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.
braidPast A V T: the structural isomorphism carryingAacross a contextV, with naturality in both the context and the tail.actAcross/tensorLeftModObj: a module tensored with an object on the left is again a module, acting through the right factor — the braided mirror oftensorRightModObj.powTailAct: the induced action ontensorPow D X (n + 1).modPowAct/modPowModObj: over a commutative monoid the tail action descends to the module power; the slot relations away from the top factor pass by naturality alone, and the top slot passes by one slot relation together with commutativity.modPowAct_perm/modPowAct_alg: the descended action commutes with the permutation action and itsℂ-linear extension.symPowAct/symPowModObj: the action descends to the symmetric power, withsymPowσa module map.modPowMod/symPowMod: the bundled modules.
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 carrying isomorphism is natural in the tail.
The carrying isomorphism is natural in the tail.
The carrying isomorphism is natural in the context.
The carrying isomorphism is natural in the context.
Carrying past a tensor context is carrying past the factors in turn.
Carrying a tensor pair past a context is carrying the factors past it in turn.
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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The action through the right factor, through the carrying isomorphism.
The action through the right factor is natural in the context.
The action through the right factor is natural in the context.
Unitality of the action through the right factor.
Associativity of the action through the right factor.
A left module tensored with an object on the left: the action
of A on V ⊗ X through the right factor.
Equations
- RS.tensorLeftModObj A V X = { smul := RS.actAcross A V X, one_smul := ⋯, mul_smul := ⋯ }
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The action through the right factor decomposes over a tensor context: braid past the outer factor, then act through the inner one.
The action through the right factor decomposes over a tensor context: braid past the outer factor, then act through the inner one.
The action through the right factor of a two-step context, conjugated by the associator: braid past the outer factor, act inside the last two.
An action through the right factor, precomposed with a reassociated whiskered morphism into the context.
Over a commutative monoid the braided self-crossing of the action agrees with the plain iterated action.
The key commutation: over a commutative monoid the external action slides across a slot leg acting on the inner factor.
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.
Unitality of the tail action.
Unitality of the tail action.
Associativity of the tail action.
Associativity of the tail action.
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.
The raw action on the ambient power of the module power.
Equations
- RS.modPowActRaw A X n = CategoryTheory.CategoryStruct.comp (RS.powTailAct A X n) (RS.modPowπ A X (n + 1))
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The raw action coequalizes the whiskered relation legs.
The monoid action on the module power, descended from the tail action through the whiskered coequalizer.
Equations
- RS.modPowAct A X n = RS.modPowWhiskerLeftDesc A X A (n + 1) (RS.modPowActRaw A X n) ⋯
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Defining equation of the descended action.
Defining equation of the descended action.
Unitality of the descended action.
Associativity of the descended action.
The module power of a module is a module, in every positive arity.
Equations
- RS.modPowModObj A X n = { smul := RS.modPowAct A X n, one_smul := ⋯, mul_smul := ⋯ }
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The module power of a module, bundled as a module.
Equations
- RS.modPowMod A X n = { X := RS.modPow A X (n + 1), mod := RS.modPowModObj A X n }
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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 descended action commutes with every permutation.
The descended action commutes with the group-algebra action, by linear extension of the permutation case.
The symmetric power as a module #
The monoid action on the symmetric power, through the section and the descended action.
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Defining equation of the symmetric-power action.
Defining equation of the symmetric-power action.
Unitality of the symmetric-power action.
Associativity of the symmetric-power action.
The symmetric power of a module is a module, in every positive arity.
Equations
- RS.symPowModObj A X n = { smul := RS.symPowAct A X n, one_smul := ⋯, mul_smul := ⋯ }
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The section of the symmetric power is a module map.
The symmetric power of a module, bundled as a module.
Equations
- RS.symPowMod A X n = { X := RS.symPow A X (n + 1), mod := RS.symPowModObj A X n }