 #### Congruences of Setoid Algebras

This is the Setoid.Algebras.Congruences module of the Agda Universal Algebra Library.

```
{-# OPTIONS --without-K --exact-split --safe #-}

open import Overture using (𝓞 ; 𝓥 ; Signature)

module Setoid.Algebras.Congruences {𝑆 : Signature 𝓞 𝓥} where

-- Imports from the Agda Standard Library ---------------------------------------
open import Agda.Primitive   using () renaming ( Set to Type )
open import Data.Product     using ( _,_ ; Σ-syntax )
open import Function         using ( id ; Func )
open import Level            using ( Level ; _⊔_ )
open import Relation.Binary  using ( Setoid ; IsEquivalence )
renaming ( Rel to BinRel )

open import Relation.Binary.PropositionalEquality using ( refl )

-- Imports from the Agda Universal Algebras Library ------------------------------
open import Overture          using ( ∣_∣  ; ∥_∥  )
open import Base.Relations    using ( 0[_] ; _|:_ ; Equivalence )
open import Setoid.Relations  using ( ⟪_⟫ ; _/_ ; ⟪_∼_⟫-elim )
open import Setoid.Algebras.Basic {𝑆 = 𝑆} using ( ov ; Algebra ; 𝕌[_] ; _̂_ )

private variable α ρ ℓ : Level

```

We now define the function `compatible` so that, if `𝑨` denotes an algebra and `R` a binary relation, then `compatible 𝑨 R` will represent the assertion that `R` is compatible with all basic operations of `𝑨`. The formal definition is immediate since all the work is done by the relation `|:`, which we defined above (see Setoid.Relations.Discrete).

```
-- Algebra compatibility with binary relation
_∣≈_ : (𝑨 : Algebra α ρ) → BinRel 𝕌[ 𝑨 ] ℓ → Type _
𝑨 ∣≈ R = ∀ 𝑓 → (𝑓 ̂ 𝑨) |: R

```

A congruence relation of an algebra `𝑨` is defined to be an equivalence relation that is compatible with the basic operations of `𝑨`. This concept can be represented in a number of alternative but equivalent ways. Formally, we define a record type (`IsCongruence`) to represent the property of being a congruence, and we define a Sigma type (`Con`) to represent the type of congruences of a given algebra.

Congruences should obviously contain the equality relation on the underlying setoid. That is, they must be reflexive. Unfortunately this doesn’t come for free (e.g., as part of the definition of `IsEquivalence` on Setoid), so we must add the field `reflexive` to the definition of `IsCongruence`. (In fact, we should probably redefine equivalence relation on setiods to be reflexive with respect to the underying setoid equality (and not just with respect to ).)

```
module _ (𝑨 : Algebra α ρ) where
open Algebra 𝑨  using ()  renaming (Domain to A )
open Setoid A   using ( _≈_ )

record IsCongruence (θ : BinRel 𝕌[ 𝑨 ] ℓ) : Type (𝓞 ⊔ 𝓥 ⊔ ρ ⊔ ℓ ⊔ α)  where
constructor mkcon
field
reflexive : ∀ {a₀ a₁} → a₀ ≈ a₁ → θ a₀ a₁
is-equivalence : IsEquivalence θ
is-compatible  : 𝑨 ∣≈ θ

Eqv : Equivalence 𝕌[ 𝑨 ] {ℓ}
Eqv = θ , is-equivalence

open IsCongruence public

Con : {ℓ : Level} → Type (α ⊔ ρ ⊔ ov ℓ)
Con {ℓ} = Σ[ θ ∈ ( BinRel 𝕌[ 𝑨 ] ℓ ) ] IsCongruence θ

```

Each of these types captures what it means to be a congruence and they are equivalent in the sense that each implies the other. One implication is the “uncurry” operation and the other is the second projection.

```
IsCongruence→Con : {𝑨 : Algebra α ρ}(θ : BinRel 𝕌[ 𝑨 ] ℓ) → IsCongruence 𝑨 θ → Con 𝑨
IsCongruence→Con θ p = θ , p

Con→IsCongruence : {𝑨 : Algebra α ρ}((θ , _) : Con 𝑨 {ℓ}) → IsCongruence 𝑨 θ
Con→IsCongruence θ = ∥ θ ∥
```

#### Quotient algebras

In many areas of abstract mathematics the quotient of an algebra `𝑨` with respect to a congruence relation `θ` of `𝑨` plays an important role. This quotient is typically denoted by `𝑨 / θ` and Agda allows us to define and express quotients using this standard notation.

```
open Algebra  using ( Domain ; Interp )
open Setoid   using ( Carrier )
open Func     using ( cong ) renaming ( f to _⟨\$⟩_  )

_╱_ : (𝑨 : Algebra α ρ) → Con 𝑨 {ℓ} → Algebra α ℓ
Domain (𝑨 ╱ θ) = 𝕌[ 𝑨 ] / (Eqv ∥ θ ∥)
(Interp (𝑨 ╱ θ)) ⟨\$⟩ (f , a) = (f ̂ 𝑨) a
cong (Interp (𝑨 ╱ θ)) {f , u} {.f , v} (refl , a) = is-compatible ∥ θ ∥ f a

module _ (𝑨 : Algebra α ρ) where
open Algebra 𝑨  using ( )      renaming (Domain to A )
open Setoid A   using ( _≈_ )  renaming (refl to refl₁)

_/∙_ : 𝕌[ 𝑨 ] → (θ : Con 𝑨{ℓ}) → Carrier (Domain (𝑨 ╱ θ))
a /∙ θ = a

/-≡ :  (θ : Con 𝑨{ℓ}){u v : 𝕌[ 𝑨 ]}
→     ⟪ u ⟫{Eqv ∥ θ ∥} ≈ ⟪ v ⟫{Eqv ∥ θ ∥} → ∣ θ ∣ u v

/-≡ θ {u}{v} uv = reflexive ∥ θ ∥ uv
```