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LeanPool.RegtsSevenster.RS.Classical.Deligne.ModTensor

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.

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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    @[reducible]

    The regular module, with underlying object reducibly A; it is definitionally Mathlib's Mod.regular A.

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      Associativity of an action transported to a right tensor factor.

      @[implicit_reducible]

      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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        @[implicit_reducible]

        The free module on an object: A ⊗ V with the action given by multiplication on the left factor.

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          The free module on an object, bundled.

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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 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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              Second leg of the module-tensor parallel pair on (M.X ⊗ A) ⊗ N.X: associate and act on N.

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                Morphisms out of the tensor product of modules are determined by their composite with the projection.

                theorem RS.modTensorDescAct_mul {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (A : D) [CategoryTheory.MonObj A] [CategoryTheory.BraidedCategory D] (M N : CategoryTheory.Mod D A) [CategoryTheory.Limits.HasCoequalizers D] [∀ (X : D), CategoryTheory.Limits.PreservesColimitsOfShape CategoryTheory.Limits.WalkingParallelPair (CategoryTheory.MonoidalCategory.tensorLeft X)] (P : D) [CategoryTheory.MonObj P] (act : CategoryTheory.MonoidalCategoryStruct.tensorObj P M.X ⟶ M.X) (compat : CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerLeft P (actRight A M.X)) act = CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator P M.X A).inv (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerRight act A) (actRight A M.X))) (hmul : CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerRight CategoryTheory.MonObj.mul M.X) act = CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator P P M.X).hom (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerLeft P act) act)) :

                Associativity descends to the tensor product of modules.

                theorem RS.modTensorDescAct_desc {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (A : D) [CategoryTheory.MonObj A] [CategoryTheory.BraidedCategory D] (M N : CategoryTheory.Mod D A) [CategoryTheory.Limits.HasCoequalizers D] [∀ (X : D), CategoryTheory.Limits.PreservesColimitsOfShape CategoryTheory.Limits.WalkingParallelPair (CategoryTheory.MonoidalCategory.tensorLeft X)] (P : D) (act : CategoryTheory.MonoidalCategoryStruct.tensorObj P M.X ⟶ M.X) (compat : CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerLeft P (actRight A M.X)) act = CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator P M.X A).inv (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerRight act A) (actRight A M.X))) {W : D} (k : CategoryTheory.MonoidalCategoryStruct.tensorObj M.X N.X ⟶ W) (hk : CategoryTheory.CategoryStruct.comp (modTensorLegM A M N) k = CategoryTheory.CategoryStruct.comp (modTensorLegN A M N) k) (w : CategoryTheory.MonoidalCategoryStruct.tensorObj P W ⟶ W) (hw : CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator P M.X N.X).inv (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerRight act N.X) k) = CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerLeft P k) w) :

                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.

                @[implicit_reducible]

                Descend a compatible monoid action on M.X to a module structure on the tensor product.

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                  @[implicit_reducible]

                  The A-module structure on the tensor product of modules over a commutative monoid: the action on the M-factor descends.

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                    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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                        Functoriality of the module tensor product in both slots.

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                          The relative tensor product of two module isomorphisms.

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                            @[reducible]

                            B as an A-module by restriction along φ, with underlying object reducibly B.

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                              Base change along φ: the extension B ⊗[A] M of an A-module M.

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