Documentation

LeanPool.RegtsSevenster.RS.Classical.Deligne.ModMulti

The multi-tensor of internal modules over a monoid object #

The replacement for associativity of the binary module tensor product of ModTensor.lean: for a list Xs of left modules over a monoid object A in a braided monoidal category, the multi-tensor X₁ ⊗_A ⋯ ⊗_A Xₙ is presented in one step, as the coequalizer of a single wide relation pair

`⊕ₛ mid s ⇉ X₁ ⊗ (X₂ ⊗ (⋯ ⊗ Xₙ))`

whose source is the finite biproduct, over the adjacent slots s of the list, of the relation objects obtained by inserting A between the two factors of the slot, and whose legs act on the slot through the braided right action and the left action respectively — the two legs of ModTensor.lean at the slot's window, in every adjacent slot at once.

The further descent of modMultiConcat through the middle A-action — the comparison with modTensor of two multi-tensors — is outside this module's scope; its substrate (the head modules, the concatenation map, and the slot relations) is complete.

The underlying tensor fold #

The tensor fold of the underlying objects of a list of modules, folded to the right with the monoidal unit as seed.

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    Transport of the tensor fold along an equality of lists. It is an eqToHom, so it composes and cancels by eqToHom simp lemmas.

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      Whiskering a list transport is a list transport.

      The slot relations #

      A slot of the list is a decomposition Xs = pre ++ M :: N :: post. Its relation object inserts A between the two factors of the slot, nested exactly as the ambient fold, so that the legs below are typed at the fold on the nose, with no transport.

      The relation object of a slot: the ambient fold with the monoid inserted between the two factors of the slot.

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        Assemble a window morphism on (M.X ⊗ A) ⊗ N.X into a relation leg over a prefix: resolve the window, reassociate the second factor onto the suffix, and whisker through the prefix.

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          The first relation leg at a slot: act on the left factor of the window through the braided right action.

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            The second relation leg at a slot: associate and act on the right factor of the window.

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              Enumeration of the slots #

              An adjacent slot of a list of modules: a decomposition into a prefix, two adjacent factors, and a suffix.

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                Extend a slot by one factor below.

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                  The list of all adjacent slots of a list of modules.

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                    Extension by a factor preserves membership in the slot list.

                    The slot enumeration is complete: every decomposition of the list occurs among its slots.

                    The relation pair of the multi-tensor #

                    @[reducible, inline]

                    The relation object of a slot, in slot form.

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                      The first relation leg of a slot, transported to the ambient list.

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                        The second relation leg of a slot, transported to the ambient list.

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                          @[reducible, inline]

                          The source of the relation pair: the biproduct of the relation objects over all adjacent slots. An abbreviation, so that the biproduct API applies to the legs without unfolding.

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                            The second leg of the relation pair, assembled over all slots.

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                              The multi-tensor and its universal property #

                              The multi-tensor of a list of modules over A: the coequalizer of the wide relation pair, identifying (x·c) ⊗ y ~ x ⊗ (c·y) in every adjacent slot simultaneously. No binary module tensor product and no associativity enter.

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                                The empty and singleton multi-tensors #

                                Below two factors there are no adjacent slots: the relation source is the empty biproduct, the legs agree, and the projection is an isomorphism.

                                On a slot-free list the projection is an isomorphism.

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                                  The empty multi-tensor is the monoidal unit of the ambient category. With this presentation the empty fold is 𝟙_ D, not the regular module: consumers wanting A as the empty product — the unit of the module category — should treat the empty list as a separate case.

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                                    Comparison with the binary module tensor product #

                                    For a two-element list the wide relation pair has a single slot, whose window legs are exactly the parallel pair of ModTensor.lean; the two coequalizer presentations agree, up to the right unitor absorbing the unit seed of the fold.

                                    theorem RS.pair_decomp {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (A : D) [CategoryTheory.MonObj A] {X Y M N : CategoryTheory.Mod D A} {pre post : List (CategoryTheory.Mod D A)} (h : [X, Y] = pre ++ M :: N :: post) :
                                    pre = [] ∧ X = M ∧ Y = N ∧ post = []

                                    The only slot of a two-element list is the full decomposition.

                                    The resolution of the two-element fold onto the plain tensor product: absorb the unit seed. A bridge morphism with a modList-typed source, so that statements through it stay type-correct at low transparency.

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                                      Comparison with the binary tensor product: the forward direction, descending the binary projection.

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                                        Comparison with the binary tensor product: the backward direction, descending the wide projection.

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                                          The two-element multi-tensor is the binary module tensor product: the one-slot wide presentation and the parallel-pair presentation coequalize the same relations.

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                                            Concatenation of folds #

                                            The fold of a concatenated list against the tensor product of the two folds, with the bridges that carry a relation slot of one block into the concatenated list. Casts are quantified, as in the slot relations, so consumers never meet a transported proof they cannot name.

                                            Concatenation of tensor folds: the two-block fold reassociates onto the fold of the concatenated list.

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                                              The relation object of a slot, concatenated on the right: the suffix grows by the second block.

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                                                The right leg-concatenation: a slot leg of the first block, whiskered by the second block, is the slot leg of the concatenated list at the widened suffix.

                                                The right leg-concatenation: a slot leg of the first block, whiskered by the second block, is the slot leg of the concatenated list at the widened suffix.

                                                The relation object of a slot, concatenated on the left: the prefix grows by the first block.

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                                                  The left leg-concatenation: a slot leg of the second block, whiskered by the first block, is the slot leg of the concatenated list at the widened prefix.

                                                  The left leg-concatenation: a slot leg of the second block, whiskered by the first block, is the slot leg of the concatenated list at the widened prefix.

                                                  Whiskered descent #

                                                  Morphisms out of a whiskered multi-tensor, by descent along the whiskered projection. The relation source is a biproduct, so the whiskered relation condition reduces to the slots through the distributors of the monoidal preadditive structure.

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                                                  The right-whiskered descent factors the given morphism through the whiskered projection.

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                                                  The left-whiskered descent factors the given morphism through the whiskered projection.

                                                  The concatenation map #

                                                  The projection of the concatenated list descends through the tensor product of the two multi-tensors, one block at a time: first the relations of the first block through the right-whiskered coequalizer, then those of the second block through the left-whiskered coequalizer.

                                                  The head action #

                                                  On a non-empty list the monoid acts through the head factor; the action descends to the multi-tensor, making it a module. The slot compatibilities are the two cases: the slot at the head, through the binary window compatibilities of ModTensor.lean, and a slot in the tail, disjoint from the action.

                                                  The head action on the fold of a non-empty list: act on the head factor.

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                                                    The shuffle of the head slot: the monoid moves inside the window and acts on the left module factor there.

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                                                      The head-slot compatibility: a window morphism compatible with the binary action commutes the head action past the head slot's leg.

                                                      Descent of the head action #