Library UniMath.CategoryTheory.precomp_ess_surj


**********************************************************
Benedikt Ahrens, Chris Kapulkin, Mike Shulman
january 2013
**********************************************************
Contents : Precomposition with a fully faithful and essentially surjective functor yields an essentially surjective functor

Require Import UniMath.Foundations.PartD.
Require Import UniMath.Foundations.Propositions.
Require Import UniMath.Foundations.Sets.

Require Import UniMath.MoreFoundations.Tactics.
Require Import UniMath.MoreFoundations.Univalence.

Require Import UniMath.CategoryTheory.Core.Categories.
Require Import UniMath.CategoryTheory.Core.Isos.
Require Import UniMath.CategoryTheory.Core.Univalence.
Require Import UniMath.CategoryTheory.Core.Functors.
Require Import UniMath.CategoryTheory.FunctorCategory.
Require Import UniMath.CategoryTheory.whiskering.

Local Open Scope cat.

Local Notation "FF ^-1" := (fully_faithful_inv_hom FF _ _ ).
Local Notation "F '^-i'" := (iso_from_fully_faithful_reflection F) (at level 20).
Local Notation "G 'O' F" := (functor_compose _ _ _ F G) (at level 25).

Ltac inv_functor HF x y :=
   let H:=fresh in
   set (H:= homotweqinvweq (weq_from_fully_faithful HF x y));
     simpl in H;
     unfold fully_faithful_inv_hom; simpl;
     rewrite H; clear H.

Lengthy preparation for the main result of this file


Section precomp_w_ess_surj_ff_is_ess_surj.

Section variables


Variables A B C : precategory.
Hypothesis Ccat : is_univalent C.
Variable H : functor A B.
Hypothesis p : essentially_surjective H.
Hypothesis fH : fully_faithful H.

We prove that precomposition with a H yields an essentially surjective functor

Section essentially_surjective.

Specification of preimage G of a functor F

Given a functor F from A to C, we construct G such that F = G O H

Variable F : functor A C.

Section preimage.

The type X b will be contractible, and G is defined as the first component of its center.

Local Definition X (b : B) := total2 (
 fun ck :
  total2 (λ c : C,
                ∏ a : A,
                     iso (H a) b → iso (F a) c) ⇒
    ∏ t t' : total2 (λ a : A, iso (H a) b),
          ∏ f : pr1 t --> pr1 t',
             (#H f · pr2 t' = pr2 t →
                    #F f · pr2 ck (pr1 t') (pr2 t') = pr2 ck (pr1 t) (pr2 t))).

Local Definition kX {b : B} (t : X b) := (pr2 (pr1 t)).

The following is the third component of the center of X b

Lemma X_aux_type_center_of_contr_proof (b : B) (anot : A) (hnot : iso (H anot) b) :
  ∏ (t t' : total2 (λ a : A, iso (H a) b))
    (f : pr1 t --> pr1 t'),
  #H f· pr2 t' = pr2 t →
  #F f·
  #F (fH^-1 (pr2 t'· inv_from_iso hnot)) =
  #F (fH^-1 (pr2 t· inv_from_iso hnot)).
Proof.
  intros t t' f.
  destruct t as [a h].
  destruct t' as [a' h'].
  simpl in ×.
  intro star.
  rewrite <- functor_comp.
  apply maponpaths.
  apply (invmaponpathsweq
          (weq_from_fully_faithful fH a anot)).
  simpl.
  rewrite functor_comp.
  inv_functor fH a' anot.
  rewrite assoc.
  inv_functor fH a anot.
  rewrite <- star.
  apply idpath.
Qed.

The center of X b

Definition X_aux_type_center_of_contr (b : B)
    (anot : A)(hnot : iso (H anot) b) : X b.
Proof.
  set (cnot := F anot).
  set (g := fun (a : A)(h : iso (H a) b) ⇒
              (fH^-i (iso_comp h (iso_inv_from_iso hnot)))).
  set (knot := fun (a : A)(h : iso (H a) b) ⇒
                    functor_on_iso F (g a h)).
  simpl in ×.
  ∃ (tpair _ (F anot) knot).
  simpl.
  apply X_aux_type_center_of_contr_proof.
Defined.

Any inhabitant of X b is equal to the center of X b.

Lemma X_aux_type_contr_eq (b : B) (anot : A) (hnot : iso (H anot) b) :
   ∏ t : X b, t = X_aux_type_center_of_contr b anot hnot.
Proof.
  intro t.
  assert (Hpr1 : pr1 (X_aux_type_center_of_contr b anot hnot) = pr1 t).
  set (w := isotoid _ Ccat ((pr2 (pr1 t)) anot hnot) :
          pr1 (pr1 (X_aux_type_center_of_contr b anot hnot)) = pr1 (pr1 t)).
  apply (total2_paths_f w).
  simpl.
  destruct t as [[c1 k1] q1].
  simpl in ×.
  apply funextsec; intro a.
  apply funextsec; intro h.
  set (gah := fH^-i (iso_comp h (iso_inv_from_iso hnot))).
  set (qhelp := q1 (tpair _ a h)(tpair _ anot hnot) gah).
  simpl in ×.

  assert (feedtoqhelp :
        #H (fH^-1 (h· inv_from_iso hnot))· hnot = h).
    inv_functor fH a anot.
    rewrite <- assoc.
    rewrite iso_after_iso_inv.
    apply id_right.
  assert (quack := qhelp feedtoqhelp).
  simpl in ×.
  intermediate_path (iso_comp (functor_on_iso F
       (fH^-i (iso_comp h (iso_inv_from_iso hnot)))) (idtoiso w) ).
  generalize w; intro w0.
  induction w0.
  simpl. apply eq_iso. simpl.
  simpl. rewrite id_right.
  apply idpath.

  apply eq_iso.
  simpl.
  unfold w.
  rewrite idtoiso_isotoid.
  apply quack.

  apply pathsinv0.
  apply (total2_paths_f Hpr1).
  apply proofirrelevance.

  repeat (apply impred; intro).
  apply (pr2 Ccat).
Qed.

Putting everything together: X b is contractible.

Definition iscontr_X : ∏ b : B, iscontr (X b).
Proof.
  intro b.
  assert (HH : isaprop (iscontr (X b))).
  apply isapropiscontr.
  apply (p b (tpair (λ x, isaprop x) (iscontr (X b)) HH)).
  intro t.
  ∃ (X_aux_type_center_of_contr b (pr1 t) (pr2 t)).
  apply (X_aux_type_contr_eq b (pr1 t) (pr2 t)).
Defined.

The object part of G, Go b, is defined as the first component of the center of X b.

G on objects


Definition Go : B → C :=
   λ b : B, pr1 (pr1 (pr1 (iscontr_X b))).

Local Definition k (b : B) :
     ∏ a : A, iso (H a) b → iso (F a) (Go b) :=
              pr2 (pr1 (pr1 (iscontr_X b))).

Local Definition q (b : B) := pr2 (pr1 (iscontr_X b)).

Given any inhabitant of X b, its first component is equal to Go b.

Definition Xphi (b : B) (t : X b) : pr1 (pr1 t) = Go b.
Proof.
  set (p1 := pr2 (iscontr_X b) t).
  exact (base_paths _ _ (base_paths _ _ p1)).
Defined.

Given any inhabitant t : X b, its second component is equal to k b, modulo transport along Xphi b t.

Definition Xkphi_transp (b : B) (t : X b) :
     ∏ a : A, ∏ h : iso (H a) b,
  transportf _ (Xphi b t) (kX t) a h = k b a h.
Proof.
  unfold k.
  rewrite <- (fiber_paths (base_paths _ _ (pr2 (iscontr_X b) t))).
  intros ? ?.
  apply maponpaths, idpath.
Qed.

Similarly to the lemma before, the second component of t is the same as k b, modulo postcomposition with an isomorphism.

Definition Xkphi_idtoiso (b : B) (t : X b) :
    ∏ a : A, ∏ h : iso (H a) b,
   k b a h · idtoiso (!Xphi b t) = kX t a h.
Proof.
  intros a h.
  rewrite <- (Xkphi_transp _ t).
  generalize (Xphi b t).
  intro i; destruct i.
  apply id_right.
Qed.


Preparation for G on morphisms

G f will be defined as the first component of the center of contraction of Y f.

Local Definition Y {b b' : B} (f : b --> b') :=
  total2 (fun g : Go b --> Go b' ⇒
      ∏ a : A,
        ∏ h : iso (H a) b,
          ∏ a' : A,
            ∏ h' : iso (H a') b',
              ∏ l : a --> a',
                #H l · h' = h · f → #F l · k b' a' h' = k b a h · g).

Lemma Y_inhab_proof (b b' : B) (f : b --> b') (a0 : A) (h0 : iso (H a0) b)
    (a0' : A) (h0' : iso (H a0') b') :
  ∏ (a : A) (h : iso (H a) b) (a' : A) (h' : iso (H a') b')
    (l : a --> a'),
  #H l· h' = h· f →
  #F l· k b' a' h' =
    k b a h· ((inv_from_iso (k b a0 h0)·
  #F (fH^-1 ((h0· f)· inv_from_iso h0')))· k b' a0' h0').
Proof.
  intros a h a' h' l alpha.
  set (m := fH^-i (iso_comp h0 (iso_inv_from_iso h))).
  set (m' := fH^-i (iso_comp h0' (iso_inv_from_iso h'))).
  assert (sss : iso_comp (functor_on_iso F m) (k b a h) =
                   k b a0 h0).
    apply eq_iso.
    apply (q b (tpair _ a0 h0) (tpair _ a h) m).
    simpl.
    inv_functor fH a0 a.
    rewrite <- assoc.
    rewrite iso_after_iso_inv.
    apply id_right.
  assert (ssss : iso_comp (functor_on_iso F m') (k b' a' h') =
                   k b' a0' h0').
    apply eq_iso.
    apply (q b' (tpair _ a0' h0') (tpair _ a' h') m').
    simpl;
    inv_functor fH a0' a'.
    rewrite <- assoc.
    rewrite iso_after_iso_inv.
    apply id_right.

  set (hfh := h0 · f · inv_from_iso h0').
  set (l0 := fH^-1 hfh).
  set (g0 := inv_from_iso (k b a0 h0) · #F l0 · k b' a0' h0').

  assert (sssss : #H (l0 · m') = #H (m · l)).
    rewrite functor_comp .
    unfold m'. simpl.
    inv_functor fH a0' a'.
    unfold l0.
    inv_functor fH a0 a0'.
    unfold hfh.
    intermediate_path (h0 · f · (inv_from_iso h0' · h0') · inv_from_iso h').
      repeat rewrite assoc; apply idpath.
    rewrite iso_after_iso_inv, id_right, functor_comp.
    inv_functor fH a0 a.
    repeat rewrite <- assoc.
    apply maponpaths, pathsinv0, iso_inv_on_right.
    rewrite assoc.
    apply iso_inv_on_left, pathsinv0, alpha.

  assert (star5 : inv_from_iso m · l0 = l · inv_from_iso m').
    apply iso_inv_on_right.
    rewrite assoc.
    apply iso_inv_on_left,
          (invmaponpathsweq (weq_from_fully_faithful fH a0 a' )),
          pathsinv0,
          sssss.
  clear sssss.
  unfold g0.
  assert (sss'' : k b a h · inv_from_iso (k b a0 h0) =
             inv_from_iso (functor_on_iso F m)).
    apply pathsinv0, iso_inv_on_left, pathsinv0.
    apply iso_inv_on_right.
    unfold m; simpl.
    apply pathsinv0, (base_paths _ _ sss).
  repeat rewrite assoc.
  rewrite sss''. clear sss'' sss.

  rewrite <- functor_on_inv_from_iso.
  rewrite <- functor_comp.
  rewrite star5; clear star5 .
  rewrite functor_comp, functor_on_inv_from_iso.
  assert (star4 :
        inv_from_iso (functor_on_iso F m')· k b' a0' h0'
           = k b' a' h' ).
    apply iso_inv_on_right.
    apply pathsinv0, (base_paths _ _ ssss).
  rewrite <- assoc.
  rewrite star4.
  apply idpath.
Qed.

The center of Y b b' f.

Definition Y_inhab (b b' : B) (f : b --> b')
      (a0 : A) (h0 : iso (H a0) b) (a0' : A) (h0' : iso (H a0') b') : Y f.
Proof.
  set (hfh := h0 · f · inv_from_iso h0').
  set (l0 := fH^-1 hfh).
  set (g0 := inv_from_iso (k b a0 h0) · #F l0 · k b' a0' h0').
  ∃ g0.
  apply Y_inhab_proof.
Defined.

Any inhabitant of Y b b' f is equal to the center.

Lemma Y_contr_eq (b b' : B) (f : b --> b')
     (a0 : A) (h0 : iso (H a0) b)
     (a0' : A) (h0' : iso (H a0') b') :
  ∏ t : Y f, t = Y_inhab b b' f a0 h0 a0' h0'.
Proof.
  intro t.
  apply pathsinv0.
  assert (Hpr : pr1 (Y_inhab b b' f a0 h0 a0' h0') = pr1 t).
    destruct t as [g1 r1]; simpl in ×.
    rewrite <- assoc.
    apply iso_inv_on_right.
    set (hfh := h0 · f · inv_from_iso h0').
    set (l0 := fH^-1 hfh).
    apply (r1 a0 h0 a0' h0' l0).
    unfold l0.
    inv_functor fH a0 a0' .
    unfold hfh.
    repeat rewrite <- assoc.
    rewrite iso_after_iso_inv, id_right.
    apply idpath.
  apply (total2_paths_f Hpr).
  apply proofirrelevance.
  repeat (apply impred; intro).
  apply (pr2 Ccat).
Qed.

The type Y b b' f is contractible.

Definition Y_iscontr (b b' : B) (f : b --> b') :
   iscontr (Y f).
Proof.
  assert (HH : isaprop (iscontr (Y f))).
    apply isapropiscontr.
  apply (p b (tpair (λ x, isaprop x) (iscontr (Y f)) HH)).
  intros [a0 h0].
  apply (p b' (tpair (λ x, isaprop x) (iscontr (Y f)) HH)).
  intros [a0' h0'].
  ∃ (Y_inhab b b' f a0 h0 a0' h0').
  apply Y_contr_eq.
Defined.

G on morphisms

We now have the data necessary to define the functor G.

Definition preimage_functor_data : functor_data B C.
Proof.
  ∃ Go.
  intros b b' f.
  exact (pr1 (pr1 (Y_iscontr b b' f))).
Defined.

Local Notation "'G' f" := (pr1 (pr1 (Y_iscontr _ _ f))) (at level 3).

The above data is indeed functorial.

Lemma is_functor_preimage_functor_data : is_functor preimage_functor_data.
Proof.
  split. unfold functor_idax. simpl.
  intro b.

  assert (PR2 : ∏ (a : A) (h : iso (H a) b) (a' : A)
          (h' : iso (H a') b)
    (l : a --> a'),
  #H l· h' = h· identity b →
  #F l· k b a' h' = k b a h· identity (Go b)).
    intros a h a' h' l LL.
    rewrite id_right.
    apply (q b (tpair _ a h) (tpair _ a' h') l).
    rewrite id_right in LL.
    apply LL.
    set (Gbrtilde :=
           tpair _ (identity (Go b)) PR2 : Y (identity b)).

    set (H' := pr2 (Y_iscontr b b (identity b)) Gbrtilde).
    set (H'' := base_paths _ _ H').
    simpl in H'.
    rewrite <- H'.
    apply idpath.

composition

  intros b b' b'' f f'.

  assert (HHHH : isaprop (pr1 (pr1 (Y_iscontr b b'' (f· f'))) =
                        pr1 (pr1 (Y_iscontr b b' f))· pr1 (pr1 (Y_iscontr b' b'' f')))).
    apply (pr2 Ccat).
  apply (p b (tpair (λ x, isaprop x) (pr1 (pr1 (Y_iscontr b b'' (f· f'))) =
           pr1 (pr1 (Y_iscontr b b' f))· pr1 (pr1 (Y_iscontr b' b'' f'))) HHHH)).
  intros [a0 h0]; simpl.
  apply (p b' (tpair (λ x, isaprop x) (pr1 (pr1 (Y_iscontr b b'' (f· f'))) =
           pr1 (pr1 (Y_iscontr b b' f))· pr1 (pr1 (Y_iscontr b' b'' f'))) HHHH)).
  intros [a0' h0']; simpl.
  apply (p b'' (tpair (λ x, isaprop x) (pr1 (pr1 (Y_iscontr b b'' (f· f'))) =
           pr1 (pr1 (Y_iscontr b b' f))· pr1 (pr1 (Y_iscontr b' b'' f'))) HHHH)).
  intros [a0'' h0''].
  simpl; clear HHHH.

  set (l0 := fH^-1 (h0 · f · inv_from_iso h0')).
  set (l0' := fH^-1 (h0' · f' · inv_from_iso h0'')).
  set (l0'' := fH^-1 (h0 · (f· f') · inv_from_iso h0'')).

  assert (L : l0 · l0' = l0'').
    apply (invmaponpathsweq (weq_from_fully_faithful fH a0 a0'')).
    simpl; rewrite functor_comp.
    unfold l0'.
    inv_functor fH a0' a0''.
    unfold l0.
    inv_functor fH a0 a0'.
    intermediate_path (h0 · f · (inv_from_iso h0' · h0') · f' · inv_from_iso h0'').
      repeat rewrite assoc; apply idpath.
    rewrite iso_after_iso_inv, id_right.
    unfold l0''.
    inv_functor fH a0 a0''.
    repeat rewrite assoc; apply idpath.

  assert (PR2 : ∏ (a : A) (h : iso (H a) b)(a' : A)
          (h' : iso (H a') b') (l : a --> a'),
           #H l· h' = h· f →
           #F l· k b' a' h' =
            k b a h· ((inv_from_iso (k b a0 h0)· #F l0)· k b' a0' h0') ).
    intros a h a' h' l.
    intro alpha.
    set (m := fH^-i (iso_comp h0 (iso_inv_from_iso h))).
    set (m' := fH^-i (iso_comp h0' (iso_inv_from_iso h'))).
    assert (sss : iso_comp (functor_on_iso F m) (k b a h) =
                   k b a0 h0).
      apply eq_iso; simpl.
      apply (q b (tpair _ a0 h0) (tpair _ a h) m).
      simpl.
      inv_functor fH a0 a.
      rewrite <- assoc.
      rewrite iso_after_iso_inv.
      apply id_right.
    assert (ssss : iso_comp (functor_on_iso F m') (k b' a' h') =
                   k b' a0' h0').
      apply eq_iso; simpl.
      apply (q b' (tpair _ a0' h0') (tpair _ a' h') m'); simpl.
      inv_functor fH a0' a'.
      rewrite <- assoc.
      rewrite iso_after_iso_inv.
      apply id_right.
    assert (sssss : #H (l0 · m') = #H (m · l)).
      rewrite functor_comp.
      unfold m'; simpl.
      inv_functor fH a0' a'.
      unfold l0.
      inv_functor fH a0 a0'.
      intermediate_path (h0 · f · (inv_from_iso h0' · h0') · inv_from_iso h').
        repeat rewrite assoc; apply idpath.
      rewrite iso_after_iso_inv, id_right, functor_comp.
      inv_functor fH a0 a.
      repeat rewrite <- assoc.
      apply maponpaths.
      apply pathsinv0.
      apply iso_inv_on_right.
      rewrite assoc.
      apply iso_inv_on_left.
      apply pathsinv0.
      apply alpha.
    assert (star5 : inv_from_iso m · l0 = l · inv_from_iso m').
      apply iso_inv_on_right.
      rewrite assoc.
      apply iso_inv_on_left.
      apply (invmaponpathsweq (weq_from_fully_faithful fH a0 a' )).
      apply pathsinv0.
      apply sssss.
    clear sssss.
    set (sss':= base_paths _ _ sss); simpl in sss'.
    assert (sss'' : k b a h · inv_from_iso (k b a0 h0) =
             inv_from_iso (functor_on_iso F m)).
      apply pathsinv0.
      apply iso_inv_on_left.
      apply pathsinv0.
      apply iso_inv_on_right.
      unfold m; simpl.
      apply pathsinv0.
      apply sss'.
    repeat rewrite assoc.
    rewrite sss''. clear sss'' sss' sss.
    rewrite <- functor_on_inv_from_iso.
    rewrite <- functor_comp.
    rewrite star5, functor_comp, functor_on_inv_from_iso.
    clear star5.
    assert (star4 :
        inv_from_iso (functor_on_iso F m')· k b' a0' h0'
           = k b' a' h' ).
      apply iso_inv_on_right.
      set (ssss' := base_paths _ _ ssss).
      apply pathsinv0.
      simpl in ssss'. simpl.
      apply ssss'; clear ssss'.
    rewrite <- assoc.
    rewrite star4.
    apply idpath.

  assert (HGf : G f = inv_from_iso (k b a0 h0) · #F l0 · k b' a0' h0').
    set (Gbrtilde :=
           tpair _ (inv_from_iso (k b a0 h0) · #F l0 · k b' a0' h0') PR2 : Y f).
    set (H' := pr2 (Y_iscontr b b' f) Gbrtilde).
    set (H'' := base_paths _ _ H').
    simpl in H'.
    rewrite <- H'.
    apply idpath.

  clear PR2.
  assert (PR2 : ∏ (a : A) (h : iso (H a) b') (a' : A)
            (h' : iso (H a') b'') (l : a --> a'),
         #H l· h' = h· f' →
           #F l· k b'' a' h' =
         k b' a h· ((inv_from_iso (k b' a0' h0')· #F l0')· k b'' a0'' h0'')).
    intros a' h' a'' h'' l'.
    intro alpha.
    set (m := fH^-i (iso_comp h0' (iso_inv_from_iso h'))).
    set (m' := fH^-i (iso_comp h0'' (iso_inv_from_iso h''))).
    assert (sss : iso_comp (functor_on_iso F m) (k b' a' h') =
                   k b' a0' h0').
      apply eq_iso; simpl.
      apply (q b' (tpair _ a0' h0') (tpair _ a' h') m); simpl.
      inv_functor fH a0' a'.
      rewrite <- assoc.
      rewrite iso_after_iso_inv.
      apply id_right.
    assert (ssss : iso_comp (functor_on_iso F m') (k b'' a'' h'') =
                   k b'' a0'' h0'').
      apply eq_iso; simpl.
      apply (q b'' (tpair _ a0'' h0'') (tpair _ a'' h'') m'); simpl.
      inv_functor fH a0'' a''.
      rewrite <- assoc.
      rewrite iso_after_iso_inv.
      apply id_right.
    assert (sssss : #H (l0' · m') = #H (m · l')).
      rewrite functor_comp.
      unfold m'. simpl.
      inv_functor fH a0'' a''.
      unfold l0'.
      inv_functor fH a0' a0''.
      intermediate_path (h0' · f' · (inv_from_iso h0'' · h0'') · inv_from_iso h'').
        repeat rewrite assoc; apply idpath.
      rewrite iso_after_iso_inv, id_right, functor_comp.
      inv_functor fH a0' a'.
      repeat rewrite <- assoc.
      apply maponpaths, pathsinv0, iso_inv_on_right.
      rewrite assoc.
      apply iso_inv_on_left, pathsinv0, alpha.
    assert (star5 : inv_from_iso m · l0' = l' · inv_from_iso m').
      apply iso_inv_on_right.
      rewrite assoc.
      apply iso_inv_on_left,
        (invmaponpathsweq (weq_from_fully_faithful fH a0' a'' )),
        pathsinv0,
        sssss.
    set (sss':= base_paths _ _ sss); simpl in sss'.
    assert (sss'' : k b' a' h' · inv_from_iso (k b' a0' h0') =
             inv_from_iso (functor_on_iso F m)).
      apply pathsinv0, iso_inv_on_left, pathsinv0, iso_inv_on_right.
      unfold m; simpl;
      apply pathsinv0, sss'.
    repeat rewrite assoc.
    rewrite sss''. clear sss'' sss' sss.
    rewrite <- functor_on_inv_from_iso.
    rewrite <- functor_comp.
    rewrite star5. clear star5 sssss.
    rewrite functor_comp, functor_on_inv_from_iso.
    assert (star4 :
        inv_from_iso (functor_on_iso F m')· k b'' a0'' h0''
           = k b'' a'' h'' ).
      apply iso_inv_on_right.
      set (ssss' := base_paths _ _ ssss).
      apply pathsinv0.
      simpl in *; apply ssss'.
    rewrite <- assoc.
    rewrite star4.
    apply idpath.
  assert (HGf' : G f' = inv_from_iso (k b' a0' h0') · #F l0' · k b'' a0'' h0'').
    set (Gbrtilde :=
       tpair _ (inv_from_iso (k b' a0' h0') · #F l0' · k b'' a0'' h0'') PR2 :
                      Y f').
    set (H' := pr2 (Y_iscontr b' b'' f') Gbrtilde).
    rewrite <-(base_paths _ _ H').
    apply idpath.

  clear PR2.
  assert (PR2 : ∏ (a : A) (h : iso (H a) b) (a' : A)
             (h' : iso (H a') b'') (l : a --> a'),
          #H l· h' = h· (f· f') →
          #F l· k b'' a' h' =
           k b a h· ((inv_from_iso (k b a0 h0)· #F l0'')· k b'' a0'' h0'')).
    intros a h a'' h'' l.
    intro alpha.
    set (m := fH^-i (iso_comp h0 (iso_inv_from_iso h))).
    set (m' := fH^-i (iso_comp h0'' (iso_inv_from_iso h''))).
    assert (sss : iso_comp (functor_on_iso F m) (k b a h) = k b a0 h0).
      apply eq_iso.
      apply (q b (tpair _ a0 h0) (tpair _ a h) m); simpl.
      inv_functor fH a0 a.
      rewrite <- assoc.
      rewrite iso_after_iso_inv.
      apply id_right.
    assert (ssss : iso_comp (functor_on_iso F m') (k b'' a'' h'') =
                   k b'' a0'' h0'').
      apply eq_iso.
      apply (q b'' (tpair _ a0'' h0'') (tpair _ a'' h'') m').
      simpl; inv_functor fH a0'' a''.
      rewrite <- assoc.
      rewrite iso_after_iso_inv.
      apply id_right.
    assert (sssss : #H (l0'' · m') = #H (m · l)).
      rewrite functor_comp.
      unfold m'. simpl.
      inv_functor fH a0'' a''.
      unfold l0''.
      inv_functor fH a0 a0''.
      intermediate_path (h0 · (f · f') · (inv_from_iso h0'' · h0'') · inv_from_iso h'').
        repeat rewrite assoc; apply idpath.
      rewrite iso_after_iso_inv, id_right, functor_comp.
      inv_functor fH a0 a.
      repeat rewrite <- assoc.
      apply maponpaths, pathsinv0, iso_inv_on_right.
      repeat rewrite assoc.
      apply iso_inv_on_left, pathsinv0.
      repeat rewrite <- assoc.
      apply alpha.
    assert (star5 : inv_from_iso m · l0'' = l · inv_from_iso m').
      apply iso_inv_on_right.
      rewrite assoc.
      apply iso_inv_on_left.
      apply (invmaponpathsweq (weq_from_fully_faithful fH a0 a'' )).
      apply pathsinv0, sssss.
    set (sss':= base_paths _ _ sss); simpl in sss'.
    assert (sss'' : k b a h · inv_from_iso (k b a0 h0) =
             inv_from_iso (functor_on_iso F m)).
      apply pathsinv0, iso_inv_on_left, pathsinv0, iso_inv_on_right.
      unfold m; simpl.
      apply pathsinv0, sss'.
    repeat rewrite assoc.
    rewrite sss''. clear sss'' sss' sss.
    rewrite <- functor_on_inv_from_iso.
    rewrite <- functor_comp.
    rewrite star5. clear star5 sssss.
    rewrite functor_comp, functor_on_inv_from_iso.
    assert (star4 :
        inv_from_iso (functor_on_iso F m')· k b'' a0'' h0''
           = k b'' a'' h'' ).
      apply iso_inv_on_right, pathsinv0, (base_paths _ _ ssss).
    rewrite <- assoc.
    rewrite star4.
    apply idpath.
  assert (HGff' : G (f · f') =
       inv_from_iso (k b a0 h0) · #F l0'' · k b'' a0'' h0'').
    set (Gbrtilde :=
           tpair _ (inv_from_iso (k b a0 h0) · #F l0'' · k b'' a0'' h0'') PR2 :
               Y (f · f')).
    rewrite <- (pr2 (Y_iscontr b b'' (f · f')) Gbrtilde).
    apply idpath.
  clear PR2.
  rewrite HGf, HGf'.
  intermediate_path (inv_from_iso (k b a0 h0)· #F l0· (k b' a0' h0'·
              inv_from_iso (k b' a0' h0'))· #F l0'· k b'' a0'' h0'').
    rewrite iso_inv_after_iso, id_right.
    rewrite HGff'.
    repeat rewrite <- assoc.
    apply maponpaths.
    rewrite <- L.
    rewrite functor_comp.
    repeat rewrite <- assoc.
    apply idpath.
  repeat rewrite <- assoc.
  apply idpath.
Qed.

We call the functor GG ...

Definition GG : [B, C, pr2 Ccat] := tpair _ preimage_functor_data
                    is_functor_preimage_functor_data.

G is the preimage of F under _ O H

Given any a : A, we produce an element in X (H a), whose first component is F a. This allows to prove G (H a) = F a.

Lemma qF (a0 : A) :
  ∏ (t t' : total2 (λ a : A, iso (H a) (H a0)))
    (f : pr1 t --> pr1 t'),
  #H f· pr2 t' = pr2 t →
  #F f· #F (fH^-1 (pr2 t')) =
  #F (fH^-1 (pr2 t)).
Proof.
  simpl.
  intros [a h] [a' h'] f L.
  simpl in L; simpl.
  rewrite <- functor_comp.
  apply maponpaths.
  apply (invmaponpathsweq (weq_from_fully_faithful fH a a0)
                 (f· fH^-1 h') (fH^-1 h) ).
  inv_functor fH a a0.
  rewrite functor_comp.
  inv_functor fH a' a0.
  apply L.
Qed.

Definition kFa (a0 : A) : ∏ a : A,
  iso (H a) (H a0) → iso (F a) (F a0) :=
 fun (a : A) (h : iso (H a) (H a0)) ⇒
   functor_on_iso F
                  (iso_from_fully_faithful_reflection fH h).

Definition XtripleF (a0 : A) : X (H a0) :=
   tpair _ (tpair _ (F a0) (kFa a0)) (qF a0).

Lemma phi (a0 : A) : pr1 (pr1 (functor_composite H GG)) a0 = pr1 (pr1 F) a0.
Proof.
  exact (!Xphi _ (XtripleF a0)).
Defined.

Lemma extphi : pr1 (pr1 (functor_composite H GG)) = pr1 (pr1 F).
Proof.
  apply funextsec.
  unfold homot.
  apply phi.
Defined.

Now for the functor as a whole. It remains to prove equality on morphisms, modulo transport.

Lemma is_preimage_for_pre_composition : functor_composite H GG = F.
Proof.
  apply (functor_eq _ _ (pr2 Ccat) (functor_composite H GG) F).
  apply (total2_paths_f extphi).
  apply funextsec; intro a0;
  apply funextsec; intro a0';
  apply funextsec; intro f.
  rewrite transport_of_functor_map_is_pointwise.
  unfold extphi.
  unfold double_transport.
  rewrite toforallpaths_funextsec.
  rewrite <- idtoiso_postcompose.
  rewrite <- idtoiso_precompose.
  rewrite idtoiso_inv.
  rewrite <- assoc.
  assert (PSIf : ∏ (a : A) (h : iso (H a) (H a0)) (a' : A)
  (h' : iso (H a') (H a0')) (l : a --> a'),
         #H l· h' = h· #H f →
         #F l· k (H a0') a' h' =
         k (H a0) a h·
         ((idtoiso (phi a0)· #F f)· inv_from_iso (idtoiso (phi a0')))).
    intros a h a' h' l alpha.
    rewrite assoc.
    apply iso_inv_on_left.
    unfold phi.
    repeat rewrite assoc.
    rewrite (Xkphi_idtoiso (H a0) (XtripleF a0)).
    repeat rewrite <- assoc.
    rewrite (Xkphi_idtoiso (H a0') (XtripleF a0')).
    simpl.
    assert (HH4 : fH^-1 h · f = l · fH^-1 h').
      apply (invmaponpathsweq (weq_from_fully_faithful fH a a0')).
      simpl; repeat rewrite functor_comp.
      inv_functor fH a a0.
      inv_functor fH a' a0'.
      apply pathsinv0, alpha.
    intermediate_path (#F (fH^-1 h· f)).
      rewrite functor_comp.
      apply idpath.
    rewrite HH4.
    rewrite functor_comp.
    apply idpath.
  set (Ybla := tpair _ (idtoiso (phi a0) · #F f · inv_from_iso (idtoiso (phi a0')))
                    PSIf : Y (#H f)).
  set (Ycontr := pr2 (Y_iscontr _ _ (#(pr1 H) f)) Ybla).
  set (Ycontr2 := base_paths _ _ Ycontr); simpl in ×.
  change (G (#H f)) with (G (#(pr1 H) f)).
  rewrite <- Ycontr2.
  repeat rewrite assoc.
  rewrite iso_after_iso_inv, id_left.
  repeat rewrite <- assoc.
  rewrite iso_after_iso_inv, id_right.
  apply idpath.
Qed.

End preimage.

End essentially_surjective.

Precomposition with an ess. surj. and f. f. functor is ess. surj.

Abstracting from F by closing the previous section, we can prove essential surjectivity of _ O H.

Lemma pre_composition_essentially_surjective (hsB: has_homsets B) :
       essentially_surjective (pre_composition_functor A B C hsB (pr2 Ccat) H).
Proof.
  intros F p' f.
  apply f.
  ∃ (GG F).
  apply idtoiso.
  apply is_preimage_for_pre_composition.
Qed.

End precomp_w_ess_surj_ff_is_ess_surj.