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LeanPool.RegtsSevenster.RS.Classical.Deligne.IndCoeq

Exactness of the tensor product on ind-objects, (co)equalizer half #

Deligne's 2.2, finite-(co)limit half: for a small rigid abelian monoidal C with preadditive tensor, the transported tensor product of Ind C preserves finite colimits and finite limits in each variable. This closes the gap documented in RS.Classical.Deligne.IndTensorExact, whose additivity results cover the finite-(co)product halves; what remains, and is proved here, is preservation of coequalizers and of equalizers.

The route is a two-stage descent from the exactness of the tensor of C itself (RS.Classical.Deligne.TensorExact):

Deliverables, for every A : Ind C:

The acceptance tests confirm that the cokernel and kernel comparison isomorphisms of both tensoring functors synthesize.

Preservation of the colimit of a composed diagram K ⋙ F by T, given that F preserves the colimit of K and the composite functor F ⋙ T does: the colimit cocone of K ⋙ F may be taken to be the image under F of the colimit cocone of K.

Transpose of the interchange property, flipped-diagram form: if K-indexed colimits preserve J-limits, the J-limit functor preserves the colimit of G.flip for any bifunctor G. The colimit comparison is identified with the canonical interchange isomorphism Limits.colimitLimitIso.

The embedding-tensor comparison as a natural isomorphism, left version: composing the embedding with left tensoring by an embedded object is left tensoring in C followed by the embedding.

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    Left tensoring by an embedded object commutes with Ind.lim: the composite Ind.lim I ⋙ tensorLeft (indOf.obj a) is the whiskering of tensorLeft a followed by Ind.lim I. The chain mirrors Mathlib's Ind.limCompInclusion, with the embedded-tensor isomorphism in place of Ind.yonedaCompInclusion.

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      Right tensoring by an embedded object commutes with Ind.lim.

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        Embedded stage, left version: left tensoring by an embedded object preserves the colimit of every parallel pair of Ind C. The pair is presented as a filtered colimit of parallel pairs of C; tensoring commutes with the presentation by RS.indLimCompTensorLeftIso, and on the functor-category side the whiskered tensorLeft a preserves coequalizers pointwise since the tensor of the rigid C is exact.

        Embedded stage for limits, left version: left tensoring by an embedded object preserves the limit of every parallel pair of Ind C — the dual of the coequalizer argument, using that Ind.lim preserves finite limits and that the tensor of the rigid C is left exact.

        @[reducible]

        The parallel pair (A ◁ f, A ◁ g) as a functor of the tensoring object A. The @[reducible] marking is deliberate: the object field must reduce at instance transparency for the show-retyped colimit proofs below to be stateable.

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          @[reducible]

          The parallel pair (f ▷ A, g ▷ A) as a functor of the tensoring object A.

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            Evaluating RS.whiskerLeftPairFunctor at a stage of the walking parallel pair gives right tensoring by the corresponding object.

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              Evaluating RS.whiskerRightPairFunctor at a stage gives left tensoring by the corresponding object.

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                The coequalizer comparison for left tensoring, naturally in the tensoring object: at A it is the canonical morphism from the colimit of the pair (A ◁ f, A ◁ g) to A ⊗ coeq (f, g).

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                  The coequalizer comparison for right tensoring, naturally in the tensoring object.

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                    Descent stage, left version: the colimit comparison of any parallel pair under left tensoring is invertible, for every tensoring ind-object. Filtered descent from the embedded stage.

                    Filtered colimits commute with WalkingParallelPair-limits in Ind C: the parallel-pair limit functor preserves filtered colimits. The property holds for types, lifts to presheaves pointwise, transposes through RS.preservesColimitsOfShape_lim, and transports to Ind C along the inclusion, which creates the limits and preserves the filtered colimits involved.

                    The equalizer comparison for left tensoring, naturally in the tensoring object: at A it is the canonical morphism from A ⊗ lim (f, g) to the limit of the pair (A ◁ f, A ◁ g).

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                      The equalizer comparison for right tensoring, naturally in the tensoring object.

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                        Descent stage for limits, left version: the limit comparison of any parallel pair under left tensoring is invertible, for every tensoring ind-object.

                        Deligne 2.2, right-exactness, left version: left tensoring on Ind C preserves finite colimits — coequalizers by the descent above, finite coproducts by the additivity of RS.Classical.Deligne.IndTensorExact.

                        Left-exactness, left version: left tensoring on Ind C preserves finite limits — equalizers by the dual descent, finite products by additivity. With the right-exactness above, the tensor of Ind C is exact in each variable.