Tensor product of internal modules over a commutative monoid #
Module theory over a monoid object, after Deligne (2002), §§2.2–2.3.
Throughout, D is a monoidal category and A : D a monoid object
in Mathlib's internal sense: MonObj A, with internal left modules
given by ModObj A X over the self-action of D and bundled as
Mod D A.
actLeft: the action morphism of a module object, typed at the tensor productA ⊗ Xrather than at the action synonym⊙ₗ, with the module laws restated in this form.tensorRightModObj: a left module tensored with an object on the right is again a left module;freeModObj/freeModspecialize to the free moduleA ⊗ V. The regular module is Mathlib'sMod.regular A.actRight: on a left module over a commutative monoid in a braided category, the braiding induces a right action; the compatibility lemmasactLeft_actRightandactRight_actRightexpress that left and right actions commute and thatactRightis associative.modTensor A M N: the tensor product of modules, the coequalizer of the pair(M.X ⊗ A) ⊗ N.X ⇉ M.X ⊗ N.Xwhose first legmodTensorLegMacts onMthroughactRightand whose second legmodTensorLegNassociates and acts onN.modTensorModObj: theA-action descends to the coequalizer when everytensorLeft Xpreserves coequalizers; more generallymodTensorDescModObjdescends any monoid action onM.Xthat commutes withactRight.modTensorUnitLeft/modTensorUnitRight: the regular module is a two-sided unit, compatibly with the actions.modTensorMap: functoriality in both slots.restrictRegular/baseChange: base change along a morphism of commutative monoid objects.
The development is scoped to the structures above; associativity of
modTensor is outside this module's scope.
The action morphism of a module object, typed at the tensor product.
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Unitality of the action, in tensor form.
Unitality of the action, in tensor form.
Associativity of the action, in tensor form.
Associativity of the action, in tensor form.
Associativity of the action, associator on the right.
Associativity of the action, associator on the right.
actLeft is natural in module morphisms.
actLeft is natural in module morphisms.
The regular module, with underlying object reducibly A; it is
definitionally Mathlib's Mod.regular A.
Equations
- RS.regularMod A = { X := A, mod := CategoryTheory.ModObj.regular A }
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Unitality of an action transported to a right tensor factor.
Associativity of an action transported to a right tensor factor.
A left module tensored with an object on the right: the action
of A on X ⊗ V through the left factor.
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The free module on an object: A ⊗ V with the action given by
multiplication on the left factor.
Equations
- RS.freeModObj A V = RS.tensorRightModObj A A V
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Inserting the unit of A and then acting on the free module
is the identity.
The free module on an object, bundled.
Equations
- RS.freeMod A V = { X := CategoryTheory.MonoidalCategoryStruct.tensorObj A V, mod := RS.freeModObj A V }
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The carrier of the free module. This is definitional, and is
stated for use by name: as a simp rule it would rewrite the type
arguments of every application of the module interface at a free
module and so stop that interface firing.
The right action of A on a left module, induced by the
braiding.
Equations
- RS.actRight A X = CategoryTheory.CategoryStruct.comp (β_ X A).hom (RS.actLeft A X)
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Unitality of the braided right action.
Unitality of the braided right action.
actRight is natural in module morphisms.
actRight is natural in module morphisms.
actRight is natural in maps of module objects.
actRight is natural in maps of module objects.
The shuffle of two free modules: multiply the two algebra factors, having carried the first generator past the second algebra factor. This is at once the head absorption that folds an incoming free letter into an accumulated head.
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Two-sided compatibility for a commutative monoid: the left action and the braided right action on a module commute.
Two-sided compatibility for a commutative monoid: the left action and the braided right action on a module commute.
For a commutative monoid, the braided right action is associative.
For a commutative monoid, the braided right action is associative.
First leg of the module-tensor parallel pair on
(M.X ⊗ A) ⊗ N.X: act on M through the braided right action.
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Second leg of the module-tensor parallel pair on
(M.X ⊗ A) ⊗ N.X: associate and act on N.
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A P-action on M.X commuting with the braided right
A-action intertwines the first leg with the induced actions on
(M.X ⊗ A) ⊗ N.X and M.X ⊗ N.X.
Any morphism P ⊗ M.X ⟶ M.X intertwines the second leg with
the induced maps on (M.X ⊗ A) ⊗ N.X and M.X ⊗ N.X.
The tensor product of two modules over A: the coequalizer of
modTensorLegM and modTensorLegN.
Equations
- RS.modTensor A M N = CategoryTheory.Limits.coequalizer (RS.modTensorLegM A M N) (RS.modTensorLegN A M N)
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The projection onto the tensor product of modules.
Equations
- RS.modTensorπ A M N = CategoryTheory.Limits.coequalizer.π (RS.modTensorLegM A M N) (RS.modTensorLegN A M N)
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The two legs agree after the projection.
The two legs agree after the projection.
Descend a morphism coequalizing the two legs to the tensor product of modules.
Equations
- RS.modTensorDesc A M N k h = CategoryTheory.Limits.coequalizer.desc k h
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The descent factors the given morphism through the projection.
The descent factors the given morphism through the projection.
Morphisms out of the tensor product of modules are determined by their composite with the projection.
Whiskering the module-tensor coequalizer by tensorLeft P
yields a colimit cofork.
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Morphisms out of a whiskered tensor product of modules are determined by their composite with the whiskered projection.
Descend a morphism along the whiskered coequalizer.
Equations
- RS.modTensorWhiskerDesc A M N P k h = CategoryTheory.Limits.Cofork.IsColimit.desc (RS.modTensorWhiskerIsColimit A M N P) k h
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The whiskered descent factors the given morphism through the whiskered projection.
The whiskered descent factors the given morphism through the whiskered projection.
Whiskering the module-tensor coequalizer by tensorRight W
yields a colimit cofork.
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Morphisms out of a right-whiskered tensor product of modules are determined by their composite with the whiskered projection.
Descend a morphism along the right-whiskered coequalizer.
Equations
- RS.modTensorWhiskerRDesc A M N W k h = CategoryTheory.Limits.Cofork.IsColimit.desc (RS.modTensorWhiskerRIsColimit A M N W) k h
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The right-whiskered descent factors the given morphism through the whiskered projection.
The right-whiskered descent factors the given morphism through the whiskered projection.
Descend a compatible action along the module-tensor projection.
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Defining equation of the descended action.
Defining equation of the descended action.
Unitality descends to the tensor product of modules.
Associativity descends to the tensor product of modules.
The descended action intertwines descended morphisms with
actions on the target: if k coequalizes the legs and carries the
induced action on M.X ⊗ N.X to w, then the descent of k is
equivariant.
Descend a compatible monoid action on M.X to a module
structure on the tensor product.
Equations
- RS.modTensorDescModObj A M N P act compat hone hmul = { smul := RS.modTensorDescAct A M N P act compat, one_smul := ⋯, mul_smul := ⋯ }
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The A-module structure on the tensor product of modules over
a commutative monoid: the action on the M-factor descends.
Equations
- RS.modTensorModObj A M N = RS.modTensorDescModObj A M N A (RS.actLeft A M.X) ⋯ ⋯ ⋯
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The action of A on the tensor product of modules.
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Defining equation of the A-action on the tensor product of
modules.
Defining equation of the A-action on the tensor product of
modules.
Unitality of the descended action.
Associativity of the descended action.
The tensor product of modules, bundled as a module.
Equations
- RS.modTensorMod A M N = { X := RS.modTensor A M N, mod := RS.modTensorModObj A M N }
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The carrier of the bundled tensor product. Definitional, and
stated for use by name, for the reason given for freeMod_X.
The descended action through the second factor: over a symmetric base, the monoid braids past the first module and acts on the second. The balance relation of the coequalizer carries the first-factor action across.
The descended action through the second factor: over a symmetric base, the monoid braids past the first module and acts on the second. The balance relation of the coequalizer carries the first-factor action across.
The action of N coequalizes the legs at M = A.
The right action of M coequalizes the legs at N = A.
The regular module is a left unit for the module tensor product.
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The regular module is a right unit for the module tensor product of a commutative monoid.
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The left unit isomorphism is a morphism of modules.
The left unit isomorphism is a morphism of modules.
The right unit isomorphism is a morphism of modules.
The right unit isomorphism is a morphism of modules.
Module morphisms intertwine the first legs.
Module morphisms intertwine the second legs.
Functoriality of the module tensor product in both slots.
Equations
- RS.modTensorMap A f g = RS.modTensorDesc A M N (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.tensorHom f.hom g.hom) (RS.modTensorπ A M' N')) ⋯
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Defining equation of the functorial map.
Defining equation of the functorial map.
The functorial map preserves identities.
The functorial map preserves composition.
modTensorMap is a morphism of modules.
modTensorMap is a morphism of modules.
Functoriality, as a morphism of bundled modules.
Equations
- RS.modTensorMapMod A f g = CategoryTheory.Mod.Hom.mk' (RS.modTensorMap A f g) ⋯
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The relative tensor product of two module isomorphisms.
Equations
- RS.modTensorMapIso A e f = { hom := RS.modTensorMapMod A e.hom f.hom, inv := RS.modTensorMapMod A e.inv f.inv, hom_inv_id := ⋯, inv_hom_id := ⋯ }
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B as an A-module by restriction along φ, with underlying
object reducibly B.
Equations
- RS.restrictRegular φ = { X := B, mod := CategoryTheory.Mod.scalarRestriction φ B }
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For commutative B, the braided right A-action on the
restricted module is right multiplication through φ.
Multiplication of B commutes with the right A-action on the
restricted module.
Base change along φ: the extension B ⊗[A] M of an
A-module M.
Equations
- RS.baseChange φ M = RS.modTensor A (RS.restrictRegular φ) M
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The B-module structure on the base change: multiplication on
the left factor descends.
Equations
- RS.baseChangeModObj φ M = RS.modTensorDescModObj A (RS.restrictRegular φ) M B CategoryTheory.MonObj.mul ⋯ ⋯ ⋯
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The action of B on the base change.
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Defining equation of the B-action on the base change.
Defining equation of the B-action on the base change.
Base change, bundled as a B-module.
Equations
- RS.baseChangeMod φ M = { X := RS.baseChange φ M, mod := RS.baseChangeModObj φ M }