$ be a ring and $I \sube R$ be an ideal. Because rings have an underlying abelian group, the ideal $I \sube R$ determines a normal subgroup $I$ of $R$’s additive group, so that we may form the quotient group $R/I$. And since ideals are closed under multiplication¹, we can extend $R$’s multiplication to a multiplication operation on $R/I$ in a canonical way! In that case, we refer to $R/I$ as a quotient ring.

module _ {I : ℙ ⌞ R ⌟} (idl : is-ideal R I) where
  private module I = is-ideal idl

Really, the bulk of the construction has already been done (in the section about quotient groups), so all that remains is the following construction: We want to show that $xy$ is invariant under equivalence for both $x$ and $y$ (and we may treat them separately for comprehensibility’s sake).

  private
    quot-grp : Group _
    quot-grp = R.additive-group /ᴳ I.ideal→normal
    module R/I = Group-on (quot-grp .snd)

    quot-mul : ⌞ quot-grp ⌟ → ⌞ quot-grp ⌟ → ⌞ quot-grp ⌟
    quot-mul =
      Coeq-rec₂ squash (λ x y → inc (x R.* y))
        (λ a (_ , _ , r) → p1 a r)
        (λ a (_ , _ , r) → p2 a r)
      where

On one side, we must show that $[ax] = [ay]$ supposing that $[x] = [y]$, i.e. that $(x - y) \in I$. But since $I$ is an ideal, we have $(ax - ay) \in I$, thus $[ax] = [ay]$. And on the other side, we have the same thing: Since $(x - y) \in I$, also $(xa - ya) \in I$, so $[xa] = [ya]$.

      p1 : ∀ a {x y} (r : (x R.- y) ∈ I) → inc (x R.* a) ≡ inc (y R.* a)
      p1 a {x} {y} x-y∈I = quot $ subst (_∈ I)
        (R.*-distribr ∙ ap (x R.* a R.+_) (sym R.neg-*-l))
        (I.has-*ᵣ a x-y∈I)

      p2 : ∀ a {x y} (r : (x R.- y) ∈ I) → inc (a R.* x) ≡ inc (a R.* y)
      p2 a {x} {y} x-y∈I = quot $ subst (_∈ I)
        (R.*-distribl ∙ ap (a R.* x R.+_) (sym R.neg-*-r))
        (I.has-*ₗ a x-y∈I)

  open make-ring
  make-R/I : make-ring ⌞ quot-grp ⌟
  make-R/I .ring-is-set = squash
  make-R/I .0R = inc 0r
  make-R/I ._+_ = R/I._⋆_
  make-R/I .-_ = R/I.inverse
  make-R/I .+-idl = R/I.idl
  make-R/I .+-invr {x} = R/I.inverser {x}
  make-R/I .+-assoc {x} {y} {z} = R/I.associative {x} {y} {z}
  make-R/I .1R = inc R.1r
  make-R/I ._*_ = quot-mul
  make-R/I .+-comm {x} {y} =
    Coeq-elim-prop₂ {C = λ x y → x R/I.⋆ y ≡ y R/I.⋆ x} (λ x y → hlevel )
      (λ x y → ap Coeq.inc R.+-commutes) x y
  make-R/I .*-idl {x} =
    Coeq-elim-prop {C = λ x → quot-mul (inc R.1r) x ≡ x} (λ _ → hlevel )
      (λ x → ap Coeq.inc R.*-idl) x
  make-R/I .*-idr {x} =
    Coeq-elim-prop {C = λ x → quot-mul x (inc R.1r) ≡ x} (λ _ → hlevel )
      (λ x → ap Coeq.inc R.*-idr) x
  make-R/I .*-assoc {x} {y} {z} =
    Coeq-elim-prop₃
      {C = λ x y z → quot-mul x (quot-mul y z) ≡ quot-mul (quot-mul x y) z}
      (λ _ _ _ → hlevel ) (λ x y z → ap Coeq.inc R.*-associative) x y z
  make-R/I .*-distribl {x} {y} {z} =
    Coeq-elim-prop₃
      {C = λ x y z → quot-mul x (y R/I.⋆ z) ≡ quot-mul x y R/I.⋆ quot-mul x z}
      (λ _ _ _ → hlevel ) (λ x y z → ap Coeq.inc R.*-distribl) x y z
  make-R/I .*-distribr {x} {y} {z} =
    Coeq-elim-prop₃
      {C = λ x y z → quot-mul (y R/I.⋆ z) x ≡ quot-mul y x R/I.⋆ quot-mul z x}
      (λ _ _ _ → hlevel ) (λ x y z → ap Coeq.inc R.*-distribr) x y z

  R/I : Ring ℓ
  R/I = to-ring make-R/I

As a quick aside, if $I$ is a complemented ideal (equivalently: a decidable ideal), and $R$ is a discrete ring, then the quotient ring is also discrete. This is a specialisation of a general result about decidable quotient sets, but we mention it here regardless:

  Discrete-ring-quotient : Discrete ⌞ R ⌟ → (∀ x → Dec (x ∈ I)) → Discrete ⌞ R/I ⌟
  Discrete-ring-quotient rdisc dec∈I = Discrete-quotient
    (normal-subgroup→congruence R.additive-group I.ideal→normal)
    rdisc
    (λ x y → dec∈I (x R.- y))