The four Anno--Logvinenko triangles, as functors on H⁰ #
EnhancedAdjunctionCones holds the four dg cone choices attached to a dg
functor with a left and a right dg adjoint. Each one is a CounitConeData or
a UnitConeData, so each gives a functor into triangles of H⁰ by the
generic construction, with every value distinguished:
| triangle | in | shape |
|---|---|---|
| twist | H⁰ B | S R X ⟶ X ⟶ T X ⟶ (S R X)⟦1⟧ |
| dual twist | H⁰ B | C X⟦-1⟧ ⟶ X ⟶ S L X ⟶ C X |
| cotwist | H⁰ A | C' X⟦-1⟧ ⟶ X ⟶ R S X ⟶ C' X |
| dual cotwist | H⁰ A | L S X ⟶ X ⟶ T' X ⟶ (L S X)⟦1⟧ |
These are Anno--Logvinenko's four triangles read as triangles of functors, which is the form their argument uses.
The shift on two of the four is one inverse rotation #
EnhancedAdjunctionCones stores the dual twist and the cotwist as unshifted
cones, because the conventional functors are their ⟦-1⟧ shifts. Rather than
shift the cone functor and then rebuild a triangle, the triangle of the
unshifted cone is inversely rotated: invRotate sends X ⟶ Y ⟶ Z ⟶ X⟦1⟧ to
Z⟦-1⟧ ⟶ X ⟶ Y ⟶ Z, so it applies the shift and reorders in one step, and
Pretriangulated.inv_rot_of_distTriang carries distinguishedness across for
free.
The unshifted triangles are kept under their Cone names, since the
TwistCotwistEquivalenceConditions are stated against the unshifted cone
functors.
The conventional shifted dg functors are named dualTwistFunctor and
cotwistFunctor. Their H⁰ functors are naturally isomorphic to the
corresponding unshifted cone functor followed by the ordinary [-1] shift.
Thus the first vertices below are values of the named dg functors on the nose,
while ordinary categorical properties can be transported through a reusable
comparison.
What is not claimed #
That the functor is spherical, or that any relation among the four triangles
holds. TwistCotwistEquivalenceConditions records the usual equivalence pair
as data
and no theorem upgrades it; that implication is Anno--Logvinenko's, and it runs
through Morita quasi-functors and higher cone coherence that this repository
does not have. Exactness below does include the conventional shifted dual
twist and cotwist: it is transported from their unshifted cones through the
sign-correct shifted-functor interface.
Nothing here is new mathematics. Every declaration is a thin wrapper over the generic cone-triangle, cone-exactness/3-by-3, or shifted-functor exactness layer, which is the point of having built those layers first.
The two triangles in H⁰ B #
The twist triangle. S R X ⟶ X ⟶ T X ⟶ (S R X)⟦1⟧, from the counit
of the right adjunction.
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The first map of the twist triangle is the right adjunction's counit.
The unshifted dual-twist triangle. X ⟶ S L X ⟶ C X ⟶ X⟦1⟧, from the
unit of the left adjunction. The conventional dual twist is C⟦-1⟧.
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The first map of the unshifted dual-twist triangle is the left adjunction's unit.
The dual-twist triangle. The inverse rotation of the unshifted one,
so its first vertex is C X⟦-1⟧, the dual twist:
C X⟦-1⟧ ⟶ X ⟶ S L X ⟶ C X.
Inverse rotation applies the shift and reorders in one step, so the triangle construction itself need not be rebuilt from the separately named shifted cone functor.
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The first vertex of the dual-twist triangle is the dual twist: the value of
the dg cone functor, shifted by -1.
The H⁰ dual twist is naturally the unshifted cone followed by [-1].
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The second map of the dual-twist triangle is the left adjunction's unit: inverse rotation moves the old first map into second place.
The two triangles in H⁰ A #
The unshifted cotwist triangle. X ⟶ R S X ⟶ C' X ⟶ X⟦1⟧, from the
unit of the right adjunction. The conventional cotwist is C'⟦-1⟧.
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The first map of the unshifted cotwist triangle is the right adjunction's unit.
The cotwist triangle. The inverse rotation of the unshifted one, so
its first vertex is C' X⟦-1⟧, the cotwist:
C' X⟦-1⟧ ⟶ X ⟶ R S X ⟶ C' X.
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The first vertex of the cotwist triangle is the cotwist: the value of the
dg cone functor, shifted by -1.
The H⁰ cotwist is naturally the unshifted cone followed by [-1].
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The second map of the cotwist triangle is the right adjunction's unit.
The dual-cotwist triangle. L S X ⟶ X ⟶ T' X ⟶ (L S X)⟦1⟧, from the
counit of the left adjunction.
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The first map of the dual-cotwist triangle is the left adjunction's counit.
The equivalence conditions, spent #
The twist is an autoequivalence of H⁰ B.
TwistCotwistEquivalenceConditions records that the twist is a
quasi-equivalence, and DGFunctor.h0Equivalence turns a quasi-equivalence into
an equivalence on H⁰. This is the first categorical invertibility statement
about a twist in this repository: everything before it was either numerical, on
K₀, or a construction with no invertibility attached.
No pretriangulated hypothesis is needed: this is an equivalence of ordinary
categories, and the cone triangles above play no part in it. This definition
alone says nothing about exactness; twistH0EquivalenceIsTriangulated combines
it with the exactness package below. Nothing here calls the functor spherical:
the Anno--Logvinenko implication from this pair of conditions to all four
spherical conditions is still out of reach.
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- P.twistH0Equivalence h = P.twistFunctor.h0Equivalence ⋯
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The unshifted cotwist cone is an autoequivalence of H⁰ A.
The conventional cotwist is its ⟦-1⟧ shift; requiring the unshifted cone
functor to be a quasi-equivalence is the shift-free form of the same
condition.
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The conventional cotwist is an autoequivalence of H⁰ A.
The recorded condition is stated for the unshifted cone. The reusable
shifted-functor comparison transports its induced H⁰ equivalence across the
conventional [-1] shift. This remains an equivalence of ordinary categories;
cotwistH0EquivalenceIsTriangulated separately packages its exactness, and no
sphericality is inferred here.
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Exactness #
The twist preserves shifts.
Unconditional: every dg functor preserves shifts
(DGFunctor.preservesShifts), because a shift element is a closed two-sided
invertible element. Cone preservation is unconditional as well; the
structured accessor below retains endpoint arguments only to expose the
3-by-3 construction.
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The unshifted cotwist cone preserves shifts too.
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The unshifted dual-twist cone preserves shifts.
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The dual cotwist preserves shifts.
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The structured 3-by-3 cone witness for the twist. The endpoint arguments are redundant for existence but expose the counit-cone proof.
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- P.twistPreservesChosenCones hSc hRc = CategoryTheory.DGAdjunction.CounitConeData.preservesChosenCones P.rightAdj P.twist hSc hRc
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Structured cone preservation for the unshifted cotwist cone; the endpoint arguments are redundant.
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Structured cone preservation for the unshifted dual-twist cone; the endpoint arguments are redundant.
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Structured cone preservation for the dual cotwist; the endpoint arguments are redundant.
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The shift comparison on H⁰ of the unshifted cotwist cone.
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The unshifted cotwist cone is exact on H⁰.
The sign-correct shift comparison on the conventional cotwist.
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The conventional cotwist is exact on H⁰.
The shift comparison on H⁰ of the unshifted dual-twist cone.
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The unshifted dual-twist cone is exact on H⁰.
The sign-correct shift comparison on the conventional dual twist.
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The conventional dual twist is exact on H⁰.
The shift comparison on H⁰ of the dual cotwist.
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The dual cotwist is exact on H⁰.
The twist is exact on H⁰.
Both dg-level capabilities are available for the twist, so H⁰ of it is a
triangulated functor: it commutes with the shift and carries distinguished
triangles to distinguished triangles. With the equivalence supplied by the
twist/cotwist conditions this is an exact autoequivalence, which is what the
word "twist" is supposed to mean.
No endpoint hypotheses are required.
The twist commutes with the shift on H⁰.
This is the twist-specific name for the canonical dg-functor package.
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Exact equivalences #
Under the recorded equivalence condition, the twist is a triangulated
equivalence of H⁰ B.
The inverse shift comparison and its triangulatedness are the canonical ones transported by Mathlib from the exact forward functor.
Under the recorded equivalence condition, the conventional cotwist is a
triangulated equivalence of H⁰ A.
Its forward exactness uses the sign-correct [-1] package; Mathlib then
transports the compatible shift structure and exactness to the inverse.