Documentation

LeanPool.RegtsSevenster.RS.Classical.Deligne.IndSchur

Transport of tensor powers and the permutation action along #

C ⥤ Ind C

The embedding RS.indOf : C ⥤ Ind C is monoidal up to isomorphism (RS.indOfTensorIso, RS.Classical.Deligne.IndTensorExact). This file upgrades that comparison to the full coherent package needed to transport the symmetric-group action on tensor powers (RS.Novel.Envelope.SymPerm) across the embedding:

The Mathlib pin has Preadditive (Ind C) (for C preadditive with finite colimits) but no Linear ℂ (Ind C) instance, so Ind C carries no permAlg, so Schur vanishing cannot be stated on Ind C; the lemmas above are the permMor-level substrate, which is what the group-algebra layer would rest on were Linear ℂ (Ind C) available.

The single-element method used throughout the Day-level proofs: a morphism out of a (possibly iterated) Day tensor of corepresentables is classified, through the Kan-extension universal property and the Yoneda lemma, by one element — its value on the canonical element RS.dayCoyonedaUnitElt assembled from identities. All coherence comparisons are decided by evaluating both sides there.

@[reducible]

The Day-convolution structure of a plain presheaf pair, read through the synonym: makes the DayConvolution API available on underlying functors of the Day category.

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    The canonical element of the Day tensor of two corepresentables: the Kan-extension unit evaluated on the pair of identities.

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      The canonical element of a left-nested triple Day tensor of corepresentables.

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        The canonical element of a right-nested triple Day tensor of corepresentables.

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          The left-nested triple Day tensor of corepresentables corepresents evaluation at (a ⊗ b) ⊗ c: iterate the Kan-extension universal property twice and read off the Yoneda lemma on the external product of three corepresentables, which is definitionally the corepresentable of the triple product category.

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            The classification of RS.dayCoyonedaIso under the corepresentability of the Day tensor of two corepresentables: its value is the identity of p ⊗ q.

            Day convolution of corepresentables intertwines the braiding: under the co-Yoneda identifications, the braiding of the Day tensor of two corepresentables is precomposition with the braiding of the base.

            The embedding into the Day presheaf category carries the embedding-tensor comparison to the Day-level comparison.

            @[instance_reducible]

            The indization equivalence's forward functor is braided: the braiding of Ind C is transported across it.

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            The embedding-tensor comparison intertwines the braiding: indOf with indOfTensorIso is a braided functor up to isomorphism.

            The tensor powers of an embedded object are the embedded tensor powers: n = 0 is RS.indOfUnitIso, and each successor stage tensors the previous one with indOf.obj X and applies the embedding-tensor comparison.

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              @[simp]

              The base case of the power comparison.

              Transport of the top braiding: swapTop on the powers of an embedded object is conjugate to the embedded swapTop.

              Transport of the insertion cycle: insertTop on the powers of an embedded object is conjugate to the embedded insertTop.

              Transport of the permutation action along the embedding C ⥤ Ind C: the action of a permutation on the tensor powers of an embedded object is conjugate, under RS.indOfPowIso, to the embedded action.

              The faithfulness bridge: the embedding C ⥤ Ind C reflects and preserves vanishing of morphisms.

              Vanishing of the permutation action on an embedded object is vanishing of the embedded action.

              Vanishing of the permutation action transports faithfully along the embedding C ⥤ Ind C. This is the permMor-level form of Schur-vanishing transport: the group-algebra form waits on a Linear ℂ (Ind C) instance, which the Mathlib pin does not provide.

              Schur vanishing, read through the embedding: the shape μ kills X precisely when the embedded action of its block idempotent vanishes. Together with RS.permMor_indOf_eq_zero_iff this is the substrate for transporting RS.SchurKilled to Ind C; phrasing the Ind C side through permAlg needs Linear ℂ (Ind C), which is a mainline decision.