Documentation

Mathlib.CategoryTheory.Limits.Shapes.Biproducts

Biproducts and binary biproducts #

We introduce the notion of (finite) biproducts and binary biproducts.

These are slightly unusual relative to the other shapes in the library, as they are simultaneously limits and colimits. (Zero objects are similar; they are "biterminal".)

For results about biproducts in preadditive categories see CategoryTheory.Preadditive.Biproducts.

In a category with zero morphisms, we model the (binary) biproduct of P Q : C using a BinaryBicone, which has a cone point X, and morphisms fst : X ⟶ P, snd : X ⟶ Q, inl : P ⟶ X and inr : X ⟶ Q, such that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q. Such a BinaryBicone is a biproduct if the cone is a limit cone, and the cocone is a colimit cocone.

For biproducts indexed by a Fintype J, a bicone again consists of a cone point X and morphisms π j : X ⟶ F j and ι j : F j ⟶ X for each j, such that ι j ≫ π j' is the identity when j = j' and zero otherwise.

Notation #

As is already taken for the sum of types, we introduce the notation X ⊞ Y for a binary biproduct. We introduce ⨁ f for the indexed biproduct.

Implementation notes #

Prior to https://github.com/leanprover-community/mathlib3/pull/14046, HasFiniteBiproducts required a DecidableEq instance on the indexing type. As this had no pay-off (everything about limits is non-constructive in mathlib), and occasional cost (constructing decidability instances appropriate for constructions involving the indexing type), we made everything classical.

structure CategoryTheory.Limits.Bicone {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] (F : JC) :
Type (max (max uC uC') w)

A c : Bicone F is:

  • an object c.pt and
  • morphisms π j : pt ⟶ F j and ι j : F j ⟶ pt for each j,
  • such that ι j ≫ π j' is the identity when j = j' and zero otherwise.
  • pt : C

    A c : Bicone F is:

    • an object c.pt and
    • morphisms π j : pt ⟶ F j and ι j : F j ⟶ pt for each j,
    • such that ι j ≫ π j' is the identity when j = j' and zero otherwise.
  • π : (j : J) → self.pt F j

    A c : Bicone F is:

    • an object c.pt and
    • morphisms π j : pt ⟶ F j and ι j : F j ⟶ pt for each j,
    • such that ι j ≫ π j' is the identity when j = j' and zero otherwise.
  • ι : (j : J) → F j self.pt

    A c : Bicone F is:

    • an object c.pt and
    • morphisms π j : pt ⟶ F j and ι j : F j ⟶ pt for each j,
    • such that ι j ≫ π j' is the identity when j = j' and zero otherwise.
  • ι_π : ∀ (j j' : J), CategoryTheory.CategoryStruct.comp (self j) (self j') = if h : j = j' then CategoryTheory.eqToHom else 0

    A c : Bicone F is:

    • an object c.pt and
    • morphisms π j : pt ⟶ F j and ι j : F j ⟶ pt for each j,
    • such that ι j ≫ π j' is the identity when j = j' and zero otherwise.
Instances For

A bicone morphism between two bicones for the same diagram is a morphism of the bicone points which commutes with the cone and cocone legs.

The category of bicones on a given diagram.

Equations
def CategoryTheory.Limits.Bicones.ext {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] {F : JC} {c c' : CategoryTheory.Limits.Bicone F} (φ : c.pt c'.pt) (wι : ∀ (j : J), CategoryTheory.CategoryStruct.comp (c j) φ.hom = c' j := by aesop_cat) (wπ : ∀ (j : J), CategoryTheory.CategoryStruct.comp φ.hom (c' j) = c j := by aesop_cat) :
c c'

To give an isomorphism between cocones, it suffices to give an isomorphism between their vertices which commutes with the cocone maps.

Equations
  • CategoryTheory.Limits.Bicones.ext φ = { hom := { hom := φ.hom, := , := }, inv := { hom := φ.inv, := , := }, hom_inv_id := , inv_hom_id := }
@[simp]
theorem CategoryTheory.Limits.Bicones.ext_hom_hom {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] {F : JC} {c c' : CategoryTheory.Limits.Bicone F} (φ : c.pt c'.pt) (wι : ∀ (j : J), CategoryTheory.CategoryStruct.comp (c j) φ.hom = c' j := by aesop_cat) (wπ : ∀ (j : J), CategoryTheory.CategoryStruct.comp φ.hom (c' j) = c j := by aesop_cat) :
(CategoryTheory.Limits.Bicones.ext φ ).hom.hom = φ.hom
@[simp]
theorem CategoryTheory.Limits.Bicones.ext_inv_hom {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] {F : JC} {c c' : CategoryTheory.Limits.Bicone F} (φ : c.pt c'.pt) (wι : ∀ (j : J), CategoryTheory.CategoryStruct.comp (c j) φ.hom = c' j := by aesop_cat) (wπ : ∀ (j : J), CategoryTheory.CategoryStruct.comp φ.hom (c' j) = c j := by aesop_cat) :
(CategoryTheory.Limits.Bicones.ext φ ).inv.hom = φ.inv

A functor G : C ⥤ D sends bicones over F to bicones over G.obj ∘ F functorially.

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

Extract the cone from a bicone.

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  • One or more equations did not get rendered due to their size.
@[reducible, inline]

A shorthand for toConeFunctor.obj

Equations
  • B.toCone = CategoryTheory.Limits.Bicone.toConeFunctor.obj B

Extract the cocone from a bicone.

Equations
  • One or more equations did not get rendered due to their size.
@[reducible, inline]

A shorthand for toCoconeFunctor.obj

Equations
  • B.toCocone = CategoryTheory.Limits.Bicone.toCoconeFunctor.obj B

We can turn any limit cone over a discrete collection of objects into a bicone.

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

We can turn any colimit cocone over a discrete collection of objects into a bicone.

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

Structure witnessing that a bicone is both a limit cone and a colimit cocone.

Instances For
theorem CategoryTheory.Limits.Bicone.IsBilimit.ext {J : Type w} {C : Type uC} {inst✝ : CategoryTheory.Category.{uC', uC} C} {inst✝¹ : CategoryTheory.Limits.HasZeroMorphisms C} {F : JC} {B : CategoryTheory.Limits.Bicone F} {x y : B.IsBilimit} (isLimit : x.isLimit = y.isLimit) (isColimit : x.isColimit = y.isColimit) :
x = y

Whisker a bicone with an equivalence between the indexing types.

Equations
  • c.whisker g = { pt := c.pt, π := fun (k : K) => c (g k), ι := fun (k : K) => c (g k), ι_π := }
@[simp]
theorem CategoryTheory.Limits.Bicone.whisker_ι {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] {K : Type w'} {f : JC} (c : CategoryTheory.Limits.Bicone f) (g : K J) (k : K) :
(c.whisker g) k = c (g k)
@[simp]
theorem CategoryTheory.Limits.Bicone.whisker_π {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] {K : Type w'} {f : JC} (c : CategoryTheory.Limits.Bicone f) (g : K J) (k : K) :
(c.whisker g) k = c (g k)
@[simp]
theorem CategoryTheory.Limits.Bicone.whisker_pt {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] {K : Type w'} {f : JC} (c : CategoryTheory.Limits.Bicone f) (g : K J) :
(c.whisker g).pt = c.pt

Taking the cone of a whiskered bicone results in a cone isomorphic to one gained by whiskering the cone and postcomposing with a suitable isomorphism.

Equations

Taking the cocone of a whiskered bicone results in a cone isomorphic to one gained by whiskering the cocone and precomposing with a suitable isomorphism.

Equations
def CategoryTheory.Limits.Bicone.whiskerIsBilimitIff {J : Type w} {C : Type uC} [CategoryTheory.Category.{uC', uC} C] [CategoryTheory.Limits.HasZeroMorphisms C] {K : Type w'} {f : JC} (c : CategoryTheory.Limits.Bicone f) (g : K J) :
(c.whisker g).IsBilimit c.IsBilimit

Whiskering a bicone with an equivalence between types preserves being a bilimit bicone.

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

A bicone over F : J → C, which is both a limit cone and a colimit cocone.

  • A bicone over F : J → C, which is both a limit cone and a colimit cocone.

  • isBilimit : self.bicone.IsBilimit

    A bicone over F : J → C, which is both a limit cone and a colimit cocone.

HasBiproduct F expresses the mere existence of a bicone which is simultaneously a limit and a colimit of the diagram F.

Instances

C has biproducts of shape J if we have a limit and a colimit, with the same cone points, of every function F : J → C.

Instances

HasFiniteBiproducts C represents a choice of biproduct for every family of objects in C indexed by a finite type.

Instances

    The isomorphism between the specified limit and the specified colimit for a functor with a bilimit.

    Equations
    • One or more equations did not get rendered due to their size.
    @[reducible, inline]

    biproduct f computes the biproduct of a family of elements f. (It is defined as an abbreviation for limit (Discrete.functor f), so for most facts about biproduct f, you will just use general facts about limits and colimits.)

    Equations

    biproduct f computes the biproduct of a family of elements f. (It is defined as an abbreviation for limit (Discrete.functor f), so for most facts about biproduct f, you will just use general facts about limits and colimits.)

    Equations

    Note that as this lemma has an if in the statement, we include a DecidableEq argument. This means you may not be able to simp using this lemma unless you open scoped Classical.

    @[reducible, inline]

    Given a collection of maps into the summands, we obtain a map into the biproduct.

    Equations
    @[reducible, inline]

    Given a collection of maps out of the summands, we obtain a map out of the biproduct.

    Equations
    @[reducible, inline]

    Given a collection of maps between corresponding summands of a pair of biproducts indexed by the same type, we obtain a map between the biproducts.

    Equations
    • One or more equations did not get rendered due to their size.
    Instances For
    @[reducible, inline]

    An alternative to biproduct.map constructed via colimits. This construction only exists in order to show it is equal to biproduct.map.

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

    The canonical isomorphism between the chosen biproduct and the chosen coproduct.

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

    If a category has biproducts of a shape J, its colim and lim functor on diagrams over J are isomorphic.

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

    Given a collection of isomorphisms between corresponding summands of a pair of biproducts indexed by the same type, we obtain an isomorphism between the biproducts.

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

    Two biproducts which differ by an equivalence in the indexing type, and up to isomorphism in the factors, are isomorphic.

    Unfortunately there are two natural ways to define each direction of this isomorphism (because it is true for both products and coproducts separately). We give the alternative definitions as lemmas below.

    Equations
    • One or more equations did not get rendered due to their size.
    instance CategoryTheory.Limits.instHasBiproductSigmaFstSndOfBiproduct {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {ι : Type u_3} (f : ιType u_2) (g : (i : ι) → f iC) [∀ (i : ι), CategoryTheory.Limits.HasBiproduct (g i)] [CategoryTheory.Limits.HasBiproduct fun (i : ι) => g i] :
    CategoryTheory.Limits.HasBiproduct fun (p : (i : ι) × f i) => g p.fst p.snd
    Equations
    • =
    def CategoryTheory.Limits.biproductBiproductIso {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {ι : Type u_3} (f : ιType u_2) (g : (i : ι) → f iC) [∀ (i : ι), CategoryTheory.Limits.HasBiproduct (g i)] [CategoryTheory.Limits.HasBiproduct fun (i : ι) => g i] :
    ( fun (i : ι) => g i) fun (p : (i : ι) × f i) => g p.fst p.snd

    An iterated biproduct is a biproduct over a sigma type.

    Equations
    • One or more equations did not get rendered due to their size.
    @[simp]
    theorem CategoryTheory.Limits.biproductBiproductIso_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {ι : Type u_3} (f : ιType u_2) (g : (i : ι) → f iC) [∀ (i : ι), CategoryTheory.Limits.HasBiproduct (g i)] [CategoryTheory.Limits.HasBiproduct fun (i : ι) => g i] :
    (CategoryTheory.Limits.biproductBiproductIso f g).inv = CategoryTheory.Limits.biproduct.lift fun (i : ι) => CategoryTheory.Limits.biproduct.lift fun (x : f i) => CategoryTheory.Limits.biproduct.π (fun (p : (i : ι) × f i) => g p.fst p.snd) i, x

    The kernel of biproduct.π f i is the inclusion from the biproduct which omits i from the index set J into the biproduct over J.

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

    The kernel of biproduct.π f i is Subtype.restrict {i}ᶜ f.

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

    The cokernel of biproduct.ι f i is the projection from the biproduct over the index set J onto the biproduct omitting i.

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

    The cokernel of biproduct.ι f i is Subtype.restrict {i}ᶜ f.

    Equations
    • One or more equations did not get rendered due to their size.
    def CategoryTheory.Limits.biproduct.matrix {J : Type} [Finite J] {K : Type} [Finite K] {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] [CategoryTheory.Limits.HasFiniteBiproducts C] {f : JC} {g : KC} (m : (j : J) → (k : K) → f j g k) :
    f g

    Convert a (dependently typed) matrix to a morphism of biproducts.

    Equations

    Extract the matrix components from a morphism of biproducts.

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

    Morphisms between direct sums are matrices.

    Equations
    • CategoryTheory.Limits.biproduct.matrixEquiv = { toFun := CategoryTheory.Limits.biproduct.components, invFun := CategoryTheory.Limits.biproduct.matrix, left_inv := , right_inv := }
    @[simp]
    theorem CategoryTheory.Limits.biproduct.matrixEquiv_symm_apply {J : Type} [Finite J] {K : Type} [Finite K] {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] [CategoryTheory.Limits.HasFiniteBiproducts C] {f : JC} {g : KC} (m : (j : J) → (k : K) → f j g k) :
    CategoryTheory.Limits.biproduct.matrixEquiv.symm m = CategoryTheory.Limits.biproduct.matrix m
    @[simp]
    theorem CategoryTheory.Limits.biproduct.matrixEquiv_apply {J : Type} [Finite J] {K : Type} [Finite K] {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] [CategoryTheory.Limits.HasFiniteBiproducts C] {f : JC} {g : KC} (m : f g) (j : J) (k : K) :
    CategoryTheory.Limits.biproduct.matrixEquiv m j k = CategoryTheory.Limits.biproduct.components m j k

    Biproducts are unique up to isomorphism. This already follows because bilimits are limits, but in the case of biproducts we can give an isomorphism with particularly nice definitional properties, namely that biproduct.lift b.π and biproduct.desc b.ι are inverses of each other.

    Equations

    The limit bicone for the biproduct over an index type with exactly one term.

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

    A biproduct over an index type with exactly one term is just the object over that term.

    Equations

    A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • pt : C

      A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • fst : self.pt P

      A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • snd : self.pt Q

      A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • inl : P self.pt

      A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • inr : Q self.pt

      A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • inl_snd : CategoryTheory.CategoryStruct.comp self.inl self.snd = 0

      A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • inr_fst : CategoryTheory.CategoryStruct.comp self.inr self.fst = 0

      A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    • A binary bicone for a pair of objects P Q : C consists of the cone point X, maps from X to both P and Q, and maps from both P and Q to X, so that inlfst = 𝟙 P, inlsnd = 0, inrfst = 0, and inrsnd = 𝟙 Q

    Instances For

    A binary bicone morphism between two binary bicones for the same diagram is a morphism of the binary bicone points which commutes with the cone and cocone legs.

    The category of binary bicones on a given diagram.

    Equations
    def CategoryTheory.Limits.BinaryBicones.ext {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {P Q : C} {c c' : CategoryTheory.Limits.BinaryBicone P Q} (φ : c.pt c'.pt) (winl : CategoryTheory.CategoryStruct.comp c.inl φ.hom = c'.inl := by aesop_cat) (winr : CategoryTheory.CategoryStruct.comp c.inr φ.hom = c'.inr := by aesop_cat) (wfst : CategoryTheory.CategoryStruct.comp φ.hom c'.fst = c.fst := by aesop_cat) (wsnd : CategoryTheory.CategoryStruct.comp φ.hom c'.snd = c.snd := by aesop_cat) :
    c c'

    To give an isomorphism between cocones, it suffices to give an isomorphism between their vertices which commutes with the cocone maps.

    Equations
    • One or more equations did not get rendered due to their size.
    @[simp]
    theorem CategoryTheory.Limits.BinaryBicones.ext_inv_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {P Q : C} {c c' : CategoryTheory.Limits.BinaryBicone P Q} (φ : c.pt c'.pt) (winl : CategoryTheory.CategoryStruct.comp c.inl φ.hom = c'.inl := by aesop_cat) (winr : CategoryTheory.CategoryStruct.comp c.inr φ.hom = c'.inr := by aesop_cat) (wfst : CategoryTheory.CategoryStruct.comp φ.hom c'.fst = c.fst := by aesop_cat) (wsnd : CategoryTheory.CategoryStruct.comp φ.hom c'.snd = c.snd := by aesop_cat) :
    (CategoryTheory.Limits.BinaryBicones.ext φ winl winr wfst wsnd).inv.hom = φ.inv
    @[simp]
    theorem CategoryTheory.Limits.BinaryBicones.ext_hom_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {P Q : C} {c c' : CategoryTheory.Limits.BinaryBicone P Q} (φ : c.pt c'.pt) (winl : CategoryTheory.CategoryStruct.comp c.inl φ.hom = c'.inl := by aesop_cat) (winr : CategoryTheory.CategoryStruct.comp c.inr φ.hom = c'.inr := by aesop_cat) (wfst : CategoryTheory.CategoryStruct.comp φ.hom c'.fst = c.fst := by aesop_cat) (wsnd : CategoryTheory.CategoryStruct.comp φ.hom c'.snd = c.snd := by aesop_cat) :
    (CategoryTheory.Limits.BinaryBicones.ext φ winl winr wfst wsnd).hom.hom = φ.hom

    A functor F : C ⥤ D sends binary bicones for P and Q to binary bicones for G.obj P and G.obj Q functorially.

    Equations
    • One or more equations did not get rendered due to their size.
    Instances For
    @[simp]
    theorem CategoryTheory.Limits.BinaryBicone.toBiconeFunctor_map_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {X Y : C} {X✝ Y✝ : CategoryTheory.Limits.BinaryBicone X Y} (f : X✝ Y✝) :
    (CategoryTheory.Limits.BinaryBicone.toBiconeFunctor.map f).hom = f.hom
    @[simp]
    theorem CategoryTheory.Limits.BinaryBicone.toBiconeFunctor_obj_pt {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {X Y : C} (b : CategoryTheory.Limits.BinaryBicone X Y) :
    (CategoryTheory.Limits.BinaryBicone.toBiconeFunctor.obj b).pt = b.pt
    @[reducible, inline]

    A shorthand for toBiconeFunctor.obj

    Equations
    • b.toBicone = CategoryTheory.Limits.BinaryBicone.toBiconeFunctor.obj b

    A binary bicone is a limit cone if and only if the corresponding bicone is a limit cone.

    Equations

    A binary bicone is a colimit cocone if and only if the corresponding bicone is a colimit cocone.

    Equations

    Convert a Bicone over a function on WalkingPair to a BinaryBicone.

    Equations
    • One or more equations did not get rendered due to their size.
    @[simp]
    theorem CategoryTheory.Limits.Bicone.toBinaryBiconeFunctor_map_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {X Y : C} {X✝ Y✝ : CategoryTheory.Limits.Bicone (CategoryTheory.Limits.pairFunction X Y)} (f : X✝ Y✝) :
    (CategoryTheory.Limits.Bicone.toBinaryBiconeFunctor.map f).hom = f.hom
    @[reducible, inline]

    A shorthand for toBinaryBiconeFunctor.obj

    Equations
    • b.toBinaryBicone = CategoryTheory.Limits.Bicone.toBinaryBiconeFunctor.obj b

    A bicone over a pair is a limit cone if and only if the corresponding binary bicone is a limit cone.

    Equations

    A bicone over a pair is a colimit cocone if and only if the corresponding binary bicone is a colimit cocone.

    Equations

    Structure witnessing that a binary bicone is a limit cone and a limit cocone.

    A binary bicone is a bilimit bicone if and only if the corresponding bicone is a bilimit.

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

    A bicone over a pair is a bilimit bicone if and only if the corresponding binary bicone is a bilimit.

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

    A bicone over P Q : C, which is both a limit cone and a colimit cocone.

    • A bicone over P Q : C, which is both a limit cone and a colimit cocone.

    • isBilimit : self.bicone.IsBilimit

      A bicone over P Q : C, which is both a limit cone and a colimit cocone.

    HasBinaryBiproduct P Q expresses the mere existence of a bicone which is simultaneously a limit and a colimit of the diagram pair P Q.

    Instances

      HasBinaryBiproducts C represents the existence of a bicone which is simultaneously a limit and a colimit of the diagram pair P Q, for every P Q : C.

      Instances

        A category with finite biproducts has binary biproducts.

        This is not an instance as typically in concrete categories there will be an alternative construction with nicer definitional properties.

        The isomorphism between the specified binary product and the specified binary coproduct for a pair for a binary biproduct.

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        @[reducible, inline]

        An arbitrary choice of biproduct of a pair of objects.

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        An arbitrary choice of biproduct of a pair of objects.

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        @[reducible, inline]

        Given a pair of maps between the summands of a pair of binary biproducts, we obtain a map between the binary biproducts.

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        Instances For
        @[reducible, inline]

        An alternative to biprod.map constructed via colimits. This construction only exists in order to show it is equal to biprod.map.

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        theorem CategoryTheory.Limits.biprod.hom_ext {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {X Y Z : C} [CategoryTheory.Limits.HasBinaryBiproduct X Y] (f g : Z X Y) (h₀ : CategoryTheory.CategoryStruct.comp f CategoryTheory.Limits.biprod.fst = CategoryTheory.CategoryStruct.comp g CategoryTheory.Limits.biprod.fst) (h₁ : CategoryTheory.CategoryStruct.comp f CategoryTheory.Limits.biprod.snd = CategoryTheory.CategoryStruct.comp g CategoryTheory.Limits.biprod.snd) :
        f = g
        theorem CategoryTheory.Limits.biprod.hom_ext' {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] {X Y Z : C} [CategoryTheory.Limits.HasBinaryBiproduct X Y] (f g : X Y Z) (h₀ : CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.inl f = CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.inl g) (h₁ : CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.inr f = CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.inr g) :
        f = g

        The canonical isomorphism between the chosen biproduct and the chosen coproduct.

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        Given a pair of isomorphisms between the summands of a pair of binary biproducts, we obtain an isomorphism between the binary biproducts.

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        Binary biproducts are unique up to isomorphism. This already follows because bilimits are limits, but in the case of biproducts we can give an isomorphism with particularly nice definitional properties, namely that biprod.lift b.fst b.snd and biprod.desc b.inl b.inr are inverses of each other.

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        The fork defined in BinaryBicone.fstKernelFork is indeed a kernel.

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        The fork defined in BinaryBicone.sndKernelFork is indeed a kernel.

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        The cofork defined in BinaryBicone.inlCokernelCofork is indeed a cokernel.

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        The cofork defined in BinaryBicone.inrCokernelCofork is indeed a cokernel.

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        The kernel of biprod.fst : X ⊞ Y ⟶ X is Y.

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        The kernel of biprod.snd : X ⊞ Y ⟶ Y is X.

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        The cokernel of biprod.inl : X ⟶ X ⊞ Y is Y.

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        The cokernel of biprod.inr : Y ⟶ X ⊞ Y is X.

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        If Y is a zero object, X ≅ X ⊞ Y for any X.

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        If X is a zero object, Y ≅ X ⊞ Y for any Y.

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        The braiding isomorphism which swaps a binary biproduct.

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        An alternative formula for the braiding isomorphism which swaps a binary biproduct, using the fact that the biproduct is a coproduct.

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        @[simp]
        @[simp]
        theorem CategoryTheory.Limits.biprod.symmetry'_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] [CategoryTheory.Limits.HasBinaryBiproducts C] (P Q : C) {Z : C} (h : P Q Z) :
        CategoryTheory.CategoryStruct.comp (CategoryTheory.Limits.biprod.lift CategoryTheory.Limits.biprod.snd CategoryTheory.Limits.biprod.fst) (CategoryTheory.CategoryStruct.comp (CategoryTheory.Limits.biprod.lift CategoryTheory.Limits.biprod.snd CategoryTheory.Limits.biprod.fst) h) = h

        The associator isomorphism which associates a binary biproduct.

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        @[simp]
        theorem CategoryTheory.Limits.biprod.associator_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] [CategoryTheory.Limits.HasBinaryBiproducts C] (P Q R : C) :
        (CategoryTheory.Limits.biprod.associator P Q R).inv = CategoryTheory.Limits.biprod.lift (CategoryTheory.Limits.biprod.lift CategoryTheory.Limits.biprod.fst (CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.snd CategoryTheory.Limits.biprod.fst)) (CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.snd CategoryTheory.Limits.biprod.snd)
        @[simp]
        theorem CategoryTheory.Limits.biprod.associator_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Limits.HasZeroMorphisms C] [CategoryTheory.Limits.HasBinaryBiproducts C] (P Q R : C) :
        (CategoryTheory.Limits.biprod.associator P Q R).hom = CategoryTheory.Limits.biprod.lift (CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.fst CategoryTheory.Limits.biprod.fst) (CategoryTheory.Limits.biprod.lift (CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.biprod.fst CategoryTheory.Limits.biprod.snd) CategoryTheory.Limits.biprod.snd)