mathlib documentation

category_theory.full_subcategory

Induced categories and full subcategories #

Given a category D and a function F : C → Dfrom a type C to the objects of D, there is an essentially unique way to give C a category structure such that F becomes a fully faithful functor, namely by taking $$ Hom_C(X, Y) = Hom_D(FX, FY) $$. We call this the category induced from D along F.

As a special case, if C is a subtype of D, this produces the full subcategory of D on the objects belonging to C. In general the induced category is equivalent to the full subcategory of D on the image of F.

Implementation notes #

It looks odd to make D an explicit argument of induced_category, when it is determined by the argument F anyways. The reason to make D explicit is in order to control its syntactic form, so that instances like induced_category.has_forget₂ (elsewhere) refer to the correct form of D. This is used to set up several algebraic categories like

def CommMon : Type (u+1) := induced_category Mon (bundled.map @comm_monoid.to_monoid) -- not induced_category (bundled monoid) (bundled.map @comm_monoid.to_monoid), -- even though Mon = bundled monoid!

@[nolint]
def category_theory.induced_category {C : Type u₁} (D : Type u₂) [category_theory.category D] (F : C D) :
Type u₁

induced_category D F, where F : C → D, is a typeclass synonym for C, which provides a category structure so that the morphisms X ⟶ Y are the morphisms in D from F X to F Y.

Equations
Instances for category_theory.induced_category
@[protected, instance]
Equations
@[simp]
theorem category_theory.induced_functor_obj {C : Type u₁} {D : Type u₂} [category_theory.category D] (F : C D) (ᾰ : C) :
theorem category_theory.full_subcategory.ext {C : Type u₁} {_inst_1 : category_theory.category C} {Z : C Prop} (x y : category_theory.full_subcategory Z) (h : x.obj = y.obj) :
x = y

An implication of predicates Z → Z' induces a functor between full subcategories.

Equations
Instances for category_theory.full_subcategory.map
@[simp]
@[simp]
theorem category_theory.full_subcategory.map_map {C : Type u₁} [category_theory.category C] {Z Z' : C Prop} (h : ⦃X : C⦄, Z X Z' X) (X Y : category_theory.full_subcategory Z) (f : X Y) :
@[protected, instance]
def category_theory.full_subcategory.lift {C : Type u₁} [category_theory.category C] {D : Type u₂} [category_theory.category D] (P : D Prop) (F : C D) (hF : (X : C), P (F.obj X)) :

A functor which maps objects to objects satisfying a certain property induces a lift through the full subcategory of objects satisfying that property.

Equations
Instances for category_theory.full_subcategory.lift
@[simp]
theorem category_theory.full_subcategory.lift_obj_obj {C : Type u₁} [category_theory.category C] {D : Type u₂} [category_theory.category D] (P : D Prop) (F : C D) (hF : (X : C), P (F.obj X)) (X : C) :
@[simp]
theorem category_theory.full_subcategory.lift_map {C : Type u₁} [category_theory.category C] {D : Type u₂} [category_theory.category D] (P : D Prop) (F : C D) (hF : (X : C), P (F.obj X)) (X Y : C) (f : X Y) :

Composing the lift of a functor through a full subcategory with the inclusion yields the original functor. Unfortunately, this is not true by definition, so we only get a natural isomorphism, but it is pointwise definitionally true, see full_subcategory.inclusion_obj_lift_obj and full_subcategory.inclusion_map_lift_map.

Equations