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LeanPool.RegtsSevenster.RS.Classical.Deligne.BigTensor

The tensor product of an arbitrary family of monoid objects #

Infrastructure for Deligne's 2.11: in a braided monoidal category D with filtered colimits, an arbitrary family B : ι → D of (commutative) monoid objects has a tensor product, defined as the filtered colimit of the tensor products of its finite subfamilies.

The index type carries a linear order, which fixes the ordering of the tensor slots: the finite sub-tensor-product over s : Finset ι is the fold of B over the sorted list of s. For s ⊆ t there is an insertion morphism which places the unit of the missing factors into the extra slots; these are the transition maps of a Finset ι-shaped diagram, and the big tensor product is its colimit.

Finite tensor products #

def RS.listTensor {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (B : ι → D) :
List ι → D

The tensor product of the factors B i over a list of indices, folded to the right with the unit object as seed.

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    def RS.finTensor {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] [LinearOrder ι] (B : ι → D) (s : Finset ι) :
    D

    The tensor product of the factors B i over a finite set of indices, in the slot order given by the linear order on ι.

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      @[instance_reducible]

      The finite tensor products of a family of monoid objects are monoid objects, by folding the binary braided instance.

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      Finite tensor products of commutative monoid objects are commutative, by folding the binary instance of the symmetric category.

      Insertion of units #

      The unit η[M] : 𝟙_ D ⟶ M is a morphism of monoid objects from the trivial monoid.

      Inclusions of finite tensor products #

      The inclusion of a sub-tensor-product is defined at the level of lists: for a Boolean predicate p, the tensor product over l.filter p maps into the tensor product over l by inserting the unit of each factor whose index fails p.

      def RS.inclFilter {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] (p : ι → Bool) (l : List ι) :

      Insertion morphism from the tensor product over the filtered list into the tensor product over the full list, placing units in the slots dropped by the filter.

      Convention: all stated morphisms have listTensor-form endpoints; the tensor-shaped intermediate objects appear only between explicit eqToHom guards, so that every composition in the subsequent lemmas is well typed on the nose.

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        Equal index lists give equal (conjugated) insertions.

        theorem RS.inclFilter_congr_pred {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] {p q : ι → Bool} (h : p = q) (l : List ι) :

        Equal predicates give equal (transported) insertions.

        theorem RS.isMonHom_eqToHom {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] [CategoryTheory.BraidedCategory D] {l₁ l₂ : List ι} (h : l₁ = l₂) (q : listTensor B l₁ = listTensor B l₂) :

        Transporting along an equality of index lists is a morphism of monoid objects.

        The insertions are morphisms of monoid objects.

        theorem RS.inclFilter_of_forall {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] (p : ι → Bool) (l : List ι) (h : ∀ i ∈ l, p i = true) :

        Inserting nothing: if every index passes the filter, the insertion is the transport of the identity.

        The composition law for insertions: inserting the units of l.filter q past p and then those of l past q is the insertion past the conjunction. This is the coherence heart of the transition maps of the big tensor product.

        Bridging lemmas for the units of the fold monoids.

        On a vanishing index list, the unit of the fold monoid is the canonical identification with the monoidal unit.

        Inclusions between finite sub-tensor-products #

        theorem RS.sort_filter_of_subset {ι : Type v} [LinearOrder ι] {s t : Finset ι} (h : s ⊆ t) :
        List.filter (fun (i : ι) => decide (i ∈ s)) (t.sort fun (x1 x2 : ι) => x1 ≤ x2) = s.sort fun (x1 x2 : ι) => x1 ≤ x2

        Sorting commutes with restriction: the sorted list of a subset of t is the filtering of the sorted list of t.

        def RS.finTensorIncl {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] [LinearOrder ι] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] {s t : Finset ι} (h : s ⊆ t) :

        The inclusion of the finite sub-tensor-product over s ⊆ t, inserting the units of the factors missing from s.

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          theorem RS.finTensorIncl_trans {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] [LinearOrder ι] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] {s t u : Finset ι} (hst : s ⊆ t) (htu : t ⊆ u) :

          Functoriality of the inclusions of sub-tensor-products.

          Functoriality of the inclusions of sub-tensor-products.

          The inclusions of sub-tensor-products are morphisms of monoid objects.

          The big tensor product as a filtered colimit #

          The Finset ι-shaped diagram of finite sub-tensor-products, with the unit insertions as transition maps.

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            @[simp]
            theorem RS.finTensorDiagram_map {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] [LinearOrder ι] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] {X✝ Y✝ : Finset ι} (f : X✝ ⟶ Y✝) :
            @[simp]

            The tensor product of the whole family B, as the filtered colimit of its finite sub-tensor-products.

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              The stage inclusion of a finite sub-tensor-product into the big tensor product.

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                @[simp]

                Stage inclusions are compatible with the insertions.

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                Stage inclusions are compatible with the insertions.

                noncomputable def RS.bigTensorOf {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] [LinearOrder ι] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] [CategoryTheory.Limits.HasColimitsOfShape (Finset ι) D] (i : ι) :

                The inclusion of a single factor, through the stage at the singleton.

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                  The unit against the stages #

                  The unit of the empty stage is the canonical identification with the monoidal unit.

                  The unit of the big tensor product is reached from the unit of any finite stage.

                  Merge maps #

                  The multiplication of the big tensor product is presented on the finite stages by the merge maps: include both stages into their union, then multiply there. This section provides the merge maps together with their coherence squares.

                  Include-then-multiply is independent of the receiving stage: merging and then including into any common superset is inclusion into the superset followed by its multiplication.

                  Naturality of the merge maps in both stages.

                  The merge maps composed with the stage inclusions form a cocone in each variable: the square defining the multiplication of the big tensor product commutes.

                  theorem RS.eqToHom_eq_finTensorIncl {ι : Type v} {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] [LinearOrder ι] (B : ι → D) [(i : ι) → CategoryTheory.MonObj (B i)] {s t : Finset ι} (e : s = t) (h : s ⊆ t) :

                  The stage identification along an equality of finite sets is an inclusion.

                  The multiplication of the big tensor product #

                  With tensoring preserving Finset ι-colimits, bigTensor B ⊗ X and X ⊗ bigTensor B are colimits of the corresponding stage diagrams; maps out of them are determined by the stages, and the merge maps assemble into the multiplication.

                  The merge maps into the big tensor product form a cocone on the stage diagram tensored with a fixed finite stage.

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                    Multiplication of the big tensor product against a fixed finite stage.

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                      The partial multiplications form a cocone on the stage diagram tensored on the left with the big tensor product.

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                        The multiplication of the big tensor product.

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                          The multiplication restricted to a pair of stages is the merge map followed by the union stage: the presentation of the multiplication over pairs of finite stages.

                          The merge maps against a fixed first stage form a cocone.

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                            Multiplication of a fixed finite stage against the big tensor product.

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                              The monoid structure on the big tensor product #

                              @[instance_reducible]

                              The big tensor product of a family of monoid objects is a monoid object: the unit is the empty stage and the multiplication is assembled from the merge maps.

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                              Commutativity #