The ℤ/2-graded doubling of a category #
Deligne 2.11's device: the category of super-objects over an
ambient category A. An object is a pair of objects of A, the
even and odd components; a morphism is a pair of component
morphisms. The graded tensor product mixes the components in the
usual super pattern, the braiding carries the Koszul sign -1 on
the odd⊗odd block, and the odd copy of the unit provides an odd
invertible object — exactly the hypothesis pair of Deligne 2.9 —
to a category that may lack one.
This module mirrors, at the abstract level, the concrete
SuperVect construction of RS/Definitions.lean: the same
component bookkeeping, with binary biproducts in place of products
of vector spaces.
The monoidal coherences are not proved by hand: the summing
functor Doubled A ⥤ A, X ↦ X.even ⊞ X.odd, is faithful, and
CategoryTheory.Monoidal.induced transports the pentagon and
triangle from A along it. The braiding coherences, which the
Koszul sign prevents from transporting, are discharged by
componentwise matrix checks against the distributor calculus set
up in the Distributors section.
A super-object over A: a pair of objects of A, thought of
as the even and odd graded components.
- even : A
The even component.
- odd : A
The odd component.
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A morphism of super-objects: a pair of component morphisms, preserving the grading.
The even component of the morphism.
The odd component of the morphism.
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Super-objects form a category with componentwise identities and composition.
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The even component of a morphism of super-objects.
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- RS.Doubled.evenHom f = f.even
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The odd component of a morphism of super-objects.
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- RS.Doubled.oddHom f = f.odd
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An isomorphism of super-objects from a pair of component isomorphisms.
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- RS.Doubled.isoMk e o = { hom := RS.Doubled.homMk e.hom o.hom, inv := RS.Doubled.homMk e.inv o.inv, hom_inv_id := ⋯, inv_hom_id := ⋯ }
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The componentwise zero morphism.
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- X.homZero Y = { zero := RS.Doubled.homMk 0 0 }
Componentwise addition of morphisms.
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- X.homAdd Y = { add := fun (f g : X ⟶ Y) => RS.Doubled.homMk (RS.Doubled.evenHom f + RS.Doubled.evenHom g) (RS.Doubled.oddHom f + RS.Doubled.oddHom g) }
Componentwise negation of morphisms.
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- X.homNeg Y = { neg := fun (f : X ⟶ Y) => RS.Doubled.homMk (-RS.Doubled.evenHom f) (-RS.Doubled.oddHom f) }
The componentwise additive group of morphisms.
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The doubling of a preadditive category is preadditive, componentwise.
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- RS.Doubled.instPreadditive = { homGroup := inferInstance, add_comp := ⋯, comp_add := ⋯ }
A super-object with two zero components is a zero object.
The doubling of a category with a zero object has a zero object, with both components zero.
The componentwise ℂ-module of morphisms.
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The doubling of a ℂ-linear category is ℂ-linear, componentwise.
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- RS.Doubled.instLinear = { homModule := inferInstance, smul_comp := ⋯, comp_smul := ⋯ }
The matrix map out of a right-whiskered binary biproduct.
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The matrix map out of a left-whiskered binary biproduct.
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inl_descRight in tensor-of-morphisms shape. Not a simp
lemma: ⊗ₘ 𝟙 normalises to the whiskering, where
inl_descRight already applies.
inl_descRight in tensor-of-morphisms shape. Not a simp
lemma: ⊗ₘ 𝟙 normalises to the whiskering, where
inl_descRight already applies.
inr_descRight in tensor-of-morphisms shape. Not a simp
lemma: ⊗ₘ 𝟙 normalises to the whiskering, where
inr_descRight already applies.
inr_descRight in tensor-of-morphisms shape. Not a simp
lemma: ⊗ₘ 𝟙 normalises to the whiskering, where
inr_descRight already applies.
inl_descLeft in tensor-of-morphisms shape. Not a simp
lemma: 𝟙 ⊗ₘ normalises to the whiskering, where inl_descLeft
already applies.
inl_descLeft in tensor-of-morphisms shape. Not a simp
lemma: 𝟙 ⊗ₘ normalises to the whiskering, where inl_descLeft
already applies.
inr_descLeft in tensor-of-morphisms shape. Not a simp
lemma: 𝟙 ⊗ₘ normalises to the whiskering, where inr_descLeft
already applies.
inr_descLeft in tensor-of-morphisms shape. Not a simp
lemma: 𝟙 ⊗ₘ normalises to the whiskering, where inr_descLeft
already applies.
Maps out of a right-whiskered binary biproduct are determined by their composites with the whiskered inclusions.
Maps out of a left-whiskered binary biproduct are determined by their composites with the whiskered inclusions.
The graded tensor product of super-objects: parities add, so each component of the product is a biproduct of two mixed blocks.
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The graded tensor product of morphisms, blockwise.
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Left whiskering of super-objects, blockwise.
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Right whiskering of super-objects, blockwise.
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The even component of the associator: each of the four parity
blocks re-associates through A's associator and is routed to the
matching block of the right-nested product.
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The odd component of the associator.
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The summing functor X ↦ X.even ⊞ X.odd. It is faithful, and
the monoidal coherences of the doubling are induced along it.
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The summing functor is faithful: both components are recovered as matrix entries.
The monoidal unit of the doubling: the unit of A in even
degree, the zero object in odd degree.
Equations
- RS.Doubled.unit = { even := CategoryTheory.MonoidalCategoryStruct.tensorUnit A, odd := 0 }
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A left tensor factor which is the zero object kills the product.
A right tensor factor which is the zero object kills the product.
Collapse of a unit block against a zero block, in the shape of the left unitor components.
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Collapse of a unit block against a zero block, in the shape of the even right unitor component.
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Collapse of a unit block against a zero block, in the shape of the odd right unitor component.
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The monoidal skeleton of the doubling: graded tensor product,
unit (𝟙_ A, 0), blockwise structural isomorphisms.
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Components of the monoidal notation #
The multiplicative comparison of the summing functor: double distribution followed by the parity shuffle.
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The unit comparison of the summing functor.
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- RS.Doubled.epsIso = { hom := CategoryTheory.Limits.biprod.inl, inv := CategoryTheory.Limits.biprod.fst, hom_inv_id := ⋯, inv_hom_id := ⋯ }
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The summing functor intertwines the graded tensor of morphisms
with the ambient tensor, through the comparison muIso.
The summing functor intertwines the graded tensor of morphisms
with the ambient tensor, through the comparison muIso.
The summing functor intertwines graded left whiskering with
ambient left whiskering, through the comparison muIso.
The summing functor intertwines graded left whiskering with
ambient left whiskering, through the comparison muIso.
The summing functor intertwines graded right whiskering with
ambient right whiskering, through the comparison muIso.
The summing functor intertwines graded right whiskering with
ambient right whiskering, through the comparison muIso.
The summing functor intertwines the graded associator with the
ambient associator, through the comparison muIso.
The summing functor intertwines the graded associator with the
ambient associator, through the comparison muIso.
The inducing data exhibiting the graded structural morphisms
as the images of A's structural morphisms under the parity
distributors.
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The doubling of a monoidal category is monoidal, with the graded tensor product: the coherences are induced along the faithful summing functor.
The summing functor is monoidal.
The doubling of a monoidal preadditive category is monoidal
preadditive, componentwise. Each component equation is restated
(show) with its objects in literal biproduct form, so that the
ambient simp lemmas apply.
The doubling of a monoidal ℂ-linear category is monoidal ℂ-linear, componentwise.
The even component of the Koszul braiding: A's braiding on
each parity block, with the sign -1 on the odd⊗odd block.
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The odd component of the Koszul braiding: the two mixed blocks
swap through A's braiding, with no sign.
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The doubling of a symmetric category is braided, with the Koszul-signed braiding.
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The doubling of a symmetric category is symmetric: the Koszul sign squares away.
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- RS.Doubled.instSymmetricCategory = { toBraidedCategory := RS.Doubled.instBraidedCategory, symmetry := ⋯ }
A biproduct of two zero objects is a zero object.
The even embedding X ↦ (X, 0).
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The even embedding is faithful.
The even embedding is full.
The even embedding is additive.
The even embedding is ℂ-linear.
The even embedding is monoidal up to isomorphism: the graded tensor of two even objects collapses to the even tensor.
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The even embedding intertwines the braidings through
evenEmbedTensorIso.
The unit comparison of the even embedding: definitional.
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The odd unit: the unit of A placed in odd degree. Together
with oddUnitSq and braiding_oddUnit this is exactly the
invertible odd object required by Deligne 2.9.
Equations
- RS.Doubled.oddUnit = { even := 0, odd := CategoryTheory.MonoidalCategoryStruct.tensorUnit A }
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The odd unit squares to the monoidal unit.
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The braiding of the odd unit with itself is -1: the Koszul
sign made visible on a single object.
The componentwise binary bicone on a pair of super-objects.
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The doubling has componentwise binary biproducts.
The doubling has finite products: it has a zero object and componentwise binary biproducts.
Every super-object is the biproduct of its even part and the
odd-unit twist of its odd part: the decomposition through which
Schur-vanishing transports from A to the doubling.
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The componentwise kernel fork of a morphism of super-objects.
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The componentwise kernel fork is limiting.
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The doubling has componentwise kernels.
The componentwise cokernel cofork of a morphism of super-objects.
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The componentwise cokernel cofork is colimiting.
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The doubling has componentwise cokernels.
A morphism of super-objects whose two components are isomorphisms is an isomorphism.
Componentwise exact pairings pair the doubled objects: the evaluations and coevaluations act blockwise on matching parities, and the odd components vanish.
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The doubling of a right rigid category is right rigid, with componentwise duals.
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The doubling of a left rigid category is left rigid, with componentwise duals.
The doubling of a rigid category is rigid.
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- RS.Doubled.instRigidCategory = { toRightRigidCategory := RS.Doubled.instRightRigidCategory, toLeftRigidCategory := RS.Doubled.instLeftRigidCategory }