Documentation

Mathlib.CategoryTheory.Limits.Shapes.Multiequalizer

Multi-(co)equalizers #

A multiequalizer is an equalizer of two morphisms between two products. Since both products and equalizers are limits, such an object is again a limit. This file provides the diagram whose limit is indeed such an object. In fact, it is well-known that any limit can be obtained as a multiequalizer. The dual construction (multicoequalizers) is also provided.

Projects #

Prove that a multiequalizer can be identified with an equalizer between products (and analogously for multicoequalizers).

Prove that the limit of any diagram is a multiequalizer (and similarly for colimits).

inductive CategoryTheory.Limits.WalkingMulticospan {L : Type w} {R : Type w} (fst : RL) (snd : RL) :

The type underlying the multiequalizer diagram.

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    inductive CategoryTheory.Limits.WalkingMultispan {L : Type w} {R : Type w} (fst : LR) (snd : LR) :

    The type underlying the multiecoqualizer diagram.

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      Composition of morphisms for WalkingMulticospan.

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        Composition of morphisms for WalkingMultispan.

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          This is a structure encapsulating the data necessary to define a Multicospan.

          • L : Type w
          • R : Type w
          • fstTo : self.Rself.L
          • sndTo : self.Rself.L
          • left : self.LC
          • right : self.RC
          • fst : (b : self.R) → self.left (self.fstTo b) self.right b
          • snd : (b : self.R) → self.left (self.sndTo b) self.right b
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            structure CategoryTheory.Limits.MultispanIndex (C : Type u) [CategoryTheory.Category.{v, u} C] :
            Type (max (max u v) (w + 1))

            This is a structure encapsulating the data necessary to define a Multispan.

            • L : Type w
            • R : Type w
            • fstFrom : self.Lself.R
            • sndFrom : self.Lself.R
            • left : self.LC
            • right : self.RC
            • fst : (a : self.L) → self.left a self.right (self.fstFrom a)
            • snd : (a : self.L) → self.left a self.right (self.sndFrom a)
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              The multicospan associated to I : MulticospanIndex.

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                The induced map ∏ I.left ⟶ ∏ I.right via I.fst.

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                  The induced map ∏ I.left ⟶ ∏ I.right via I.snd.

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                    Taking the multiequalizer over the multicospan index is equivalent to taking the equalizer over the two morphsims ∏ I.left ⇉ ∏ I.right. This is the diagram of the latter.

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                      The multispan associated to I : MultispanIndex.

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                        The induced map ∐ I.left ⟶ ∐ I.right via I.fst.

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                          The induced map ∐ I.left ⟶ ∐ I.right via I.snd.

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                            Taking the multicoequalizer over the multispan index is equivalent to taking the coequalizer over the two morphsims ∐ I.left ⇉ ∐ I.right. This is the diagram of the latter.

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                              The maps from the cone point of a multifork to the objects on the left.

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                                theorem CategoryTheory.Limits.Multifork.ofι_pt {C : Type u} [CategoryTheory.Category.{v, u} C] (I : CategoryTheory.Limits.MulticospanIndex C) (P : C) (ι : (a : I.L) → P I.left a) (w : ∀ (b : I.R), CategoryTheory.CategoryStruct.comp (ι (I.fstTo b)) (I.fst b) = CategoryTheory.CategoryStruct.comp (ι (I.sndTo b)) (I.snd b)) :
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                                def CategoryTheory.Limits.Multifork.ofι {C : Type u} [CategoryTheory.Category.{v, u} C] (I : CategoryTheory.Limits.MulticospanIndex C) (P : C) (ι : (a : I.L) → P I.left a) (w : ∀ (b : I.R), CategoryTheory.CategoryStruct.comp (ι (I.fstTo b)) (I.fst b) = CategoryTheory.CategoryStruct.comp (ι (I.sndTo b)) (I.snd b)) :

                                Construct a multifork using a collection ι of morphisms.

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                                  This definition provides a convenient way to show that a multifork is a limit.

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                                    Given a multifork, we may obtain a fork over ∏ I.left ⇉ ∏ I.right.

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                                      Given a fork over ∏ I.left ⇉ ∏ I.right, we may obtain a multifork.

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                                        The category of multiforks is equivalent to the category of forks over ∏ I.left ⇉ ∏ I.right. It then follows from CategoryTheory.IsLimit.ofPreservesConeTerminal (or reflects) that it preserves and reflects limit cones.

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                                          The maps to the cocone point of a multicofork from the objects on the right.

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                                            theorem CategoryTheory.Limits.Multicofork.ofπ_pt {C : Type u} [CategoryTheory.Category.{v, u} C] (I : CategoryTheory.Limits.MultispanIndex C) (P : C) (π : (b : I.R) → I.right b P) (w : ∀ (a : I.L), CategoryTheory.CategoryStruct.comp (I.fst a) (π (I.fstFrom a)) = CategoryTheory.CategoryStruct.comp (I.snd a) (π (I.sndFrom a))) :
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                                            theorem CategoryTheory.Limits.Multicofork.ofπ_ι_app {C : Type u} [CategoryTheory.Category.{v, u} C] (I : CategoryTheory.Limits.MultispanIndex C) (P : C) (π : (b : I.R) → I.right b P) (w : ∀ (a : I.L), CategoryTheory.CategoryStruct.comp (I.fst a) (π (I.fstFrom a)) = CategoryTheory.CategoryStruct.comp (I.snd a) (π (I.sndFrom a))) (x : CategoryTheory.Limits.WalkingMultispan I.fstFrom I.sndFrom) :
                                            def CategoryTheory.Limits.Multicofork.ofπ {C : Type u} [CategoryTheory.Category.{v, u} C] (I : CategoryTheory.Limits.MultispanIndex C) (P : C) (π : (b : I.R) → I.right b P) (w : ∀ (a : I.L), CategoryTheory.CategoryStruct.comp (I.fst a) (π (I.fstFrom a)) = CategoryTheory.CategoryStruct.comp (I.snd a) (π (I.sndFrom a))) :

                                            Construct a multicofork using a collection π of morphisms.

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                                              This definition provides a convenient way to show that a multicofork is a colimit.

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                                                The category of multicoforks is equivalent to the category of coforks over ∐ I.left ⇉ ∐ I.right. It then follows from CategoryTheory.IsColimit.ofPreservesCoconeInitial (or reflects) that it preserves and reflects colimit cocones.

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                                                  For I : MulticospanIndex C, we say that it has a multiequalizer if the associated multicospan has a limit.

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                                                    For I : MultispanIndex C, we say that it has a multicoequalizer if the associated multicospan has a limit.

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                                                      Construct a morphism to the multiequalizer from its universal property.

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                                                        Construct a morphism from the multicoequalizer from its universal property.

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