module Cat.Functor.Base where

Functors🔗

This module defines the most important clases of functors: Full, faithful, fully faithful (abbreviated ff), split essentially surjective and (“merely”) essentially surjective.

A functor is full when its action on hom-sets is surjective:

is-full : Functor C D → Type _
is-full {C = C} {D = D} F =
  ∀ {x y} (g : D .Hom (F₀ F x) (F₀ F y)) → ∃[ f ∈ C .Hom x y ] (F₁ F f ≡ g)

A functor is faithful when its action on hom-sets is injective:

is-faithful : Functor C D → Type _
is-faithful F = ∀ {x y} → injective (F₁ F {x = x} {y})

ff Functors🔗

A functor is fully faithful (abbreviated ff) when its action on hom-sets is an equivalence. Since Hom-sets are sets, it suffices for the functor to be full and faithful; Rather than taking this conjunction as a definition, we use the more directly useful data as a definition and prove the conjunction as a theorem.

is-fully-faithful : Functor C D → Type _
is-fully-faithful F = ∀ {x y} → is-equiv (F₁ F {x = x} {y})

fully-faithful→faithful : {F : Functor C D} → is-fully-faithful F → is-faithful F
fully-faithful→faithful {F = F} ff {_} {_} {x} {y} p =
  x                         ≡⟨ sym (equiv→unit ff x) ⟩≡
  equiv→inverse ff (F₁ F x) ≡⟨ ap (equiv→inverse ff) p ⟩≡
  equiv→inverse ff (F₁ F y) ≡⟨ equiv→unit ff y ⟩≡
  y                         ∎

fully-faithful→full : {F : Functor C D} → is-fully-faithful F → is-full F
fully-faithful→full {F = F} ff g = inc (equiv→inverse ff g , equiv→counit ff g)

full+faithful→ff
  : (F : Functor C D) → is-full F → is-faithful F → is-fully-faithful F
full+faithful→ff {C = C} {D = D} F surj inj .is-eqv = p where
  img-is-prop : ∀ {x y} f → is-prop (fibre (F₁ F {x = x} {y}) f)
  img-is-prop f (g , p) (h , q) = Σ-prop-path (λ _ → D .Hom-set _ _ _ _) (inj (p ∙ sym q))

  p : ∀ {x y} f → is-contr (fibre (F₁ F {x = x} {y}) f)
  p f .centre = ∥-∥-elim (λ _ → img-is-prop f) (λ x → x) (surj f)
  p f .paths = img-is-prop f _

A very important property of fully faithful functors (like FF) is that they are conservative: If the image of f:x→yf : x \to y under FF is an isomorphism Fx≅FyFx \cong Fy, then ff was really an isomorphism f:x≅yf : x \cong y.

module _ {C : Precategory o h} {D : Precategory o₁ h₁} where
  private
    module C = Precategory C
    module D = Precategory D

  import Cat.Morphism C as Cm
  import Cat.Morphism D as Dm

  is-ff→is-conservative
    : {F : Functor C D} → is-fully-faithful F
    → ∀ {X Y} (f : C.Hom X Y) → Dm.is-invertible (F₁ F f)
    → Cm.is-invertible f
  is-ff→is-conservative {F = F} ff f isinv = i where
    open Cm.is-invertible
    open Cm.Inverses

Since the functor is ff, we can find a map “F1−1(f):y→xF_1^{-1}(f) : y \to x” in the domain category to serve as an inverse for ff:

    g : C.Hom _ _
    g = equiv→inverse ff (isinv .Dm.is-invertible.inv)
    module ff {a} {b} = Equiv (_ , ff {a} {b})

    Ffog =
      F₁ F (f C.∘ g)    ≡⟨ F-∘ F _ _ ⟩≡
      F₁ F f D.∘ F₁ F g ≡⟨ ap₂ D._∘_ refl (ff.ε _) ∙ isinv .Dm.is-invertible.invl ⟩≡
      D.id              ∎

    Fgof =
      F₁ F (g C.∘ f)    ≡⟨ F-∘ F _ _ ⟩≡
      F₁ F g D.∘ F₁ F f ≡⟨ ap₂ D._∘_ (ff.ε _) refl ∙ isinv .Dm.is-invertible.invr ⟩≡
      D.id              ∎

    i : Cm.is-invertible _
    i .inv = g
    i .inverses .invl =
      f C.∘ g                    ≡⟨ sym (ff.η _) ⟩≡
      ff.from ⌜ F₁ F (f C.∘ g) ⌝ ≡⟨ ap! (Ffog ∙ sym (F-id F)) ⟩≡
      ff.from (F₁ F C.id)        ≡⟨ ff.η _ ⟩≡
      C.id                       ∎
    i .inverses .invr =
      g C.∘ f                    ≡⟨ sym (ff.η _) ⟩≡
      ff.from ⌜ F₁ F (g C.∘ f) ⌝ ≡⟨ ap! (Fgof ∙ sym (F-id F)) ⟩≡
      ff.from (F₁ F C.id)        ≡⟨ ff.η _ ⟩≡
      C.id                       ∎

  is-ff→essentially-injective
    : {F : Functor C D} → is-fully-faithful F
    → ∀ {X Y} → F₀ F X Dm.≅ F₀ F Y
    → X Cm.≅ Y
  is-ff→essentially-injective {F = F} ff im = im′ where
    -- Cm.make-iso (equiv→inverse ff to) inv invl invr
    open Dm._≅_ im using (to ; from ; inverses)
    D-inv′ : Dm.is-invertible (F₁ F (equiv→inverse ff to))
    D-inv′ .Dm.is-invertible.inv = from
    D-inv′ .Dm.is-invertible.inverses =
      subst (λ e → Dm.Inverses e from) (sym (equiv→counit ff _)) inverses

    open Cm.is-invertible (is-ff→is-conservative {F = F} ff (equiv→inverse ff to) D-inv′)

    im′ : _ Cm.≅ _
    im′ .to   = equiv→inverse ff to
    im′ .from = inv
    im′ .inverses .Cm.Inverses.invl = invl
    im′ .inverses .Cm.Inverses.invr = invr

Essential Fibres🔗

The essential fibre of a functor F:C→DF : C \to D over an object y:Dy : D is the space of objects of CC which FF takes, up to isomorphism, to yy.

Essential-fibre : Functor C D → D .Ob → Type _
Essential-fibre {D = D} F y = Σ[ x ∈ _ ] (F₀ F x ≅ y)
  where open import Cat.Morphism D

A functor is split essentially surjective (abbreviated split eso) if there is a procedure for finding points in the essential fibre over any object. It’s essentially surjective if the this procedure merely, i.e. truncatedly, finds a point:

is-split-eso : Functor C D → Type _
is-split-eso F = ∀ y → Essential-fibre F y

is-eso : Functor C D → Type _
is-eso F = ∀ y → ∥ Essential-fibre F y ∥
module _ {C : Precategory o h} {D : Precategory o₁ h₁} where
  import Cat.Reasoning C as C
  import Cat.Reasoning D as D
  private module _ where
    open import Cat.Reasoning using (_≅_ ; Inverses)
    open _≅_ public
    open Inverses public

  open import Cat.Univalent

  F-map-path : ∀ {x y} (F : Functor C D) (i : x C.≅ y)
             → (ccat : is-category C)
             → (dcat : is-category D)
             → ap (F₀ F) (Univalent.iso→path ccat i) ≡ Univalent.iso→path dcat (F-map-iso F i)
  F-map-path F i ccat dcat =
    Univalent.J-iso ccat
      (λ B p → ap (F₀ F) (Univalent.iso→path ccat p) ≡ Univalent.iso→path dcat (F-map-iso F p))
      idc
      i
    where abstract
      idc : ∀ {x} → ap (F₀ F) (Univalent.iso→path ccat (C.id-iso {x}) )
          ≡ Univalent.iso→path dcat (F-map-iso F C.id-iso)
      idc =
        ap (F₀ F) ⌜ Univalent.iso→path ccat C.id-iso ⌝ ≡⟨ ap! (Univalent.iso→path-id ccat) ⟩≡
        ap (F₀ F) refl                                 ≡˘⟨ Univalent.path→iso→path dcat _ ⟩≡˘
        Univalent.iso→path dcat ⌜ path→iso refl ⌝      ≡⟨ ap! (D.≅-pathp refl refl (transport-refl _ ∙ sym (F-id F))) ⟩≡
        Univalent.iso→path dcat (F-map-iso F C.id-iso) ∎

  ap-F₀-to-iso
    : ∀ (F : Functor C D) {y z : C .Ob}
    → (p : y ≡ z) → path→iso (ap (F₀ F) p) ≡ F-map-iso F (path→iso p)
  ap-F₀-to-iso F {y} =
    J (λ _ p → path→iso (ap (F₀ F) p) ≡ F-map-iso F (path→iso p))
      (D.≅-pathp (λ _ → F .F₀ y) (λ _ → F .F₀ y)
        (Regularity.fast! (sym (F .F-id))))

  ap-F₀-iso
    : ∀ (cc : is-category C) (F : Functor C D) {y z : C.Ob}
    → (p : y C.≅ z) → path→iso (ap (F .F₀) (cc .to-path p)) ≡ F-map-iso F p
  ap-F₀-iso cc F p = ap-F₀-to-iso F (cc .to-path p)
                   ∙ ap (F-map-iso F) (Univalent.iso→path→iso cc p)

  is-ff→F-map-iso-is-equiv
    : {F : Functor C D} → is-fully-faithful F
    → ∀ {X Y} → is-equiv (F-map-iso {x = X} {Y} F)
  is-ff→F-map-iso-is-equiv {F = F} ff = is-iso→is-equiv isom where
    isom : is-iso _
    isom .is-iso.inv = is-ff→essentially-injective {F = F} ff
    isom .is-iso.rinv x = D.≅-pathp refl refl (equiv→counit ff _)
    isom .is-iso.linv x = C.≅-pathp refl refl (equiv→unit ff _)

Pseudomonic Functors🔗

A functor is pseudomonic if it is faithful and full on isomorphisms. Pseudomonic functors are arguably the correct notion of subcategory, as they ensure that we are not able to distinguish between isomorphic objects when creating a subcategory.

  is-full-on-isos : Functor C D → Type (o ⊔ h ⊔ h₁)
  is-full-on-isos F =
    ∀ {x y} → (f : (F .F₀ x) D.≅ (F .F₀ y)) → ∃[ g ∈ x C.≅ y ] (F-map-iso F g ≡ f)

  record is-pseudomonic (F : Functor C D) : Type (o ⊔ h ⊔ h₁) where
    no-eta-equality
    field
      faithful : is-faithful F
      isos-full : is-full-on-isos F

  open is-pseudomonic

Somewhat surprisingly, pseudomonic functors are conservative. As FF is full on isos, there merely exists some iso gg in the fibre of ff. However, Invertability is a property of morphisms, so we can untruncate the mere existence. Once we have our hands on the isomorphism, we perform a simple calculation to note that it yields an inverse to ff.

  pseudomonic→conservative
    : ∀ {F : Functor C D}
    → is-pseudomonic F
    → ∀ {x y} (f : C.Hom x y) → D.is-invertible (F₁ F f)
    → C.is-invertible f
  pseudomonic→conservative {F = F} pseudo {x} {y} f inv =
    ∥-∥-rec C.is-invertible-is-prop
      (λ (g , p) →
        C.make-invertible (C.from g)
          (sym (ap (C._∘ _) (pseudo .faithful (ap D.to p))) ∙ C.invl g)
          (sym (ap (_ C.∘_) (pseudo .faithful (ap D.to p))) ∙ C.invr g))
      (pseudo .isos-full (D.invertible→iso _ inv))

In a similar vein, pseudomonic functors are essentially injective. The proof follows a similar path to the prior one, hinging on the fact that faithful functors are an embedding on isos.

  pseudomonic→essentially-injective
    : ∀ {F : Functor C D}
    → is-pseudomonic F
    → ∀ {x y} → F₀ F x D.≅ F₀ F y
    → x C.≅ y
  pseudomonic→essentially-injective {F = F} pseudo f =
    ∥-∥-rec (faithful→iso-fibre-prop F (pseudo .faithful) f)
      (λ x → x)
      (pseudo .isos-full f) .fst

Fully faithful functors are pseudomonic, as they are faithful and essentially injective.

  ff→pseudomonic
    : ∀ {F : Functor C D}
    → is-fully-faithful F
    → is-pseudomonic F
  ff→pseudomonic {F} ff .faithful = fully-faithful→faithful {F = F} ff
  ff→pseudomonic {F} ff .isos-full f =
    inc (is-ff→essentially-injective {F = F} ff f ,
         D.≅-pathp refl refl (equiv→counit ff (D.to f)))