Kernel realizations of adjunction counits #
Let Φ_P : X ⥤ Y have a kernel-presented right adjoint Φ_Q. With a
convolution kernel and a unit kernel on Y, the counit has the form
Φ_{conv Q P} ⟶ Φ_{O_Δ}.
Here conv Q P is the kernel of Φ_Q ⋙ Φ_P (ConvolutionData.compIso
reads composition diagrammatically, so this is Huybrechts' P ∘ Q).
The missing geometric statement is that this natural transformation is
induced by an actual kernel morphism conv Q P ⟶ O_Δ. CounitKernelData
records that ordinary kernel arrow and the equality identifying its transform
with the counit. Any enhancement of the kernel category then supplies a
noncanonical closed representative and a noncanonical dg cone, packaged by
CounitKernelData.toConeData. Exactness in the kernel variable sends that dg
cone to a source-natural family of pointwise distinguished triangles.
CounitKernelConeData.counitTriangleInSource transports their first two
vertices to the composite adjoint transform and the identity functor, with
first map literally the adjunction counit and third vertex the Fourier--Mukai
twist candidate. The selected cone kernel is named twistKernel and
definitionally presents that twist, so the candidate is explicitly a kernel
functor without any exactness or invertibility conclusion.
The constructor CounitKernelData.ofFull produces the ordinary arrow when the
kernel transform is explicitly assumed full. No fullness theorem is asserted
here: without that strong hypothesis, producing the kernel arrow remains the
genuine Fourier--Mukai realization obligation involving convolution, the
diagonal kernel, and the adjunction trace.
An ordinary kernel arrow realizing the counit of a kernel-presented adjunction, together with the exact comparison between its transform and the counit. It deliberately contains no enhancement or cone choice.
The kernel morphism
conv Q P ⟶ O_Δ.- transform_arrow : E.transformMap self.arrow = CategoryStruct.comp (D.compIso R.adjKernel P).inv (CategoryStruct.comp R.adj.counit U.unitIso.hom)
Transforming the kernel morphism gives the adjunction counit after the convolution and unit-kernel identifications.
Instances For
A dg kernel arrow realizing the counit of a kernel-presented adjunction,
together with a chosen cone of that arrow, for an enhancement e of the
kernel category.
The arrow lives between the enhancement's lifts e.equiv.inverse.obj _ of
the convolution kernel and the unit kernel. The equality transform_arrow
includes all the changes of presentation: the comparison equivalence
identifies the transform of the arrow with a map between the two kernels,
convolution identifies the source transform with Φ_Q ⋙ Φ_P, and the
unit-kernel isomorphism identifies the target transform with 𝟭 Y.
- arrow : ↥(cocycles (H0.of e.dgCat (e.equiv.inverse.obj (D.conv R.adjKernel P))) (H0.of e.dgCat (e.equiv.inverse.obj U.unitKernel)))
The closed kernel morphism
conv Q P ⟶ O_Δ, in the enhancement. - cone : e.dgCat
Its chosen dg cone.
The cone representability witness.
- transform_arrow : E.transformMap (CategoryStruct.comp (e.equiv.counitIso.app (D.conv R.adjKernel P)).inv (CategoryStruct.comp (e.equiv.functor.map (H0.homMk self.arrow)) (e.equiv.counitIso.app U.unitKernel).hom)) = CategoryStruct.comp (D.compIso R.adjKernel P).inv (CategoryStruct.comp R.adj.counit U.unitIso.hom)
Transforming the kernel arrow gives the adjunction counit after the enhancement, convolution and unit-kernel identifications.
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Read counit-kernel data through the generic kernel-transformation realization interface.
Equations
- S.toKernelTransformationData = { arrow := S.arrow, transform_arrow := ⋯ }
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Ordinary counit-kernel data are determined by their kernel arrow.
Fullness of the kernel transform is a sufficient abstract hypothesis for lifting the adjunction counit comparison to an ordinary kernel morphism.
Equations
- CategoryTheory.Triangulated.FourierMukai.CounitKernelData.ofFull = { arrow := CategoryTheory.Triangulated.FourierMukai.KernelTransformationData.ofFull.arrow, transform_arrow := ⋯ }
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A noncanonical closed degree-zero representative of the ordinary kernel arrow in an enhancement.
Equations
- S.liftedArrow e = e.liftedCocycle S.arrow
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The selected cocycle represents the inverse image of the ordinary kernel
arrow in H⁰ of the enhancement.
Conjugating the selected representative by the counit of the enhancement equivalence recovers the original kernel arrow.
Choose a closed representative and a dg cone for an ordinary counit kernel
arrow. Both choices are noncanonical; the resulting comparison equation is
the exact equation stored in S.
Equations
- One or more equations did not get rendered due to their size.
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Read enhanced counit-kernel data through the generic kernel-transformation realization interface.
Equations
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Forget the chosen cocycle and cone while retaining the ordinary kernel arrow represented through the enhancement equivalence.
Equations
- S.toCounitKernelData = { arrow := S.toKernelTransformationConeData.toKernelTransformationData.arrow, transform_arrow := ⋯ }
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The kernel arrow and its cone as a reusable dg cone presentation.
Equations
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The ordinary kernel represented by the selected enhanced counit cone.
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The Fourier--Mukai transform of the counit-cone kernel, read in the kernel category through the enhancement. This is the kernel-presented twist candidate.
Equations
- S.twist = E.transform S.twistKernel
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The named twist kernel presents the twist definitionally.
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The twist candidate is a kernel functor, witnessed by its selected cone kernel. This asserts neither exactness nor invertibility.
Identify the enhanced lift of the convolution kernel first with the ordinary convolution kernel and then with the composite transform presenting the source of the adjunction counit.
Equations
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Identify the enhanced lift of the unit kernel with the identity transform, using the enhancement comparison and the supplied unit-kernel presentation.
Equations
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The stored kernel-transform equation, normalized to the naturality square whose bottom edge is literally the adjunction counit.
The generic endpoint-normalization package underlying the literal counit triangle.
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The counit-kernel cone as a source-natural family of transform triangles.
The first two vertices are the transforms of the enhancement's lifts of the
convolution and unit kernels; the third is S.twist.
Equations
- S.triangleInSource hE = S.normalizationData.rawTriangle hE
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Every objectwise triangle obtained from the counit-kernel cone is distinguished.
The first map of the raw transform triangle before endpoint normalization.
The source endpoint comparison, typed against the first projection of the raw triangle functor.
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The target endpoint comparison, typed against the second projection of the raw triangle functor.
Equations
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The first square for the comparison from the raw transform triangle to the literal counit triangle.
The transported second map from the identity functor to the twist candidate. It retains the supplied enhancement and cone choices.
Equations
- S.counitToTwist hE = S.normalizationData.normalizedSecond hE
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The transported connecting map from the twist candidate to the shifted composite of the right adjoint and the original transform.
Equations
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The source-natural counit triangle
R P ⋙ P ⟶ 𝟙 Y ⟶ twist ⟶ (R P ⋙ P)⟦1⟧.
Its first map and all three vertices are literal. Its remaining maps retain
the choices in S; this is only an objectwise distinguished family, not a
distinguished triangle in a functor category.
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The raw exact-family triangle and the literal counit triangle are naturally isomorphic through the two endpoint comparisons and the identity on the twist candidate.
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Every value of the literal counit triangle is distinguished.
The literal counit-triangle family with codomain restricted to distinguished triangles.
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Specialize a generic kernel realization of the adjunction counit to the stable counit-kernel interface.
Equations
- S.toCounitKernelData = { arrow := S.arrow, transform_arrow := ⋯ }
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Specialize a generic enhanced realization of the adjunction counit to the stable counit-kernel cone interface.
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Forgetting the choices made by toConeData recovers the original ordinary
kernel datum. This does not identify the selected cocycle or cone.