---
layout: default
title : "Legacy.Base.Homomorphisms.Kernels module (The Agda Universal Algebra Library)"
date : "2021-09-08"
author: "agda-algebras development team"
---

### <a id="kernels-of-homomorphisms">Kernels of Homomorphisms</a>

This is the [Legacy.Base.Homomorphisms.Kernels] module of the [Agda Universal Algebra Library][].


```agda


{-# OPTIONS --cubical-compatible --exact-split --safe #-}

open import Overture using ( Signature; 𝓞 ; 𝓥 )

module Legacy.Base.Homomorphisms.Kernels {𝑆 : Signature 𝓞 𝓥} where

-- Imports from Agda and the Agda Standard Library --------------------------------
open import Data.Product   using ( _,_ )
open import Function.Base  using ( _∘_ )
open import Level          using ( Level ; _⊔_ ; suc )

open  import Relation.Binary.PropositionalEquality
      using ( _≡_ ; module ≡-Reasoning ; refl )

-- Imports from the Agda Universal Algebras Library --------------------------------
open import Overture        using ( ∣_∣ ; ∥_∥ ; _⁻¹ )
open import Legacy.Base.Functions  using ( Image_∋_ ; IsSurjective )
open import Legacy.Base.Equality   using ( swelldef )
open import Legacy.Base.Relations  using ( ker ; ker-IsEquivalence ; ⟪_⟫ ; mkblk )

open  import Legacy.Base.Algebras {𝑆 = 𝑆}
      using ( Algebra ; compatible ; _̂_ ; Con ; mkcon ; _╱_ ; IsCongruence ; /-≡ )

open import Legacy.Base.Homomorphisms.Basic {𝑆 = 𝑆}  using ( hom ; epi ; epi→hom )

private variable α β : Level
```



#### <a id="definition">Definition</a>

The kernel of a homomorphism is a congruence relation and conversely for every
congruence relation θ, there exists a homomorphism with kernel θ (namely, that
canonical projection onto the quotient modulo θ).


```agda


module _ {𝑨 : Algebra α} where
 open ≡-Reasoning
 homker-comp :  swelldef 𝓥 β → {𝑩 : Algebra β}(h : hom 𝑨 𝑩)
  →             compatible 𝑨 (ker ∣ h ∣)

 homker-comp wd {𝑩} h f {u}{v} kuv =
  ∣ h ∣((f ̂ 𝑨) u)    ≡⟨ ∥ h ∥ f u ⟩
  (f ̂ 𝑩)(∣ h ∣ ∘ u)  ≡⟨ wd(f ̂ 𝑩)(∣ h ∣ ∘ u)(∣ h ∣ ∘ v)kuv ⟩
  (f ̂ 𝑩)(∣ h ∣ ∘ v)  ≡⟨ (∥ h ∥ f v)⁻¹ ⟩
  ∣ h ∣((f ̂ 𝑨) v)    ∎
```


(Notice, it is here that the `swelldef` postulate comes into play, and because it
is needed to prove `homker-comp`, it is postulated by all the lemmas below that
depend upon `homker-comp`.)

It is convenient to define a function that takes a homomorphism and constructs a
congruence from its kernel.  We call this function `kercon`.


```agda


 kercon : swelldef 𝓥 β → {𝑩 : Algebra β} → hom 𝑨 𝑩 → Con{α}{β} 𝑨
 kercon wd {𝑩} h = ker ∣ h ∣ , mkcon (ker-IsEquivalence ∣ h ∣)(homker-comp wd {𝑩} h)
```


With this congruence we construct the corresponding quotient, along with some
syntactic sugar to denote it.


```agda


 kerquo : swelldef 𝓥 β → {𝑩 : Algebra β} → hom 𝑨 𝑩 → Algebra (α ⊔ suc β)
 kerquo wd {𝑩} h = 𝑨 ╱ (kercon wd {𝑩} h)

ker[_⇒_]_↾_ :  (𝑨 : Algebra α)(𝑩 : Algebra β) → hom 𝑨 𝑩 → swelldef 𝓥 β
 →             Algebra (α ⊔ suc β)

ker[ 𝑨 ⇒ 𝑩 ] h ↾ wd = kerquo wd {𝑩} h
```


Thus, given `h : hom 𝑨 𝑩`, we can construct the quotient of `𝑨` modulo the kernel
of `h`, and the syntax for this quotient in the
[agda-algebras](https://github.com/ualib/agda-algebras) library is
`𝑨 [ 𝑩 ]/ker h ↾ fe`.

#### <a id="the-canonical-projection">The canonical projection</a>

Given an algebra `𝑨` and a congruence `θ`, the *canonical projection* is a map
from `𝑨` onto `𝑨 ╱ θ` that is constructed, and proved epimorphic, as follows.


```agda


module _ {α β : Level}{𝑨 : Algebra α} where
 πepi : (θ : Con{α}{β} 𝑨) → epi 𝑨 (𝑨 ╱ θ)
 πepi θ = (λ a → ⟪ a ⟫) , (λ _ _ → refl) , cπ-is-epic  where
  cπ-is-epic : IsSurjective (λ a → ⟪ a ⟫)
  cπ-is-epic (C , mkblk a refl ) =  Image_∋_.eq a refl
```


In may happen that we don't care about the surjectivity of `πepi`, in which case
would might prefer to work with the *homomorphic reduct* of `πepi`. This is
obtained by applying `epi-to-hom`, like so.


```agda


 πhom : (θ : Con{α}{β} 𝑨) → hom 𝑨 (𝑨 ╱ θ)
 πhom θ = epi→hom (𝑨 ╱ θ) (πepi θ)
```


We combine the foregoing to define a function that takes 𝑆-algebras `𝑨` and `𝑩`,
and a homomorphism `h : hom 𝑨 𝑩` and returns the canonical epimorphism from `𝑨`
onto `𝑨 [ 𝑩 ]/ker h`. (Recall, the latter is the special notation we defined above
for the quotient of `𝑨` modulo the kernel of `h`.)


```agda


 πker :  (wd : swelldef 𝓥 β){𝑩 : Algebra β}(h : hom 𝑨 𝑩)
  →      epi 𝑨 (ker[ 𝑨 ⇒ 𝑩 ] h ↾ wd)

 πker wd {𝑩} h = πepi (kercon wd {𝑩} h)
```


The kernel of the canonical projection of `𝑨` onto `𝑨 / θ` is equal to `θ`, but
since equality of inhabitants of certain types (like `Congruence` or `Rel`) can be
a tricky business, we settle for proving the containment `𝑨 / θ ⊆ θ`. Of the two
containments, this is the easier one to prove; luckily it is also the one we need
later.


```agda


 open IsCongruence

 ker-in-con :  {wd : swelldef 𝓥 (α ⊔ suc β)}(θ : Con 𝑨)
  →            ∀ {x}{y} → ∣ kercon wd {𝑨 ╱ θ} (πhom θ) ∣ x y →  ∣ θ ∣ x y

 ker-in-con θ hyp = /-≡ θ hyp
```