Kernel realizations of left-adjunction dual twists #
Let Φ_Q ⊣ Φ_P, where Φ_P : X ⥤ Y is presented by a kernel for a
correspondence C and Φ_Q : Y ⥤ X is its supplied kernel-presented left
adjoint. The adjunction unit has the form
id_Y ⟶ Φ_Q ⋙ Φ_P.
For the original functor Φ_P, the inverse rotation of the unit-cone
triangle is conventionally called the dual twist triangle
dualTwist ⟶ id_Y ⟶ Φ_Q ⋙ Φ_P ⟶ dualTwist⟦1⟧.
The implementation introduces no second cone or normalization mechanism.
LeftAdjointKernelData.toRightAdjointKernelData reads the same adjunction as a
right adjunction for Φ_Q, and all constructions below delegate to the
right-adjunction unit/cotwist interface with the two correspondences swapped.
The public names here retain the dual-twist convention of the original
functor.
The ordinary unit-kernel arrow, its enhanced representative, and its cone are still supplied or selected noncanonically. Nothing here makes the dual twist exact, invertible, canonical, or spherical, and no dual-cotwist construction is bundled into this lane.
An ordinary kernel arrow realizing the unit of a kernel-presented left adjunction, named as the dual-twist input for the original functor.
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An enhanced representative and chosen cone for a left-adjunction unit kernel arrow, named as the dual-twist input for the original functor.
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The specialized transform equation for the left-adjunction unit.
Fullness of the kernel transform is sufficient to realize the left-adjunction unit by an ordinary kernel morphism.
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Choose a closed representative and dg cone for the dual-twist unit-kernel arrow. Both choices are noncanonical.
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The unshifted cone of the left-adjunction unit.
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The choice-dependent ordinary dual twist: the pointwise [-1] shift of
the selected left-unit cone transform.
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The shifted enhanced cone kernel presenting the dual twist.
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The transform of dualTwistKernel is the dual twist.
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The dual twist is a kernel functor for the endocorrespondence on Y.
This does not assert exactness or invertibility.
The transformed enhanced arrow forms the literal left-adjunction-unit square.
The transported second map from Φ_Q ⋙ Φ_P to the unshifted
dual-twist cone.
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The transported connecting map from the unshifted dual-twist cone to the shifted identity.
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The source-natural unshifted left-unit triangle on the target Y of the
original functor Φ_P.
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Every value of the left-adjunction unit triangle is distinguished.
The left-unit triangle family valued in distinguished triangles.
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The conventional dual-twist triangle, obtained by inverse rotation of the left-adjunction unit triangle.
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The first projection of the dual-twist triangle is strictly the dual twist.
The second projection is strictly the identity.
The third projection is strictly the left-adjoint composite.
At the natural-transformation level, the second map is strictly the left-adjunction unit.
Every value of the dual-twist triangle is distinguished.
The dual-twist triangle family valued in distinguished triangles.