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LeanPool.RegtsSevenster.RS.Classical.Deligne.PowMerge

The merge isomorphism for module powers #

The relative tensor product of two module powers is the module power of the summed arity: the descended power multiplication powMulDesc of PowChain.lean is an isomorphism modTensor A (modPowMod A X a) (modPowMod A X b) ≅ modPow A X (a + 1 + b + 1), with inverse powSplit descended from the inverse of the concatenation of ambient tensor powers.

Structural shuffles of the boundary window #

Both boundary computations move a window map across the split of the ambient power into two halves. The shuffles are stated at general objects, so that no tensor-power arity enters the rewriting.

theorem RS.split_shuffle_snd {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] {P Y G S B : D} (v : CategoryTheory.MonoidalCategoryStruct.tensorObj G S ⟶ S) :
CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerRight (CategoryTheory.MonoidalCategoryStruct.whiskerLeft P (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator Y G S).hom (CategoryTheory.MonoidalCategoryStruct.whiskerLeft Y v))) B) (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerRight (CategoryTheory.MonoidalCategoryStruct.associator P Y S).inv B) (CategoryTheory.MonoidalCategoryStruct.associator (CategoryTheory.MonoidalCategoryStruct.tensorObj P Y) S B).hom) = CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator P (CategoryTheory.MonoidalCategoryStruct.tensorObj (CategoryTheory.MonoidalCategoryStruct.tensorObj Y G) S) B).hom (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerLeft P (CategoryTheory.MonoidalCategoryStruct.associator (CategoryTheory.MonoidalCategoryStruct.tensorObj Y G) S B).hom) (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator P (CategoryTheory.MonoidalCategoryStruct.tensorObj Y G) (CategoryTheory.MonoidalCategoryStruct.tensorObj S B)).inv (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerRight (CategoryTheory.MonoidalCategoryStruct.associator P Y G).inv (CategoryTheory.MonoidalCategoryStruct.tensorObj S B)) (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator (CategoryTheory.MonoidalCategoryStruct.tensorObj P Y) G (CategoryTheory.MonoidalCategoryStruct.tensorObj S B)).hom (CategoryTheory.MonoidalCategoryStruct.whiskerLeft (CategoryTheory.MonoidalCategoryStruct.tensorObj P Y) (CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator G S B).inv (CategoryTheory.MonoidalCategoryStruct.whiskerRight v B)))))))

The right-window shuffle: a window map acting on the last two window factors passes to the head of the right half of the split.

The head insertion #

The split of the big power exposes the right half with its head factor peeled: the map into the module power of the right half is the peeling inverse followed by the projection. Through the multiplication with the singleton power this insertion is a module map for the action on the head factor — the slide of the monoid from the head to the tail of a module power, obtained from modPowMul_actLeft rather than slot by slot.

The left unitor inverse, retyped so that its target is stated through the singleton tensor power — this keeps every statement about it type-correct at low transparency.

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    The peeling inverse is the concatenation with a singleton block, up to the unitor and an arity transport.

    The head insertion of a factor into a module power: peel the head off the target power and project.

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      The tail of the left half #

      The braided right action of the monoid on the left half of the split is, after the projection, the boundary window's action on the last factor of the left half.

      Braiding the monoid over a context pair: the braided right action through the last factor of a pair is the braiding of the factor alone, then the action — the monoid never crosses the context.

      The boundary reassociation, retyped so that its target is stated through the grown tensor power.

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        The boundary bridge of the concatenation #

        Detach the top factor of the first block onto the second. The associator, retyped so that its source is stated through the tensor power — the inverse bridge to powAttach.

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          The boundary split of the concatenation: undoing the split concatenation after the glued one detaches the exposed window factor onto the right half and peels it back in.

          The boundary slot #

          The slot relation straddling the two halves of the split becomes, after both projections, the coequalizer relation of the module tensor product: the boundary bridge carries the relation object onto (modPow ⊗ A) ⊗ modPow, the braided right action of the left half absorbs the M-leg, and the head insertion of the right half absorbs the N-leg.

          The boundary bridge: reassociate the window across the split and project both halves, keeping the monoid between them.

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            The interior slots #

            A slot relation lying inside one half of the split embeds across the concatenation into that half and is absorbed by the half's own projection.

            The bridge of midConcatFst, as an isomorphism.

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              The bridge of midConcatSnd, as an isomorphism.

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                The split and the merge isomorphism #

                The wide-coequalizer condition of the split: every slot relation of the big power is absorbed by the split — inside the left half, at the boundary, or inside the right half.

                The split of a module power: the inverse of the concatenation descends through the wide coequalizer onto the module tensor product of the two halves.

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                  The merge isomorphism: the relative tensor product of two module powers is the module power of the summed arity, with the descended power multiplication as the forward direction and the split as its inverse.

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