Balanced tensor composition of graded bimodules #
For graded algebras A, B, and C, this file constructs the composition
(A,B)-bimodules × (B,C)-bimodules → (A,C)-bimodules,
balanced over B. Each bimodule is an object of GradedModuleCat over its
enveloping algebra, with the tensor-product internal grading. Restricting the
enveloping action supplies the actions used to balance. The quotient has the sum
internal grading and the two outer actions.
The construction uses the existing balanced quotient, grading, map, and lift APIs. Its internal grading introduces no Koszul sign. Cohomological tensor signs come from Mathlib's totalization when this bifunctor is applied to cochain complexes. Taking all three algebras and gradings equal gives tensor composition of endobimodules; heterogeneous composition also supplies the tensors needed for transfer through a supplied Morita equivalence.
bimoduleModuleTensor gives the corresponding action on graded left modules:
an (A,B)-bimodule tensored over B with a left B-module is a left A-module.
Both constructions are linear in each variable.
GradedBimoduleCat is the category of graded bimodules of a graded algebra with an
internal grading, and GradedModuleCat.bimoduleTensor is the balanced bimodule tensor of
that setting.
References #
- B. Keller, Deriving DG categories, Section 6.1.
The left action obtained by restricting the envelope action to its first factor.
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The right action obtained by restricting the envelope action to its second factor.
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The output left action is induced from the bimodule's first factor.
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An internally graded bimodule acts on a graded left module by balanced tensor product.
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The pure tensor of a bimodule element and a left-module element.
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The pure tensor as a bilinear map over the ground ring.
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Forgetting the output grading identifies the left-module tensor carrier with the balanced quotient.
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The carrier identification preserves the output left action.
The carrier identification preserves membership in each internal degree piece.
The left-module tensor carrier identification is homogeneous of degree zero.
Balancing transfers the right action on the bimodule to the left module.
The output left action acts on the first tensor factor.
The internal degrees of the two factors add.
The quotient map from the ground-ring tensor product to the left-module tensor product.
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The internal degree piece is the image of the ground-ring tensor degree piece.
Membership in a degree piece has a homogeneous ground-ring tensor representative.
A linear map shifts internal degrees if it does so on homogeneous pure left-module tensors.
Tensor a graded bimodule map with a graded left-module map.
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Maps from a bimodule-module tensor are determined by their values on pure tensors.
Lift a balanced bilinear map respecting the left action and total internal degree.
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A homogeneous balanced left-module lift is determined by its pure-tensor values.
Balanced tensor action of graded bimodules on graded left modules.
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The left outer action on the balanced tensor quotient.
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The right outer action on the balanced tensor quotient.
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The commuting outer actions combine into an action of the output envelope.
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The graded (A,C)-bimodule obtained by balancing over B.
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The pure tensor in the graded bimodule tensor product.
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Forgetting the output grading identifies the tensor carrier with the balanced quotient. The factor actions in this quotient are the restrictions of the enveloping actions.
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The bimodule carrier identification preserves every internal degree piece.
The bimodule tensor carrier identification is homogeneous of degree zero.
Internal degrees add on pure bimodule tensors.
Every bimodule tensor is a sum of pure tensors.
The quotient map from the ground-ring tensor product.
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The internal degree piece is the image of the ground-ring tensor degree piece.
A homogeneous balanced tensor has a homogeneous ground-ring representative.
A linear map shifts the degree of every tensor if it does so on homogeneous pure tensors.
Balancing moves the right B-action to the left B-action.
A pure output enveloping scalar acts on the two outer factors.
The underlying carrier identification preserves the restricted left outer action.
The underlying carrier identification preserves the restricted right outer action.
Tensor the degree-zero bimodule maps in both factors.
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Morphisms out of a bimodule tensor product agree if they agree on pure tensors.
Lift a balanced bilinear map that respects the two outer actions and internal degrees.
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The homogeneous balanced lift is uniquely determined by its pure-tensor values.
Tensoring bimodule maps respects composition.
Balanced tensor composition of internally graded bimodules.
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The category of graded bimodules over the graded algebra A with internal grading Ξ:
the graded modules over the enveloping grading Ξ.tensorProduct Ξ.opposite.