mathlib documentation

category_theory.​limits.​limits

category_theory.​limits.​limits

@[nolint]
structure category_theory.​limits.​is_limit {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} :
category_theory.limits.cone FType (max u v)

A cone t on F is a limit cone if each cone on F admits a unique cone morphism to t.

The universal morphism from any other cone to a limit cone.

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Alternative constructor for is_limit, providing a morphism of cones rather than a morphism between the cone points and separately the factorisation condition.

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Any cone morphism between limit cones is an isomorphism.

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If the canonical morphism from a cone point to a limiting cone point is an iso, then the first cone was limiting also.

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theorem category_theory.​limits.​is_limit.​hom_lift {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} {t : category_theory.limits.cone F} (h : category_theory.limits.is_limit t) {W : C} (m : W t.X) :
m = h.lift {X := W, π := {app := λ (b : J), m t.π.app b, naturality' := _}}

theorem category_theory.​limits.​is_limit.​hom_ext {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} {t : category_theory.limits.cone F} (h : category_theory.limits.is_limit t) {W : C} {f f' : W t.X} :
(∀ (j : J), f t.π.app j = f' t.π.app j)f = f'

Two morphisms into a limit are equal if their compositions with each cone morphism are equal.

The cone points of two limit cones for naturally isomorphic functors are themselves isomorphic.

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We can prove two cone points (s : cone F).X and (t.cone F).X are isomorphic if

  • both cones are limit cones
  • their indexing categories are equivalent via some e : J ≌ K,
  • the triangle of functors commutes up to a natural isomorphism: e.functor ⋙ G ≅ F.

This is the most general form of uniqueness of cone points, allowing relabelling of both the indexing category (up to equivalence) and the functor (up to natural isomorphism).

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The universal property of a limit cone: a map W ⟶ X is the same as a cone on F with vertex W.

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The limit of F represents the functor taking W to the set of cones on F with vertex W.

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def category_theory.​limits.​is_limit.​hom_iso' {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} {t : category_theory.limits.cone F} (h : category_theory.limits.is_limit t) (W : C) :
(W t.X) {p // ∀ {j j' : J} (f : j j'), p j F.map f = p j'}

Another, more explicit, formulation of the universal property of a limit cone. See also hom_iso.

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If G : C → D is a faithful functor which sends t to a limit cone, then it suffices to check that the induced maps for the image of t can be lifted to maps of C.

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If F and G are naturally isomorphic, then F.map_cone c being a limit implies G.map_cone c is also a limit.

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If F.cones is represented by X, each morphism f : Y ⟶ X gives a cone with cone point Y.

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If F.cones is represented by X, each cone s gives a morphism s.X ⟶ X.

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If F.cones is represented by X, the cone corresponding to the identity morphism on X will be a limit cone.

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If F.cones is represented by X, the cone corresponding to a morphism f : Y ⟶ X is the limit cone extended by f.

If F.cones is represented by X, any cone is the extension of the limit cone by the corresponding morphism.

If F.cones is representable, then the cone corresponding to the identity morphism on the representing object is a limit cone.

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@[nolint]
structure category_theory.​limits.​is_colimit {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} :
category_theory.limits.cocone FType (max u v)

A cocone t on F is a colimit cocone if each cocone on F admits a unique cocone morphism from t.

The universal morphism from a colimit cocone to any other cone.

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Alternative constructor for is_colimit, providing a morphism of cocones rather than a morphism between the cocone points and separately the factorisation condition.

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Any cocone morphism between colimit cocones is an isomorphism.

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If the canonical morphism to a cocone point from a colimiting cocone point is an iso, then the first cocone was colimiting also.

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theorem category_theory.​limits.​is_colimit.​hom_desc {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} {t : category_theory.limits.cocone F} (h : category_theory.limits.is_colimit t) {W : C} (m : t.X W) :
m = h.desc {X := W, ι := {app := λ (b : J), t.ι.app b m, naturality' := _}}

theorem category_theory.​limits.​is_colimit.​hom_ext {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} {t : category_theory.limits.cocone F} (h : category_theory.limits.is_colimit t) {W : C} {f f' : t.X W} :
(∀ (j : J), t.ι.app j f = t.ι.app j f')f = f'

Two morphisms out of a colimit are equal if their compositions with each cocone morphism are equal.

We can prove two cocone points (s : cocone F).X and (t.cocone F).X are isomorphic if

  • both cocones are colimit ccoones
  • their indexing categories are equivalent via some e : J ≌ K,
  • the triangle of functors commutes up to a natural isomorphism: e.functor ⋙ G ≅ F.

This is the most general form of uniqueness of cocone points, allowing relabelling of both the indexing category (up to equivalence) and the functor (up to natural isomorphism).

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The universal property of a colimit cocone: a map X ⟶ W is the same as a cocone on F with vertex W.

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The colimit of F represents the functor taking W to the set of cocones on F with vertex W.

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def category_theory.​limits.​is_colimit.​hom_iso' {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] {F : J C} {t : category_theory.limits.cocone F} (h : category_theory.limits.is_colimit t) (W : C) :
(t.X W) {p // ∀ {j j' : J} (f : j j'), F.map f p j' = p j}

Another, more explicit, formulation of the universal property of a colimit cocone. See also hom_iso.

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If G : C → D is a faithful functor which sends t to a colimit cocone, then it suffices to check that the induced maps for the image of t can be lifted to maps of C.

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If F.cocones is corepresented by X, each morphism f : X ⟶ Y gives a cocone with cone point Y.

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If F.cocones is corepresented by X, each cocone s gives a morphism X ⟶ s.X.

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If F.cocones is corepresented by X, the cocone corresponding to the identity morphism on X will be a colimit cocone.

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If F.cocones is corepresented by X, the cocone corresponding to a morphism f : Y ⟶ X is the colimit cocone extended by f.

If F.cocones is corepresented by X, any cocone is the extension of the colimit cocone by the corresponding morphism.

If F.cocones is corepresentable, then the cocone corresponding to the identity morphism on the representing object is a colimit cocone.

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The projection from the chosen limit object to a value of the functor.

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The morphism from the cone point of any other cone to the chosen limit object.

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The isomorphism (in Type) between morphisms from a specified object W to the limit object, and cones with cone point W.

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def category_theory.​limits.​limit.​hom_iso' {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] (F : J C) [category_theory.limits.has_limit F] (W : C) :
(W category_theory.limits.limit F) {p // ∀ {j j' : J} (f : j j'), p j F.map f = p j'}

The isomorphism (in Type) between morphisms from a specified object W to the limit object, and an explicit componentwise description of cones with cone point W.

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The chosen limits of F : J ⥤ C and G : K ⥤ C are isomorphic, if there is an equivalence e : J ≌ K making the triangle commute up to natural isomorphism.

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The canonical morphism from G applied to the chosen limit of F to the chosen limit of F ⋙ G.

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Functoriality of limits.

Usually this morphism should be accessed through lim.map, but may be needed separately when you have specified limits for the source and target functors, but not necessarily for all functors of shape J.

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limit F is functorial in F, when C has all limits of shape J.

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The isomorphism between morphisms from W to the cone point of the limit cone for F and cones over F with cone point W is natural in F.

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The coprojection from a value of the functor to the chosen colimit object.

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@[simp]

We have lots of lemmas describing how to simplify colimit.ι F j ≫ _, and combined with colimit.ext we rely on these lemmas for many calculations.

However, since category.assoc is a @[simp] lemma, often expressions are right associated, and it's hard to apply these lemmas about colimit.ι.

We thus use reassoc to define additional @[simp] lemmas, with an arbitrary extra morphism. (see tactic/reassoc_axiom.lean)

The isomorphism (in Type) between morphisms from the colimit object to a specified object W, and cocones with cone point W.

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def category_theory.​limits.​colimit.​hom_iso' {J : Type v} [category_theory.small_category J] {C : Type u} [category_theory.category C] (F : J C) [category_theory.limits.has_colimit F] (W : C) :
(category_theory.limits.colimit F W) {p // ∀ {j j' : J} (f : j j'), F.map f p j' = p j}

The isomorphism (in Type) between morphisms from the colimit object to a specified object W, and an explicit componentwise description of cocones with cone point W.

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The canonical morphism from G applied to the chosen colimit of F ⋙ G to G applied to the chosen colimit of F.

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Functoriality of colimits.

Usually this morphism should be accessed through colim.map, but may be needed separately when you have specified colimits for the source and target functors, but not necessarily for all functors of shape J.

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colimit F is functorial in F, when C has all colimits of shape J.

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The isomorphism between morphisms from the cone point of the chosen colimit cocone for F to W and cocones over F with cone point W is natural in F.

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