Kernel realizations of adjunction units #
Let Φ_P : X ⥤ Y have a kernel-presented right adjoint Φ_Q. With a
convolution kernel and a diagonal unit kernel on X, the adjunction unit has
the form
Φ_{O_Δ} ⟶ Φ_{conv P Q}.
Here conv P Q presents Φ_P ⋙ Φ_Q. The abbreviations
AdjunctionUnitKernelData and AdjunctionUnitKernelConeData specialize the
generic kernel-transformation interfaces to an ordinary kernel morphism
O_Δ ⟶ conv P Q, its enhanced representative, and a chosen dg cone. They
are abbreviations rather than parallel records, since there is no legacy unit
realization API to preserve.
Exactness in the kernel variable gives a source-natural, pointwise distinguished unshifted unit triangle
𝟭 X ⟶ Φ_P ⋙ Φ_Q ⟶ cotwistCone ⟶ (𝟭 X)⟦1⟧.
The third functor is the unshifted cotwist-cone candidate. Its pointwise
[-1] shift is the conventional, choice-dependent ordinary cotwist, and
inverse rotation gives the source-natural triangle
cotwist ⟶ 𝟭 X ⟶ Φ_P ⋙ Φ_Q ⟶ cotwist⟦1⟧.
The shift and inverse rotation reuse the functor-category HasShift instance
and Mathlib's invRotate; no signed map is reconstructed here. The existing
shift comparison of a shift-coherent kernel family presents the cotwist by the
cone kernel shifted inside the enhancement's homotopy category. No
representative or cone is canonical, and no exactness, autoequivalence, or
sphericality claim is made.
AdjunctionUnitKernelData.ofFull is only a sufficient abstract constructor.
Without the explicit fullness hypothesis, producing the kernel arrow remains
the geometric unit-realization obligation.
An ordinary kernel arrow realizing the unit of a kernel-presented right adjunction. This is a definitional specialization of the generic kernel-transformation interface.
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An enhanced representative and chosen dg cone of a kernel arrow realizing the unit of a kernel-presented right adjunction.
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The specialized kernel-transform equation for the adjunction unit.
Fullness of the kernel transform is sufficient to realize the adjunction unit by an ordinary kernel morphism.
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Choose a closed representative and dg cone for the unit-kernel arrow. Both choices are noncanonical.
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The transform of the selected cone: the unshifted cotwist-cone candidate.
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The conventional, choice-dependent ordinary cotwist: the pointwise
[-1] shift of the selected cotwist-cone transform. This is a functor only;
no exactness or equivalence is asserted.
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- S.cotwist = (CategoryTheory.shiftFunctor (CategoryTheory.Functor X X) (-1)).obj S.cotwistCone
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The selected kernel presenting the conventional cotwist: shift the
enhanced unit cone by -1 before transporting it to the ordinary kernel
category.
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The kernel family's shift comparison identifies the transform of
cotwistKernel with the pointwise shifted cotwist-cone functor.
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The conventional cotwist is a Fourier--Mukai kernel functor, presented by the explicitly named shifted cone kernel. This asserts neither exactness nor invertibility of the cotwist.
The transformed enhanced arrow forms the literal adjunction-unit square.
The transported second map from Φ_P ⋙ Φ_Q to the unshifted cotwist-cone
candidate.
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The transported connecting map from the unshifted cotwist-cone candidate to the shifted identity functor.
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The source-natural unshifted unit triangle
𝟙 X ⟶ Φ_P ⋙ Φ_Q ⟶ cotwistCone ⟶ (𝟙 X)⟦1⟧.
Its first map and all three vertices are literal. It is only an objectwise distinguished family, not a distinguished triangle in a functor category.
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Every value of the literal unshifted unit triangle is distinguished.
The literal unshifted unit-triangle family with codomain restricted to distinguished triangles.
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The conventional cotwist triangle #
The inverse rotation of the unshifted unit triangle:
cotwist ⟶ 𝟭 X ⟶ Φ_P ⋙ Φ_Q ⟶ cotwist⟦1⟧.
Mathlib's inverse rotation owns the minus sign in the first map.
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The first projection of the rotated triangle is strictly the pointwise shifted cotwist-cone functor.
The second projection of the rotated triangle is strictly the identity.
The third projection of the rotated triangle is strictly the composite adjoint transform.
At the natural-transformation level, the second map is strictly the adjunction unit.
Inverse rotation moves the adjunction unit into the second map.
Every value of the conventional cotwist triangle is distinguished.
The conventional cotwist-triangle family with codomain restricted to distinguished triangles.
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