open import Cat.Prelude

import Cat.Reasoning

module Cat.Diagram.Equaliser where

module _ {o ℓ} (C : Precategory o ℓ) where
  open Cat.Reasoning C
  private variable
    A B : Ob
    f g h : Hom A B

Equalisers🔗

The equaliser of two maps $f,g:A\to B\text{,}$ when it exists, represents the largest subobject of $A$ where $f$ and $g$ agree. In this sense, the equaliser is the categorical generalisation of a solution set: The solution set of an equation in one variable is largest subset of the domain (i.e. what the variable ranges over) where the left- and right-hand-sides agree.

  record is-equaliser {E} (f g : Hom A B) (equ : Hom E A) : Type (o ⊔ ℓ) where
    field
      equal     : f ∘ equ ≡ g ∘ equ
      universal : ∀ {F} {e' : Hom F A} (p : f ∘ e' ≡ g ∘ e') → Hom F E
      factors   : ∀ {F} {e' : Hom F A} {p : f ∘ e' ≡ g ∘ e'} → equ ∘ universal p ≡ e'
      unique
        : ∀ {F} {e' : Hom F A} {p : f ∘ e' ≡ g ∘ e'} {other : Hom F E}
        → equ ∘ other ≡ e'
        → other ≡ universal p

    equal-∘ : f ∘ equ ∘ h ≡ g ∘ equ ∘ h
    equal-∘ {h = h} =
      f ∘ equ ∘ h ≡⟨ extendl equal ⟩≡
      g ∘ equ ∘ h ∎

    unique₂
      : ∀ {F} {e' : Hom F A}  {o1 o2 : Hom F E}
      → f ∘ e' ≡ g ∘ e'
      → equ ∘ o1 ≡ e'
      → equ ∘ o2 ≡ e'
      → o1 ≡ o2
    unique₂ p q r = unique {p = p} q ∙ sym (unique r)

We can visualise the situation using the commutative diagram below:

There is also a convenient bundling of an equalising arrow together with its domain:

  record Equaliser (f g : Hom A B) : Type (o ⊔ ℓ) where
    field
      {apex}  : Ob
      equ     : Hom apex A
      has-is-eq : is-equaliser f g equ

    open is-equaliser has-is-eq public

Equalisers are monic🔗

As a small initial application, we prove that equaliser arrows are always monic:

module _ {o ℓ} {C : Precategory o ℓ} where
  open Cat.Reasoning C
  private variable
    A B : Ob
    f g h : Hom A B

  is-equaliser→is-monic
    : ∀ {E} (equ : Hom E A)
    → is-equaliser C f g equ
    → is-monic equ
  is-equaliser→is-monic equ equalises g h p =
    unique₂ (extendl equal) p refl
    where open is-equaliser equalises

Uniqueness🔗

As universal constructions, equalisers are unique up to isomorphism. The proof follows the usual pattern: if $E,E^{\prime }$ both equalise $f,g:A\to B\text{,}$ then we can construct maps between $E$ and $E^{\prime }$ via the universal property of equalisers, and uniqueness ensures that these maps form an isomorphism.

  is-equaliser→iso
    : {E E' : Ob}
    → {e : Hom E A} {e' : Hom E' A}
    → is-equaliser C f g e
    → is-equaliser C f g e'
    → E ≅ E'
  is-equaliser→iso {e = e} {e' = e'} eq eq' =
    make-iso (eq' .universal (eq .equal)) (eq .universal (eq' .equal))
      (unique₂ eq' (eq' .equal) (pulll (eq' .factors) ∙ eq .factors) (idr _))
      (unique₂ eq (eq .equal) (pulll (eq .factors) ∙ eq' .factors) (idr _))
    where open is-equaliser

Properties of equalisers🔗

The equaliser of the pair $(f,f):A\to B$ always exists, and is given by the identity map $\mathrm{id}:A\to A\text{.}$

  id-is-equaliser : is-equaliser C f f id
  id-is-equaliser .is-equaliser.equal = refl
  id-is-equaliser .is-equaliser.universal {e' = e'} _ = e'
  id-is-equaliser .is-equaliser.factors = idl _
  id-is-equaliser .is-equaliser.unique p = sym (idl _) ∙ p

If $e:E\to A$ is an equaliser and an epimorphism, then $e$ is an iso.

  equaliser+epi→invertible
    : ∀ {E} {e : Hom E A}
    → is-equaliser C f g e
    → is-epic e
    → is-invertible e

Suppose that $e$ equalises some pair $(f,g):A\to B\text{.}$ By definition, this means that $f\circ e=g\circ e\text{;}$ however, $e$ is an epi, so $f=g\text{.}$ This in turn means that $\mathrm{id}:A\to A$ can be extended into a map $A\to E$ via the universal property of $e\text{,}$ and universality ensures that this extension is an isomorphism!

  equaliser+epi→invertible {f = f} {g = g} {e = e} e-equaliser e-epi =
    make-invertible
      (universal {e' = id} (ap₂ _∘_ f≡g refl))
      factors
      (unique₂ (ap₂ _∘_ f≡g refl) (pulll factors) id-comm)
    where
      open is-equaliser e-equaliser
      f≡g : f ≡ g
      f≡g = e-epi f g equal

Categories with all equalisers🔗

We also define a helper module for working with categories that have equalisers of all parallel pairs of morphisms.

has-equalisers : ∀ {o ℓ} → Precategory o ℓ → Type _
has-equalisers C = ∀ {a b} (f g : Hom a b) → Equaliser C f g
  where open Precategory C

module Equalisers
  {o ℓ}
  (C : Precategory o ℓ)
  (all-equalisers : has-equalisers C)
  where
  open Cat.Reasoning C
  module equaliser {a b} (f g : Hom a b) = Equaliser (all-equalisers f g)

  Equ : ∀ {a b} (f g : Hom a b) → Ob
  Equ = equaliser.apex