# mathlibdocumentation

category_theory.limits.shapes.reflexive

# Reflexive coequalizers #

We define reflexive pairs as a pair of morphisms which have a common section. We say a category has reflexive coequalizers if it has coequalizers of all reflexive pairs. Reflexive coequalizers often enjoy nicer properties than general coequalizers, and feature heavily in some versions of the monadicity theorem.

We also give some examples of reflexive pairs: for an adjunction F ⊣ G with counit ε, the pair (FGε_B, ε_FGB) is reflexive. If a pair f,g is a kernel pair for some morphism, then it is reflexive.

# TODO #

• If C has binary coproducts and reflexive coequalizers, then it has all coequalizers.
• If T is a monad on cocomplete category C, then algebra T is cocomplete iff it has reflexive coequalizers.
• If C is locally cartesian closed and has reflexive coequalizers, then it has images: in fact regular epi (and hence strong epi) images.
@[class]
structure category_theory.is_reflexive_pair {C : Type u} {A B : C} (f g : A B) :
Prop

The pair f g : A ⟶ B is reflexive if there is a morphism B ⟶ A which is a section for both.

Instances of this typeclass
@[class]
structure category_theory.is_coreflexive_pair {C : Type u} {A B : C} (f g : A B) :
Prop

The pair f g : A ⟶ B is coreflexive if there is a morphism B ⟶ A which is a retraction for both.

theorem category_theory.is_reflexive_pair.mk' {C : Type u} {A B : C} {f g : A B} (s : B A) (sf : s f = 𝟙 B) (sg : s g = 𝟙 B) :
theorem category_theory.is_coreflexive_pair.mk' {C : Type u} {A B : C} {f g : A B} (s : B A) (fs : f s = 𝟙 A) (gs : g s = 𝟙 A) :
noncomputable def category_theory.common_section {C : Type u} {A B : C} (f g : A B)  :
B A

Get the common section for a reflexive pair.

Equations
@[simp]
theorem category_theory.section_comp_left_assoc {C : Type u} {A B : C} (f g : A B) {X' : C} (f' : B X') :
f f' = f'
@[simp]
theorem category_theory.section_comp_left {C : Type u} {A B : C} (f g : A B)  :
= 𝟙 B
@[simp]
theorem category_theory.section_comp_right_assoc {C : Type u} {A B : C} (f g : A B) {X' : C} (f' : B X') :
g f' = f'
@[simp]
theorem category_theory.section_comp_right {C : Type u} {A B : C} (f g : A B)  :
= 𝟙 B
noncomputable def category_theory.common_retraction {C : Type u} {A B : C} (f g : A B)  :
B A

Get the common retraction for a coreflexive pair.

Equations
@[simp]
theorem category_theory.left_comp_retraction {C : Type u} {A B : C} (f g : A B)  :
= 𝟙 A
@[simp]
theorem category_theory.left_comp_retraction_assoc {C : Type u} {A B : C} (f g : A B) {X' : C} (f' : A X') :
f = f'
@[simp]
theorem category_theory.right_comp_retraction {C : Type u} {A B : C} (f g : A B)  :
= 𝟙 A
@[simp]
theorem category_theory.right_comp_retraction_assoc {C : Type u} {A B : C} (f g : A B) {X' : C} (f' : A X') :
g = f'
theorem category_theory.is_kernel_pair.is_reflexive_pair {C : Type u} {A B R : C} {f g : R A} {q : A B} (h : g) :

If f,g is a kernel pair for some morphism q, then it is reflexive.

theorem category_theory.is_reflexive_pair.swap {C : Type u} {A B : C} {f g : A B}  :

If f,g is reflexive, then g,f is reflexive.

theorem category_theory.is_coreflexive_pair.swap {C : Type u} {A B : C} {f g : A B}  :

If f,g is coreflexive, then g,f is coreflexive.

@[protected, instance]
def category_theory.app.is_reflexive_pair {C : Type u} {D : Type u₂} {F : C D} {G : D C} (adj : F G) (B : D) :

For an adjunction F ⊣ G with counit ε, the pair (FGε_B, ε_FGB) is reflexive.

@[class]
structure category_theory.limits.has_reflexive_coequalizers (C : Type u)  :
Prop
• has_coeq : ∀ ⦃A B : C⦄ (f g : A B) [_inst_3 : ,

C has reflexive coequalizers if it has coequalizers for every reflexive pair.

Instances of this typeclass
@[class]
structure category_theory.limits.has_coreflexive_equalizers (C : Type u)  :
Prop
• has_eq : ∀ ⦃A B : C⦄ (f g : A B) [_inst_3 : ,

C has coreflexive equalizers if it has equalizers for every coreflexive pair.

Instances of this typeclass
theorem category_theory.limits.has_coequalizer_of_common_section (C : Type u) {A B : C} {f g : A B} (r : B A) (rf : r f = 𝟙 B) (rg : r g = 𝟙 B) :
theorem category_theory.limits.has_equalizer_of_common_retraction (C : Type u) {A B : C} {f g : A B} (r : B A) (fr : f r = 𝟙 A) (gr : g r = 𝟙 A) :
@[protected, instance]

If C has coequalizers, then it has reflexive coequalizers.

@[protected, instance]

If C has equalizers, then it has coreflexive equalizers.