The Yoneda embedding for preadditive categories

The Yoneda embedding for preadditive categories sends an object Y to the presheaf sending an object X to the group of morphisms X ⟶ Y. At each point, we get an additional End Y-module structure.

We also show that this presheaf is additive and that it is compatible with the normal Yoneda embedding in the expected way and deduce that the preadditive Yoneda embedding is fully faithful.

TODO

• The Yoneda embedding is additive itself

@[simp]

theorem CategoryTheory.preadditiveYonedaObj_obj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (Y : C) (X : Cᵒᵖ) : (CategoryTheory.preadditiveYonedaObj Y).obj X = ModuleCat.of (CategoryTheory.End Y) (X.unop ⟶ Y)

@[simp]

theorem CategoryTheory.preadditiveYonedaObj_map {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (Y : C) : ∀ {X Y_1 : Cᵒᵖ} (f : X ⟶ Y_1), (CategoryTheory.preadditiveYonedaObj Y).map f = ModuleCat.ofHom { toAddHom := { toFun := fun (g : X.unop ⟶ Y) => CategoryTheory.CategoryStruct.comp f.unop g, map_add' := ⋯ }, map_smul' := ⋯ }

def CategoryTheory.preadditiveYonedaObj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (Y : C) : CategoryTheory.Functor Cᵒᵖ (ModuleCat (CategoryTheory.End Y))

The Yoneda embedding for preadditive categories sends an object Y to the presheaf sending an object X to the End Y-module of morphisms X ⟶ Y.

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@[simp]

theorem CategoryTheory.preadditiveYoneda_obj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (Y : C) : CategoryTheory.preadditiveYoneda.obj Y = CategoryTheory.Functor.comp (CategoryTheory.preadditiveYonedaObj Y) (CategoryTheory.forget₂ (ModuleCat (CategoryTheory.End Y)) AddCommGroupCat)

@[simp]

theorem CategoryTheory.preadditiveYoneda_map_app_apply {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : ∀ {X Y : C} (f : X ⟶ Y) (X_1 : Cᵒᵖ) (g : ↑(((fun (Y : C) => CategoryTheory.Functor.comp (CategoryTheory.preadditiveYonedaObj Y) (CategoryTheory.forget₂ (ModuleCat (CategoryTheory.End Y)) AddCommGroupCat)) X).obj X_1)), ((CategoryTheory.preadditiveYoneda.map f).app X_1) g = CategoryTheory.CategoryStruct.comp g f

def CategoryTheory.preadditiveYoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor C (CategoryTheory.Functor Cᵒᵖ AddCommGroupCat)

The Yoneda embedding for preadditive categories sends an object Y to the presheaf sending an object X to the group of morphisms X ⟶ Y. At each point, we get an additional End Y-module structure, see preadditiveYonedaObj.

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• One or more equations did not get rendered due to their size.

@[simp]

theorem CategoryTheory.preadditiveCoyonedaObj_obj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : Cᵒᵖ) (Y : C) : (CategoryTheory.preadditiveCoyonedaObj X).obj Y = ModuleCat.of (CategoryTheory.End X) (X.unop ⟶ Y)

@[simp]

theorem CategoryTheory.preadditiveCoyonedaObj_map {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : Cᵒᵖ) : ∀ {X_1 Y : C} (f : X_1 ⟶ Y), (CategoryTheory.preadditiveCoyonedaObj X).map f = ModuleCat.ofHom { toAddHom := { toFun := fun (g : X.unop ⟶ X_1) => CategoryTheory.CategoryStruct.comp g f, map_add' := ⋯ }, map_smul' := ⋯ }

def CategoryTheory.preadditiveCoyonedaObj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : Cᵒᵖ) : CategoryTheory.Functor C (ModuleCat (CategoryTheory.End X))

The Yoneda embedding for preadditive categories sends an object X to the copresheaf sending an object Y to the End X-module of morphisms X ⟶ Y.

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• One or more equations did not get rendered due to their size.

@[simp]

theorem CategoryTheory.preadditiveCoyoneda_obj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : Cᵒᵖ) : CategoryTheory.preadditiveCoyoneda.obj X = CategoryTheory.Functor.comp (CategoryTheory.preadditiveCoyonedaObj X) (CategoryTheory.forget₂ (ModuleCat (CategoryTheory.End X)) AddCommGroupCat)

@[simp]

theorem CategoryTheory.preadditiveCoyoneda_map_app_apply {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : ∀ {X Y : Cᵒᵖ} (f : X ⟶ Y) (Y_1 : C) (g : ↑(((fun (X : Cᵒᵖ) => CategoryTheory.Functor.comp (CategoryTheory.preadditiveCoyonedaObj X) (CategoryTheory.forget₂ (ModuleCat (CategoryTheory.End X)) AddCommGroupCat)) X).obj Y_1)), ((CategoryTheory.preadditiveCoyoneda.map f).app Y_1) g = CategoryTheory.CategoryStruct.comp f.unop g

def CategoryTheory.preadditiveCoyoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor Cᵒᵖ (CategoryTheory.Functor C AddCommGroupCat)

The Yoneda embedding for preadditive categories sends an object X to the copresheaf sending an object Y to the group of morphisms X ⟶ Y. At each point, we get an additional End X-module structure, see preadditiveCoyonedaObj.

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instance CategoryTheory.additive_yonedaObj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : C) : CategoryTheory.Functor.Additive (CategoryTheory.preadditiveYonedaObj X)

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• ⋯ = ⋯

instance CategoryTheory.additive_yonedaObj' {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : C) : CategoryTheory.Functor.Additive (CategoryTheory.preadditiveYoneda.obj X)

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• ⋯ = ⋯

instance CategoryTheory.additive_coyonedaObj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : Cᵒᵖ) : CategoryTheory.Functor.Additive (CategoryTheory.preadditiveCoyonedaObj X)

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• ⋯ = ⋯

instance CategoryTheory.additive_coyonedaObj' {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] (X : Cᵒᵖ) : CategoryTheory.Functor.Additive (CategoryTheory.preadditiveCoyoneda.obj X)

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• ⋯ = ⋯

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theorem CategoryTheory.whiskering_preadditiveYoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor.comp CategoryTheory.preadditiveYoneda ((CategoryTheory.whiskeringRight Cᵒᵖ AddCommGroupCat (Type v)).obj (CategoryTheory.forget AddCommGroupCat)) = CategoryTheory.yoneda

Composing the preadditive yoneda embedding with the forgetful functor yields the regular Yoneda embedding.

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theorem CategoryTheory.whiskering_preadditiveCoyoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor.comp CategoryTheory.preadditiveCoyoneda ((CategoryTheory.whiskeringRight C AddCommGroupCat (Type v)).obj (CategoryTheory.forget AddCommGroupCat)) = CategoryTheory.coyoneda

Composing the preadditive yoneda embedding with the forgetful functor yields the regular Yoneda embedding.

instance CategoryTheory.full_preadditiveYoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor.Full CategoryTheory.preadditiveYoneda

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instance CategoryTheory.full_preadditiveCoyoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor.Full CategoryTheory.preadditiveCoyoneda

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instance CategoryTheory.faithful_preadditiveYoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor.Faithful CategoryTheory.preadditiveYoneda

Equations

• ⋯ = ⋯

instance CategoryTheory.faithful_preadditiveCoyoneda {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] : CategoryTheory.Functor.Faithful CategoryTheory.preadditiveCoyoneda

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• ⋯ = ⋯