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LeanPool.RegtsSevenster.RS.Classical.Deligne.IndTensorExact

Right-exactness of the tensor product on ind-objects #

Deligne's 2.2, exactness half: the transported tensor product of Ind C preserves colimits in each variable, and the monoidal structure is preadditive.

For a small monoidal C the file proves, unconditionally:

For C additionally preadditive with finite colimits and a preadditive tensor ([Preadditive C] [HasFiniteColimits C] [MonoidalPreadditive C] — Deligne's setting, where C is abelian ℂ-linear with exact tensor):

Filtered colimits are the only colimits Ind C possesses for general C; under [HasFiniteColimits C] it is cocomplete, and the finite-coproduct half of general right-exactness follows from the additivity above (see the acceptance tests at the bottom). Preservation of coequalizers demands genuine right-exactness of the tensor of C and is left to a follow-up lane.

The @[reducible] marking on the small coprodDiagram/ coprodCocone/coprodPairFunctor helpers is deliberate: their object fields must reduce at instance transparency for the show-retyped colimit proofs below to be stateable.

The embedding of Ind C into the Day presheaf category: the indization equivalence onto the full monoidal subcategory of ind-objects, followed by the subcategory inclusion.

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    @[instance_reducible]

    The forward functor of the indization equivalence is monoidal: the monoidal structure of Ind C is transported across it.

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    @[instance_reducible]

    The embedding into the Day presheaf category is monoidal.

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    The embedding of Ind C into the Day presheaf category is the inclusion into plain presheaves followed by the tautological equivalence with the Day synonym.

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      The embedding into the Day presheaf category preserves small filtered colimits: the inclusion into plain presheaves creates them and the tautological Day equivalence preserves everything.

      Deligne 2.2, filtered half, left version: tensoring on the left in Ind C preserves small filtered colimits. The embedding into the Day presheaf category is monoidal, so it intertwines tensorLeft X with the Day-convolution tensorLeft of the image, which preserves all small colimits; the embedding preserves and reflects filtered colimits, so tensorLeft X preserves them.

      The embedding into the Day presheaf category, restricted along indOf, is the Day synonym of the Yoneda embedding.

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        The embedding sends indOf.obj z to the Day representable at z.

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          The embedding calculus for the tensor of two embedded objects: under indToDay, the tensor indOf.obj x ⊗ indOf.obj y is the Day tensor of the representables at x and y, which is the representable at x ⊗ y — that is, the image of indOf.obj (x ⊗ y).

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            The fully faithful structure of the embedding into the Day presheaf category.

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              The tensor of two embedded objects of Ind C is the embedding of the tensor: indOf is monoidal up to isomorphism.

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                The embedding C ⥤ Ind C is monoidal up to isomorphism, in the orientation used downstream.

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                  Characterisation of the canonical isomorphism between two corepresenting objects: its classification under the first corepresentability structure is the universal element of the second.

                  The universal transformation classified by the Day tensor of two corepresentables: on a pair of morphisms it takes the tensor, (f, g) ↦ f ⊗ₘ g.

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                    RS.dayCoyonedaIso classifies as the universal transformation (f, g) ↦ f ⊗ₘ g: composing the Day unit with its underlying natural transformation is RS.coyonedaTensorHom.

                    Naturality of RS.indOfTensorIso in the right variable: the embedding-tensor comparison intertwines whiskering by an embedded object with the embedded whiskering.

                    The unit of Ind C is the embedded unit: the transported Day unit, identified through RS.dayUnitIso with the representable at 𝟙_ C and pulled back through the fully faithful monoidal embedding RS.indToDay.

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                      A cocone leg, with its type stated at the cocone point.

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                        @[reducible]

                        The pointwise binary coproduct of two diagrams of the same shape.

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                          @[simp]

                          A leg of a cocone over a pointwise coproduct, with its type stated at the coproduct.

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                            @[reducible]

                            The coproduct of two cocones: a cocone over the pointwise coproduct diagram, with the coproduct of the two points as its point.

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                              @[reducible]

                              A cocone over the pointwise coproduct, restricted along the first inclusion to a cocone over the first diagram.

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                                @[reducible]

                                A cocone over the pointwise coproduct, restricted along the second inclusion to a cocone over the second diagram.

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                                  The coproduct of two colimit cocones is a colimit cocone over the pointwise coproduct diagram: colimits commute with binary coproducts.

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                                    @[reducible]

                                    The pointwise binary coproduct of two functors.

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                                      Filtered descent for pointwise-invertible transformations: a natural transformation between endofunctors of Ind C that both preserve filtered colimits is invertible everywhere as soon as it is invertible on the embedded objects, since every ind-object is a filtered colimit of embedded objects.

                                      Abstract form of the embedded base case, left version: a morphism satisfying the two defining equations of the binary coproduct comparison for tensorLeft (indOf.obj a) at a pair of embedded objects is invertible, provided the corresponding comparison in C is.

                                      The embedded base case, left version: the binary coproduct comparison for tensoring on the left by an embedded object is invertible at a pair of embedded objects.

                                      Stage two, left version: the comparison for tensoring on the left by an embedded object, at one embedded and one arbitrary argument. Filtered descent in the second coproduct argument.

                                      An object that is both initial and terminal is a zero object.

                                      The embedding C ⥤ Ind C carries zero objects to zero objects: it preserves the initial and the terminal object.

                                      Tensoring a vanishing ind-object on the left kills it: descend along a presentation of the other factor and use that tensoring in C is additive.

                                      Deligne 2.2, preadditive half: the transported monoidal structure on Ind C is preadditive — whiskering is additive in each variable.