Orbit push-down of dual corepresentable modules #
This file develops the injective-representable half of Gabriel's Nakayama
comparison. The coefficient dual of Hom(-, X) is a covariant module, and
shifting its source reindexes the orbit Hom decomposition by negation.
The coefficient dual of the contravariant representable Hom(-, X),
regarded as a covariant linear module.
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The dual corepresentable bundled as an additive linear module.
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A morphism of representing objects acts covariantly on coefficient-dual corepresentables.
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Bundled linear-module form of the map induced by a morphism of representing objects.
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Right composition with the inverse of an isomorphism, bundled as the linear equivalence in the direction used by coefficient duality.
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Coefficient-dual corepresentables are naturally invariant under changing their representing object by an isomorphism.
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A dual corepresentable known to be pointwise finite-dimensional with finite object support.
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Bundled finite-dimensional form of the map induced by a morphism of representing objects.
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Moving a shift from the source of a Hom space to the target gives a linear equivalence, with the degree written explicitly for later dependent reindexing.
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The source-shift equivalence is composition in the orbit category with the canonical morphism from an object to its shift.
The source-shifted Hom direct sum is the usual target-shifted orbit Hom
direct sum. The reindexing is b ↦ -b.
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On the summand indexed by -a, the source-shifted Hom equivalence is
the homogeneous degree-a inclusion.
Inverse formula on a homogeneous orbit morphism.
Projection of an orbit Hom to its ordinary degree-zero component.
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Degree-zero projection intertwines left composition by an ordinary upstairs morphism with left composition in the orbit category.
Degree-zero projection intertwines right composition by an ordinary upstairs morphism with right composition in the orbit category.
Precomposing a homogeneous orbit morphism of degree a = -b by the
canonical morphism from the b-shift recovers the inverse source-shift
adjunction in degree zero.
The b-component of the inverse source-shift decomposition is obtained
by precomposing downstairs with the canonical morphism from the b-shift
and then taking degree zero.
A functional on ordinary Hom(Y, X) extends to orbit Hom by reading its
degree-zero component.
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The degree-zero extension is natural for ordinary upstairs morphisms.
The statement is pointwise because its source and target module categories
live in the Hom universes of C and of the orbit category, respectively.
The degree-zero extension is natural in the representing object.
The adjointly assembled comparison from push-down of a dual corepresentable to the downstairs dual corepresentable.
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The assembled comparison is natural for a homogeneous orbit morphism.
The assembled comparison is natural for every orbit morphism.
Orbit push-down of a dual corepresentable maps naturally to the downstairs dual corepresentable.
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The push-down comparison commutes with morphisms of representing objects. This is the square needed to transport a projective-presentation differential through the Nakayama construction.
The objectwise finite-duality comparison underlying push-down of a dual corepresentable.
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Under finite support, the naturally assembled comparison is exactly the basis-free direct-sum duality equivalence.
If the translated dual Hom values have finite support at every object, orbit push-down of a dual corepresentable is naturally isomorphic to the downstairs dual corepresentable.
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The forward map of the finite dual-corepresentable isomorphism is the unrestricted natural comparison constructed above.
The finite push-down isomorphisms commute with morphisms of representing objects.
Changing an upstairs representing object and then moving it to the chosen orbit representative agrees with first moving both objects and then applying the induced skeletal morphism.
Restricting the orbit dual corepresentable at a chosen representative gives the literal dual corepresentable on the induced orbit skeleton.
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Restriction from the shift-orbit category to the chosen orbit skeleton commutes with morphisms of representing objects.
For a finite dual corepresentable, the objectwise comparison is available at every upstairs object from literal finite support and freeness of the deck action.
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Deck freeness and finite support make the objectwise comparison a natural isomorphism on the full shift-orbit category.
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The deck-finite orbit-category comparison commutes with morphisms of representing objects.
Skeletal Gabriel push-down sends a finite dual corepresentable to the dual corepresentable at the strict orbit of its representing object.
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The skeletal dual-corepresentable isomorphisms commute with the morphism between strict deck orbits induced by an upstairs representing morphism.
Bundled linear-module form of skeletal push-down preserving a finite dual corepresentable.
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The skeletal comparison is natural in the representing object inside the literal category of additive linear modules.
The downstairs skeletal dual corepresentable, with finiteness transported from its finite upstairs source through skeletal push-down.
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The finite-dimensional skeletal dual corepresentables inherit the map induced by a morphism of upstairs representing objects.
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Literal finite-dimensional skeletal push-down preserves a finite dual corepresentable.
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The literal finite-dimensional skeletal push-down comparison commutes with maps of representing objects.