Comparing presented dg and Fourier--Mukai cotwists #
Suppose a strict dg adjunction is presented on ordinary categories by a pair of Fourier--Mukai transforms. A chosen dg cone on its unit and an independently chosen enhanced Fourier--Mukai cone then give two pointwise distinguished unit triangles with the same first two vertices and the same first map.
Mathlib's isoTriangleOfIso₁₂ therefore supplies a noncanonical
isomorphism between the two triangles at each source object. Applying
invRotate gives the corresponding comparison between the conventional
cotwist triangles, with Mathlib retaining ownership of the sign and shift
cancellation. The first component compares the transported dg cotwist object
with the Fourier--Mukai cotwist object.
The unconditional completion chosen by isoTriangleOfIso₁₂ is made
separately at each object and therefore supplies no naturality. A separate
PresentedUnitComparisonData interface records a genuinely supplied natural
cone isomorphism and its two remaining triangle squares; from that input the
file constructs natural unit- and cotwist-triangle isomorphisms. It does not
construct this input or prove independence of either cone choice. Under an
explicit H⁰ equivalence hypothesis on the unshifted dg unit cone, the
comparison packages the selected shifted kernel as a KernelAutoequivalence;
it does not prove that hypothesis or sphericality. A further ShiftCompatibility
refinement can select a Fourier--Mukai CommShift
structure compatible with the conventional cotwist comparison; exactness
then transfers through Mathlib's existing Functor.isTriangulated_of_iso
theorem. Compatibility of the composite comparison from the actual shifted
dg cone is derived from that refinement rather than stored separately.
At each object, the presented dg unit triangle is noncanonically isomorphic to the independently chosen Fourier--Mukai unit triangle.
The isomorphism is the identity on the identity and adjunction-composite
vertices. Its third component is the choice made by triangulated-category
completion and is not asserted to be natural in Z.
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The first component of the unit-triangle comparison is the identity.
The second component of the unit-triangle comparison is the identity.
The resulting noncanonical objectwise comparison between the transported dg unit cone and the Fourier--Mukai unit cone.
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Inverse rotation of the unit-triangle comparison gives a noncanonical objectwise comparison of the conventional cotwist triangles.
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The second component of the cotwist-triangle comparison is the identity.
The third component of the cotwist-triangle comparison is the identity.
The resulting noncanonical objectwise comparison between the transported dg cotwist and the Fourier--Mukai cotwist.
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The actual shifted dg cone, after H⁰ and ordinary transport, is
noncanonically objectwise isomorphic to the Fourier--Mukai cotwist.
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Supplied natural comparison data #
The genuinely additional data required to supply a separate natural unit-cone comparison of the same triangle families considered objectwise above.
This is a semantic abbreviation of the generic comparison between two first-map normalizations. It stores a natural isomorphism of the unit-cone functors and compatibility with the two remaining triangle maps.
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Supply a natural comparison of the transported dg and Fourier--Mukai unit cones, together with exactly the two triangle-map squares not fixed by the common adjunction unit.
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- CategoryTheory.Triangulated.FourierMukai.AdjunctionUnitKernelConeData.PresentedUnitComparisonData.ofConeIso adj H K S hE iso second third = { thirdIso := iso, second := second, third := third }
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The supplied comparison data determine a natural isomorphism between the presented dg and Fourier--Mukai unit-triangle families.
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The natural unit-triangle comparison is the identity on the identity vertex.
The natural unit-triangle comparison is the identity on the adjunction composite vertex.
The third component of the natural unit-triangle comparison is the supplied unit-cone isomorphism.
The supplied natural isomorphism between the two unshifted unit-cone functors.
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Inverse rotation upgrades the natural unit-triangle comparison to a natural comparison of the conventional cotwist triangles.
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The natural cotwist-triangle comparison is the identity on its identity vertex.
The natural cotwist-triangle comparison is the identity on its adjunction composite vertex.
The induced natural isomorphism from the conventional transported dg cotwist to the Fourier--Mukai cotwist.
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The transport of the actual shifted dg unit cone is naturally isomorphic to the Fourier--Mukai cotwist.
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The conventional transported dg cotwist is a kernel functor once a natural comparison with the selected Fourier--Mukai cotwist has been supplied.
The transport of the actual shifted dg unit cone is a kernel functor through the composite natural comparison.
An explicit equivalence hypothesis on H⁰ of the unshifted dg unit cone
transfers to the selected conventional Fourier--Mukai cotwist.
The selected shifted cotwist kernel, packaged as a kernel autoequivalence
under the explicit equivalence hypothesis on H⁰ of the unshifted dg unit
cone.
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Compatibility with selected shift structures #
The cotwist-specific specialization of the shared comparison-shift
package. The source is fixed to the canonical sign-correct shift structure on
the conventional pointwise [-1] cotwist. Compatibility of the intermediate
comparison from the actual shifted dg cone is derived canonically and is not
stored here.
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The sign-correct shift structure on the conventional pointwise [-1] cotwist.
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The selected shift structure on the Fourier--Mukai cotwist.
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The conventional cotwist comparison respects its selected source and target shift structures.
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- ⋯ = ⋯
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The composite comparison from the actual shifted dg cotwist to the Fourier--Mukai cotwist respects the canonical source shift package and the selected Fourier--Mukai shift package.
The Fourier--Mukai cotwist is triangulated relative to the selected target shift structure when the conventional cotwist comparison respects shifts.
The kernel autoequivalence of the selected Fourier--Mukai cotwist is an exact equivalence relative to the selected target shift package.