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LeanPool.RegtsSevenster.RS.Classical.Deligne.PowAct

The monoid action on module powers and symmetric powers #

Over an internal commutative monoid A and a left module X in a braided category, the tensor powers of X carry an A-action through their last factor, with A braided past the lower factors. This module descends that action through the coequalizers of SymAlg.lean, making every positive module power and every positive symmetric power a module again.

Braiding a monoid past a context #

Carry an object across a context: the isomorphism A ⊗ (V ⊗ T) ≅ V ⊗ (A ⊗ T) braiding A past V.

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    The action through the right tensor factor #

    The action of a monoid on V ⊗ X through the right factor: braid A past V, then act on X.

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      @[implicit_reducible]

      A left module tensored with an object on the left: the action of A on V ⊗ X through the right factor.

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        The tail action on tensor powers #

        The raw tail action on a positive tensor power: the monoid acts on the last factor, braided past the lower power.

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          The tail action is the action through the right factor at the definitional fold of the tensor power.

          Descent of the tail action to the module power #

          The tail action coequalizes the whiskered relation legs: at a slot away from the top factor the action and the leg touch disjoint factors and pass one another by naturality, and at the top slot one slot relation together with commutativity of the monoid closes the square. Every equation crossing the definitional fold tensorPow D X (n + 1) = tensorPow D X n ⊗ X is applied by exact term-level composition, never by rewriting inside a foreign frame.

          Permutation equivariance of the descended action #

          The descended action commutes with the permutation action: for a top-fixing generator by naturality alone, and for the top transposition by carrying the acting monoid into the top slot and applying the slot relation there — the same slot the descent itself used. Generation by the adjacent transpositions extends both to the full symmetric group, and linearity to the group algebra.

          The window shuffle carrying the acting monoid into the top slot: d ⊗ (y ⊗ z) ↦ (z ⊗ d) ⊗ y.

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            The braided window identity for the first leg: acting on the top factor and braiding is shuffling and acting through the braided right action.

            The braided window identity for the second leg: the shuffle followed by the second leg is braiding first, then acting on the top factor.

            The symmetric power as a module #