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LeanPool.RegtsSevenster.RS.Classical.Deligne.Rappel210Reduce

Exactness of tensoring with a dualizable object #

The first stage of the reduction of the local splitting statement: tensoring with a two-sided dualizable object is exact, because the exact pairings make the tensor functor a left and a right adjoint at once. A short exact sequence therefore stays short exact after tensoring, which produces the internal-hom extension that the pullback stage consumes.

Tensoring on the left with an object with a left dual preserves colimits: the exact pairing makes it a left adjoint.

Tensoring on the left with an object with a right dual preserves limits: the exact pairing makes it a right adjoint.

Tensoring with a two-sided dualizable object is exact: a short exact sequence stays short exact after tensoring on the left.

The middle object of the unit-form extension: the pullback of the internal-hom epimorphism along the name of the identity.

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    The unit-form extension: the given sequence, internally hommed and pulled back along the name of the identity, now with unit quotient.

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      Epimorphisms dualise to monomorphisms: the right adjoint mate of an epimorphism is monic.

      The free section carrier: the point section, folded into the free module through the multiplication.

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        noncomputable def RS.freeModExtend {D : Type u} [CategoryTheory.Category.{v, u} D] [CategoryTheory.MonoidalCategory D] (B : D) [CategoryTheory.MonObj B] {V : D} (M : CategoryTheory.Mod D B) (q : V ⟶ M.X) :
        freeMod B V ⟶ M

        Extend a point to the free module: any morphism into the carrier of a module extends to a linear map from the free module, through the action.

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          Contract the dual against the argument: braid the payload out and evaluate.

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            The element of the free section: the unit, pushed through the free section and out of the pullback.

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              The transferred point: the free-section element, contracted against the argument.

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                The section of the statement of record: the transferred point, extended to the free module.

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