Library UniMath.CategoryTheory.Limits.Opp

Duality between C and C^op

Contents

  • From C^op to C
    • Monics and Epis
    • Initial, Terminal, and Zero
    • Equalizers and Coequalizers
    • Kernels and Cokernels
    • Pullbacks and Pushouts
    • BinProducts and BinCoproducts
  • From C to C^op
    • Monics and Epis
    • Initial, Terminal, and Zero
    • Equalizers and Coequalizers
    • Kernels and Cokernels
    • Pullbacks and Pushouts
    • BinProducts and BinCoproducts

Translation of structures from C^op to C

Section def_opposites.

  Variable C : category.
  Let hs : has_homsets C := homset_property C.

Monic and Epi


  Definition opp_isMonic {a b : C} (f : a --> b) (H : @isMonic (op_category C) _ _ f) : @isEpi C _ _ f := H.
  Opaque opp_isMonic.

  Definition opp_Monic {a b : C} (f : @Monic (op_category C) a b) : @Epi C b a :=
    @make_Epi C _ _ f (opp_isMonic f (pr2 f)).

  Definition opp_isEpi {a b : C} (f : a --> b) (H : @isEpi (C^op) _ _ f) : @isMonic C _ _ f := H.
  Opaque opp_isEpi.

  Definition opp_Epi {a b : C} (f : @Epi (C^op) a b) : @Monic C b a :=
    @make_Monic C _ _ f (opp_isEpi f (pr2 f)).

Initial, Terminal, and Zero


  Definition opp_isInitial {x : C} (H : @isInitial (C^op) x) : @isTerminal C x := H.

  Definition opp_Initial (I : @Initial (C^op)) : @Terminal C :=
    @make_Terminal C _ (opp_isInitial (pr2 I)).

  Definition opp_isTerminal {x : C} (H : @isTerminal (C^op) x) : @isInitial C x := H.

  Definition opp_Terminal (T : @Terminal (C^op)) : @Initial C :=
    @make_Initial C _ (opp_isTerminal (pr2 T)).

  Lemma opp_isZero {x : C} (H : @isZero (C^op) x) : @isZero C x.
  Show proof.
    use make_isZero.
    - intros a. exact (dirprod_pr2 H a).
    - intros a. exact (dirprod_pr1 H a).

  Definition opp_Zero (Z : @Zero (C^op)) : @Zero C := @make_Zero C _ (opp_isZero (pr2 Z)).

Equality on ZeroArrows


  Lemma opp_ZeroArrowTo {x : C} (Z : @Zero (C^op)) :
    @ZeroArrowTo (C^op) Z x = @ZeroArrowFrom C (opp_Zero Z) x.
  Show proof.
    apply ArrowsToZero.

  Lemma opp_ZeroArrowFrom {x : C} (Z : @Zero (C^op)) :
    @ZeroArrowFrom (C^op) Z x = @ZeroArrowTo C (opp_Zero Z) x.
  Show proof.
    apply ArrowsFromZero.

  Lemma opp_ZeroArrow {x y : C} (Z : @Zero (C^op)) :
    @ZeroArrow (C^op) Z x y = @ZeroArrow C (opp_Zero Z) y x.
  Show proof.
    unfold ZeroArrow.
    rewrite opp_ZeroArrowTo.
    rewrite opp_ZeroArrowFrom.
    apply idpath.

  Local Opaque ZeroArrow.

Equalizers and Coequalizers


  Lemma opp_isEqualizer {x y z : C} (f g : (C^op)⟦y, z) (e : (C^op)⟦x, y) (H : e · f = e · g)
    (H' : @isEqualizer (op_category C) _ _ _ f g e H) : @isCoequalizer C _ _ _ f g e H.
  Show proof.
    exact H'.

  Lemma opp_isCoequalizer {x y z : C} (f g : (C^op)⟦x, y) (e : (C^op)⟦y, z)
        (H : f · e = g · e) (H' : @isCoequalizer (C^op) _ _ _ f g e H) :
    @isEqualizer C _ _ _ f g e H.
  Show proof.
    exact H'.

  Definition opp_Equalizer {y z : C} (f g : (C^op)⟦y, z) (E : @Equalizer (op_category C) y z f g) :
    @Coequalizer C z y f g := @make_Coequalizer C _ _ _ f g (EqualizerArrow E) (EqualizerEqAr E)
                                              (opp_isEqualizer f g (EqualizerArrow E)
                                                               (EqualizerEqAr E)
                                                               (isEqualizer_Equalizer E)).

  Definition opp_Coequalizer {y z : C} (f g : (C^op)⟦y, z) (E : @Coequalizer (C^op) y z f g) :
    @Equalizer C z y f g := @make_Equalizer C _ _ _ f g (CoequalizerArrow E) (CoequalizerEqAr E)
                                          (opp_isCoequalizer f g (CoequalizerArrow E)
                                                             (CoequalizerEqAr E)
                                                             (isCoequalizer_Coequalizer E)).

  Definition opp_Equalizers (E : @Equalizers (op_category C)) : @Coequalizers C.
  Show proof.
    intros x y f g.
    use opp_Equalizer.
    exact (E y x f g).

  Definition opp_Coequalizers (E : @Coequalizers (C^op)) : @Equalizers C.
  Show proof.
    intros x y f g.
    use opp_Coequalizer.
    exact (E y x f g).

Kernels and Cokernels


  Local Lemma opp_isCokernel_eq {x y z : C^op} (f : (C^op)⟦x, y) (g : C^opy, z) (Z : Zero (C^op))
        (H : f · g = ZeroArrow Z _ _) (Z' : Zero C) :
    (g : Cz, y) · (f : Cy, x) = ZeroArrow Z' _ _.
  Show proof.
    cbn in *. rewrite H. rewrite opp_ZeroArrow.
    exact (ZerosArrowEq C (opp_Zero Z) Z' z x).

  Lemma opp_isCokernel {x y z : C^op} {f : (C^op)⟦x, y} {g : C^opy, z} {Z : Zero (C^op)}
        {H : f · g = ZeroArrow Z _ _} (K' : isKernel (C:=op_category C) Z f g H) {Z' : Zero C} :
    isCokernel Z' (g : Cz, y) (f : Cy, x) (opp_isCokernel_eq f g Z H Z').
  Show proof.
    set (K := make_Kernel _ _ _ _ K').
    use make_isCokernel.
    - intros w h H'.
      rewrite <- (ZerosArrowEq C (opp_Zero Z) Z' z w) in H'.
      rewrite <- opp_ZeroArrow in H'.
      use unique_exists.
      + exact (KernelIn (C:=op_category _ )Z K w h H').
      + use (KernelCommutes (C:=op_category _) Z K).
      + intros y0. apply hs.
      + cbn. intros y0 X. use (KernelInsEq (C:=op_category _) Z K). rewrite (KernelCommutes (C:=op_category _) Z K). cbn. rewrite X.
        apply idpath.

  Local Lemma opp_Kernel_eq {y z : C} (f : (C^op)⟦y, z) (Z : Zero (C^op))
             (K : @Kernel (op_category C) Z y z f) :
    @compose C^op _ _ _ (KernelArrow K) f = ZeroArrow (opp_Zero Z) z K.
  Show proof.
    cbn. rewrite <- opp_ZeroArrow. apply (KernelCompZero (C:= op_category _) Z K).

  Lemma opp_Kernel_isCokernel {y z : C} (f : (C^op)⟦y, z) (Z : Zero (C^op))
             (K : @Kernel (C^op) Z y z f) :
    isCokernel (opp_Zero Z) f (KernelArrow K) (opp_Kernel_eq f Z K).
  Show proof.
    use make_isCokernel.
    - intros w h H'. rewrite <- opp_ZeroArrow in H'.
      use unique_exists.
      + exact (KernelIn Z K w h H').
      + use (KernelCommutes Z K).
      + intros y0. apply hs.
      + cbn. intros y0 X. use (@KernelInsEq C^op). rewrite (KernelCommutes Z K). cbn. rewrite X.
        apply idpath.

  Definition opp_Kernel {y z : C} (f : (C^op)⟦y, z) (Z : Zero (C^op))
             (K : @Kernel (C^op) Z y z f) : @Cokernel C (opp_Zero Z) z y f.
  Show proof.
    use make_Cokernel.
    - exact K.
    - exact (KernelArrow K).
    - exact (opp_Kernel_eq f Z K).
    - exact (opp_Kernel_isCokernel f Z K).

  Lemma opp_isKernel {x y z : op_category C} {f : (C^op)⟦x, y} {g : C^opy, z} {Z : Zero (C^op)}
        {H : f · g = ZeroArrow Z _ _} (CK' : isCokernel Z f g H) {Z' : Zero C} :
    isKernel Z' (g : Cz, y) (f : Cy, x) (opp_isCokernel_eq f g Z H Z').
  Show proof.
    set (CK := make_Cokernel _ _ _ _ CK').
    use make_isKernel.
    intros w h H'.
    rewrite <- (ZerosArrowEq C (opp_Zero Z) Z' w x) in H'.
    rewrite <- opp_ZeroArrow in H'.
    use unique_exists.
    + exact (CokernelOut Z CK w h H').
    + use (CokernelCommutes Z CK).
    + intros y0. apply hs.
    + cbn. intros y0 X. use (CokernelOutsEq _ CK). rewrite (CokernelCommutes Z CK). cbn. rewrite X.
      apply idpath.

  Local Lemma opp_Cokernel_eq {y z : C} (f : (C^op)⟦y, z) (Z : Zero (C^op))
             (CK : @Cokernel (C^op) Z y z f) :
    @compose (C^op) _ _ _ f (CokernelArrow CK) = ZeroArrow (opp_Zero Z) CK y.
  Show proof.
    cbn. rewrite <- opp_ZeroArrow. apply (CokernelCompZero Z CK).

  Lemma opp_Cokernel_isKernel {y z : C} (f : (C^op)⟦y, z) (Z : Zero (C^op))
             (CK : @Cokernel (C^op) Z y z f) :
    isKernel (opp_Zero Z) (CokernelArrow CK) f (opp_Cokernel_eq f Z CK).
  Show proof.
    use make_isKernel.
    intros w h H'. rewrite <- opp_ZeroArrow in H'.
    use unique_exists.
    + exact (CokernelOut Z CK w h H').
    + use (CokernelCommutes Z CK).
    + intros y0. apply hs.
    + cbn. intros y0 X. use (@CokernelOutsEq C^op). rewrite (CokernelCommutes Z CK). cbn. rewrite X.
      apply idpath.

  Definition opp_Cokernel {y z : C} (f : (C^op)⟦y, z) (Z : Zero (C^op))
             (CK : @Cokernel (C^op) Z y z f) : @Kernel C (opp_Zero Z) z y f.
  Show proof.
    use make_Kernel.
    - exact CK.
    - exact (CokernelArrow CK).
    - exact (opp_Cokernel_eq f Z CK).
    - exact (opp_Cokernel_isKernel f Z CK).

  Definition opp_Kernels (Z : Zero (C^op)) (K : @Kernels (C^op) Z) : @Cokernels C (opp_Zero Z).
  Show proof.
    intros x y f.
    use opp_Kernel.
    apply (K y x f).

  Definition opp_Cokernels (Z : Zero (C^op)) (CK : @Cokernels (C^op) Z) : @Kernels C (opp_Zero Z).
  Show proof.
    intros x y f.
    use opp_Cokernel.
    apply (CK y x f).

Pushouts and pullbacks


  Lemma opp_isPushout {a b c d : C} (f : (C^op)⟦a, b) (g : (C^op)⟦a, c)
        (in1 : (C^op)⟦b, d) (in2 : (C^op)⟦c, d) (H : f · in1 = g · in2)
        (iPo : @isPushout (C^op) a b c d f g in1 in2 H) : @isPullback C a b c d f g in1 in2 H.
  Show proof.
    exact iPo.

  Lemma opp_isPullback {a b c d : C} (f : (C^op)⟦b, a) (g : (C^op)⟦c, a)
        (p1 : (C^op)⟦d, b) (p2 : (C^op)⟦d, c) (H : p1 · f = p2 · g)
        (iPb : @isPullback (C^op) a b c d f g p1 p2 H) : @isPushout C a b c d f g p1 p2 H.
  Show proof.
    exact iPb.

  Definition opp_Pushout {a b c : C} (f : (C^op)⟦a, b) (g : (C^op)⟦a, c)
             (Po : @Pushout (C^op) a b c f g) : @Pullback C a b c f g.
  Show proof.
    exact Po.

  Definition opp_Pullback {a b c : C} (f : (C^op)⟦b, a) (g : (C^op)⟦c, a)
             (Pb : @Pullback (C^op) a b c f g) : @Pushout C a b c f g.
  Show proof.
    exact Pb.

  Definition opp_Pushouts (Pos : @Pushouts (C^op)) : @Pullbacks C.
  Show proof.
    exact Pos.

  Definition opp_Pullbacks (Pbs : @Pushouts (C^op)) : @Pullbacks C.
  Show proof.
    exact Pbs.

BinProducts and BinCoproducts


  Definition opp_isBinProduct (c d p : C) (p1 : (C^op)⟦p, c) (p2 : (C^op)⟦p, d)
             (iBPC : @isBinProduct (C^op) c d p p1 p2) : @isBinCoproduct C c d p p1 p2 :=
    iBPC.

  Definition opp_isBinCoproduct (a b co : C) (ia : (C^op)⟦a, co) (ib : (C^op)⟦b, co)
             (iBCC : @isBinCoproduct (C^op) a b co ia ib) :
    @isBinProduct C a b co ia ib := iBCC.

  Definition opp_BinProduct (c d : C) (BPC : @BinProduct (C^op) c d) :
    @BinCoproduct C c d := BPC.

  Definition opp_BinCoproduct (c d : C) (BCC : @BinCoproduct (C^op) c d) :
    @BinProduct C c d := BCC.

  Definition opp_BinProducts (BP : @BinProducts (C^op)) : @BinCoproducts C := BP.

  Definition opp_BinCoproducts (BC : @BinCoproducts (C^op)) : @BinProducts C := BC.

End def_opposites.

Translation of structures from C to C^op

Section def_opposites'.

  Variable C : category.
  Let hs : has_homsets C := homset_property C.

Monic and Epi


  Definition isMonic_opp {a b : C} {f : Ca, b} (H : @isMonic C a b f) : @isEpi (C^op) b a f := H.
  Opaque isMonic_opp.

  Definition Monic_opp {a b : C} (f : @Monic C a b) : @Epi (C^op) b a :=
    @make_Epi (C^op) b a f (isMonic_opp (pr2 f)).

  Definition isEpi_opp {a b : C} {f : Ca, b} (H : @isEpi C a b f) : @isMonic (C^op) b a f := H.
  Opaque isEpi_opp.

  Definition Epi_opp {a b : C} (f : @Epi C a b) : @Monic (C^op) b a :=
    @make_Monic (C^op) b a f (isEpi_opp (pr2 f)).

Initial, Terminal, and Zero


  Definition isInitial_opp {x : C} (H : @isInitial C x) : @isTerminal (C^op) x := H.

  Definition Initial_opp (I : @Initial C) : @Terminal (C^op) :=
    @make_Terminal (C^op) _ (isInitial_opp (pr2 I)).

  Definition isTerminal_opp {x : C} (H : @isTerminal C x) : @isInitial (C^op) x := H.

  Definition Terminal_opp (T : @Terminal C) : @Initial (C^op) :=
    @make_Initial (C^op) _ (isTerminal_opp (pr2 T)).

  Lemma isZero_opp {x : C} (H : @isZero C x) : @isZero (C^op) x.
  Show proof.
    use make_isZero.
    - intros a. apply (pr2 H a).
    - intros a. apply (pr1 H a).

  Definition Zero_opp (T : @Zero C) : @Zero (C^op) := @make_Zero (C^op) _ (isZero_opp (pr2 T)).

Equality on ZeroArrows


  Lemma ZeroArrowTo_opp {x : C} (Z : @Zero C) :
    @ZeroArrowTo C Z x = @ZeroArrowFrom (C^op) (Zero_opp Z) x.
  Show proof.
    apply ArrowsToZero.

  Lemma ZeroArrowFrom_opp {x : C} (Z : @Zero C) :
    @ZeroArrowFrom C Z x = @ZeroArrowTo (C^op) (Zero_opp Z) x.
  Show proof.
    apply ArrowsFromZero.

  Lemma ZeroArrow_opp {x y : C} (Z : @Zero C) :
    @ZeroArrow C Z x y = @ZeroArrow (C^op) (Zero_opp Z) y x.
  Show proof.
    unfold ZeroArrow.
    rewrite ZeroArrowTo_opp.
    rewrite ZeroArrowFrom_opp.
    apply idpath.

  Local Opaque ZeroArrow.

Equalizers and Coequalizers


  Definition isEqualizer_opp {x y z : C} (f g : Cy, z) (e : Cx, y) (H : e · f = e · g)
             (isE : @isEqualizer C _ _ _ f g e H) : @isCoequalizer (C^op) _ _ _ f g e H := isE.

  Definition isCoequalizer_opp {x y z : C} (f g : Cx, y) (e : Cy, z) (H : f · e = g · e)
             (isC : @isCoequalizer C _ _ _ f g e H) : @isEqualizer (C^op) _ _ _ f g e H := isC.

  Definition Equalizer_opp {y z : C} (f g : Cy, z) (E : @Equalizer C y z f g) :
    @Coequalizer (C^op) z y f g := @make_Coequalizer (C^op) _ _ _ f g (EqualizerArrow E)
                                                   (EqualizerEqAr E)
                                                   (isEqualizer_opp f g (EqualizerArrow E)
                                                                    (EqualizerEqAr E)
                                                                    (isEqualizer_Equalizer E)).

  Definition Coequalizer_opp {y z : C} (f g : Cy, z) (CE : @Coequalizer C y z f g) :
    @Equalizer (C^op) z y f g := @make_Equalizer (C^op) _ _ _ f g (CoequalizerArrow CE)
                                               (CoequalizerEqAr CE)
                                               (isCoequalizer_opp f g (CoequalizerArrow CE)
                                                                  (CoequalizerEqAr CE)
                                                                  (isCoequalizer_Coequalizer CE)).

  Definition Equalizers_opp (E : @Equalizers C) : @Coequalizers (C^op).
  Show proof.
    intros x y f g.
    use Equalizer_opp.
    exact (E y x f g).

  Definition Coequalizers_opp (CE : @Coequalizers C) : @Equalizers (C^op).
  Show proof.
    intros x y f g.
    use Coequalizer_opp.
    exact (CE y x f g).

Kernels and Cokernels


  Local Lemma isCokernel_opp_eq {x y z : C} (f : Cx, y) (g : Cy, z) (Z : Zero C)
        (H : f · g = ZeroArrow Z _ _) (Z' : Zero C^op) :
    (g : C^opz, y) · (f : C^opy, x) = ZeroArrow Z' _ _.
  Show proof.
    cbn in *. rewrite H. rewrite ZeroArrow_opp.
    exact (ZerosArrowEq C^op (Zero_opp Z) Z' z x).

  Lemma isCokernel_opp {x y z : C} {f : Cx, y} {g : Cy, z} {Z : Zero C}
        {H : f · g = ZeroArrow Z _ _} (K' : isKernel Z f g H) {Z' : Zero C^op} :
    isCokernel Z' (g : C^opz, y) (f : C^opy, x) (isCokernel_opp_eq f g Z H Z').
  Show proof.
    set (K := make_Kernel _ _ _ _ K').
    use make_isCokernel.
    - intros w h H'. cbn in H'.
      set (XXX := (ZerosArrowEq C^op (Zero_opp Z) Z' z w)).
      use unique_exists.
      + use (KernelIn Z K w h _).
        rewrite ZeroArrow_opp.
        rewrite XXX.
        apply H'.
      + cbn. use (KernelCommutes Z K).
      + intros y0. apply (has_homsets_opp hs).
      + cbn. intros y0 X. use (KernelInsEq Z K). rewrite KernelCommutes. exact X.

  Local Lemma Kernel_opp_eq {y z : C} (f : Cy, z) (Z : Zero C) (K : @Kernel C Z y z f) :
    @compose C^op _ _ _ f (KernelArrow K) = ZeroArrow (Zero_opp Z) z K.
  Show proof.
    cbn. rewrite (KernelCompZero Z K). apply ZeroArrow_opp.

  Lemma Kernel_opp_isCokernel {y z : C} (f : Cy, z) (Z : Zero C) (K : @Kernel C Z y z f) :
    isCokernel (Zero_opp Z) f (KernelArrow K) (Kernel_opp_eq f Z K).
  Show proof.
    use make_isCokernel.
    - intros w h H'. cbn in H'.
      use unique_exists.
      + rewrite <- ZeroArrow_opp in H'. exact (KernelIn Z K w h H').
      + cbn. use KernelCommutes.
      + intros y0. apply (has_homsets_opp hs).
      + cbn. intros y0 X. use KernelInsEq. rewrite KernelCommutes. exact X.

  Definition Kernel_opp {y z : C} (f : Cy, z) (Z : Zero C) (K : @Kernel C Z y z f) :
    @Cokernel (C^op) (Zero_opp Z) z y f.
  Show proof.
    use make_Cokernel.
    - exact K.
    - exact (KernelArrow K).
    - exact (Kernel_opp_eq f Z K).
    - exact (Kernel_opp_isCokernel f Z K).

  Lemma isKernel_opp {x y z : C^op} {f : Cx, y} {g : Cy, z} {Z : Zero C}
        {H : f · g = ZeroArrow Z _ _} (CK' : isCokernel Z f g H) {Z' : Zero C^op} :
    isKernel Z' (g : C^opz, y) (f : C^opy, x) (isCokernel_opp_eq f g Z H Z').
  Show proof.
    set (CK := make_Cokernel _ _ _ _ CK').
    use make_isKernel.
    - intros w h H'.
      rewrite <- (ZerosArrowEq C^op (Zero_opp Z) Z' w x) in H'.
      rewrite <- ZeroArrow_opp in H'.
      use unique_exists.
      + exact (CokernelOut Z CK w h H').
      + use (CokernelCommutes Z CK).
      + intros y0. apply hs.
      + cbn. intros y0 X. use (CokernelOutsEq _ CK). rewrite (CokernelCommutes Z CK). cbn. rewrite X.
        apply idpath.

  Local Lemma Cokernel_opp_eq {y z : C} (f : Cy, z) (Z : Zero C) (CK : @Cokernel C Z y z f) :
    @compose C^op _ _ _ (CokernelArrow CK) f = ZeroArrow (Zero_opp Z) CK y.
  Show proof.
    cbn. rewrite (CokernelCompZero Z CK). apply ZeroArrow_opp.

  Lemma Cokernel_opp_isKernel {y z : C} (f : Cy, z) (Z : Zero C)
        (CK : @Cokernel C Z y z f) :
    isKernel (Zero_opp Z) (CokernelArrow CK) f (Cokernel_opp_eq f Z CK).
  Show proof.
    use make_isKernel.

    - intros w h H'. cbn in H'.
      use unique_exists.
      + rewrite <- ZeroArrow_opp in H'. exact (CokernelOut Z CK w h H').
      + cbn. use CokernelCommutes.
      + intros y0. apply (has_homsets_opp hs).
      + cbn. intros y0 X. use CokernelOutsEq. rewrite CokernelCommutes. exact X.

  Definition Cokernel_opp {y z : C} (f : Cy, z) (Z : Zero C) (CK : @Cokernel C Z y z f) :
    @Kernel (C^op) (Zero_opp Z) z y f.
  Show proof.
    use make_Kernel.
    - exact CK.
    - exact (CokernelArrow CK).
    - exact (Cokernel_opp_eq f Z CK).
    - exact (Cokernel_opp_isKernel f Z CK).

  Definition Kernels_opp (Z : Zero C) (K : @Kernels C Z) : @Cokernels (C^op) (Zero_opp Z).
  Show proof.
    intros x y f.
    use Kernel_opp.
    apply (K y x f).

  Definition Cokernels_opp (Z : Zero C) (CK : @Cokernels C Z) : @Kernels (C^op) (Zero_opp Z).
  Show proof.
    intros x y f.
    use Cokernel_opp.
    apply (CK y x f).

Pushouts and pullbacks


  Definition isPushout_opp {a b c d : C} (f : Ca, b) (g : Ca, c) (in1 : Cb, d) (in2 : Cc, d)
             (H : f · in1 = g · in2) (iPo : @isPushout C a b c d f g in1 in2 H) :
    @isPullback (C^op) a b c d f g in1 in2 H := iPo.

  Definition isPullback_opp {a b c d : C} (f : Cb, a) (g : Cc, a) (p1 : Cd, b) (p2 : Cd, c)
        (H : p1 · f = p2 · g) (iPb : @isPullback C a b c d f g p1 p2 H) :
    @isPushout (C^op) a b c d f g p1 p2 H := iPb.

  Definition Pushout_opp {a b c : C} (f : Ca, b) (g : Ca, c) (Po : @Pushout C a b c f g) :
    @Pullback (C^op) a b c f g := Po.

  Definition Pullback_opp {a b c : C} (f : Cb, a) (g : Cc, a) (Pb : @Pullback C a b c f g) :
    @Pushout (C^op) a b c f g := Pb.

  Definition Pushouts_opp (Pos : @Pushouts C) : @Pullbacks (C^op) := Pos.

  Definition Pullbacks_opp (Pbs : @Pushouts C) : @Pullbacks (C^op) := Pbs.

BinProducts and BinCoproducts


  Definition isBinProduct_opp (c d p : C) (p1 : Cp, c) (p2 : Cp, d)
             (iBPC : @isBinProduct C c d p p1 p2) :
    @isBinCoproduct (C^op) c d p p1 p2 := iBPC.

  Definition isBinCoproduct_opp (a b co : C) (ia : Ca, co) (ib : Cb, co)
             (iBCC : @isBinCoproduct C a b co ia ib) :
    @isBinProduct (C^op) a b co ia ib := iBCC.

  Definition BinProduct_opp (c d : C) (iBPC : @BinProduct C c d) :
    @BinCoproduct (C^op) c d := iBPC.

  Definition BinCoproduct_opp (c d : C) (iBCC : @BinCoproduct C c d) :
    @BinProduct (C^op) c d := iBCC.

  Definition BinProducts_opp (BP : @BinProducts C) : @BinCoproducts (C^op) := BP.

  Definition BinCoproducts_opp (BC : @BinCoproducts C) : @BinProducts (C^op) := BC.

End def_opposites'.

Definition opp_zero_lifts {C:category} {X:Type} (j : X -> ob C) :
  zero_lifts C j -> zero_lifts C^op j.
Show proof.
  apply hinhfun; intros [z iz]. exists z. exact (isZero_opp C iz).