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:
RS.indToDay— the monoidal embedding ofInd Cinto the Day presheaf categoryCᵒᵖ ⊛⥤ Type v, with itsFunctor.Monoidalinstance transported fromRS.indDayEquivalence;RS.tensorLeft_ind_preservesColimitsOfShapeand the right-hand and packaged (PreservesFilteredColimits) versions —tensorLeft XandtensorRight XonInd Cpreserve all small filtered colimits, for everyX : Ind C;RS.indOfTensorIso/RS.indOfTensorIsoSymm— the embeddingindOf : C ⥤ Ind Cis monoidal up to isomorphism, with naturality in each variable (RS.indOfTensorIso_hom_natural_right/_left); this rests on the corepresentability calculus forRS.dayCoyonedaIso(RS.eta_comp_dayCoyonedaIso_homand the naturality lemmas forRS.dayYonedaIso).
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):
RS.isIso_coprodComparison_tensorLeft/_tensorRight— the binary coproduct comparisons of both tensoring functors onInd Care invertible, by a three-stage filtered descent (RS.isIso_app_of_isIso_indOf) from the embedded case, which is conjugate underindOfto additivity of the tensor ofC;RS.isZero_tensor_left_ind/_right_ind— tensoring kills zero objects;RS.tensorLeft_ind_additive/RS.tensorRight_ind_additiveandMonoidalPreadditive (Ind C)— whiskering inInd Cis additive in each variable.
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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The forward functor of the indization equivalence is monoidal:
the monoidal structure of Ind C is transported across it.
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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.
Deligne 2.2, filtered half, right version: tensoring on the
right in Ind C preserves small filtered colimits.
Filtered-colimit preservation by left tensoring, packaged.
Filtered-colimit preservation by right tensoring, packaged.
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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- RS.indOfTensorIsoSymm x y = (RS.indOfTensorIso x y).symm
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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.coyonedaTensorHom a b = { app := fun (X : D × D) => TypeCat.ofHom fun (fg : (a, b) ⟶ X) => CategoryTheory.MonoidalCategoryStruct.tensorHom fg.1 fg.2, naturality := ⋯ }
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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.dayCoyonedaIso in the right variable.
Naturality of RS.dayCoyonedaIso in the left variable.
Naturality of Coyoneda.objOpOp, inverse form.
Naturality of Coyoneda.objOpOp, forward form, at a left
whiskering of Cᵒᵖ.
Naturality of Coyoneda.objOpOp, forward form, at a right
whiskering of Cᵒᵖ.
Naturality of RS.dayYonedaIso in the right variable.
Naturality of RS.dayYonedaIso in the left variable.
Naturality of RS.indToDayIndOfIso in its object.
Inverse form of RS.indToDay_map_comp_indToDayIndOfIso_hom.
Naturality of RS.indOfTensorIso in the right variable: the
embedding-tensor comparison intertwines whiskering by an embedded
object with the embedded whiskering.
Naturality of RS.indOfTensorIso in the left variable.
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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- RS.coconeLeg c j = c.ι.app j
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The pointwise binary coproduct of two diagrams of the same shape.
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A leg of a cocone over a pointwise coproduct, with its type stated at the coproduct.
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- RS.coprodLeg s j = s.ι.app j
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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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- RS.coprodCocone c₁ c₂ = { pt := c₁.pt ⨿ c₂.pt, ι := { app := fun (j : J) => CategoryTheory.Limits.coprod.map (RS.coconeLeg c₁ j) (RS.coconeLeg c₂ j), naturality := ⋯ } }
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A cocone over the pointwise coproduct, restricted along the first inclusion to a cocone over the first diagram.
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- RS.coprodCoconeFst s = { pt := s.pt, ι := { app := fun (j : J) => CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.coprod.inl (RS.coprodLeg s j), naturality := ⋯ } }
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A cocone over the pointwise coproduct, restricted along the second inclusion to a cocone over the second diagram.
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- RS.coprodCoconeSnd s = { pt := s.pt, ι := { app := fun (j : J) => CategoryTheory.CategoryStruct.comp CategoryTheory.Limits.coprod.inr (RS.coprodLeg s j), naturality := ⋯ } }
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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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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.
Abstract form of the embedded base case, right version.
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.
The embedded base case, right version.
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.
Stage two, right version.
Stage three, left version: descent in the first coproduct argument.
Stage three, right version.
The binary coproduct comparison for left tensoring in Ind C
is invertible: final descent in the tensoring object.
The binary coproduct comparison for right tensoring in
Ind C is invertible.
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.
A colimit all of whose stages vanish vanishes.
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.
Tensoring a vanishing ind-object on the right kills it.
Left tensoring on Ind C preserves zero morphisms.
Right tensoring on Ind C preserves zero morphisms.
Left tensoring on Ind C preserves binary coproducts.
Right tensoring on Ind C preserves binary coproducts.
Left tensoring on Ind C is additive.
Right tensoring on Ind C is additive.
Deligne 2.2, preadditive half: the transported monoidal
structure on Ind C is preadditive — whiskering is additive in each
variable.