The chain complex of a finite resolution #
A finite P-resolution of X in an exact category is a chain of conflations
K₁ ↪ Q₀ ↠ X, K₂ ↪ Q₁ ↠ K₁, …, Kₙ ↪ Qₙ₋₁ ↠ Kₙ₋₁
ending at a syzygy Kₙ satisfying P. Splicing the conflations together gives the bounded
chain complex
0 ⟶ Kₙ ⟶ Qₙ₋₁ ⟶ ⋯ ⟶ Q₁ ⟶ Q₀
augmented by the deflation Q₀ ↠ X: the differential Qₖ₊₁ ⟶ Qₖ is the composite
Qₖ₊₁ ↠ Kₖ₊₁ ↪ Qₖ, and the terms beyond Kₙ are zero. This file constructs that complex from the
recursive data, so that the comparison theory of resolutions can be phrased with Mathlib's chain
maps and chain homotopies.
The construction commutes with conflation-exact functors: the complex of the image of a
resolution under such a functor F is the image under F of its complex. This is what makes the
comparison theory functorial, and in particular compatible with the grading shift of a graded
exact category.
Main definitions #
TauCeti.ExactStructure.FiniteResolution.term: then-th termQₙof the complex, withKₙin degreenand zero beyond.TauCeti.ExactStructure.FiniteResolution.aug: the augmentationQ₀ ⟶ X.TauCeti.ExactStructure.FiniteResolution.d: the differentialQₙ₊₁ ⟶ Qₙ.TauCeti.ExactStructure.FiniteResolution.toChainComplex: theℕ-indexed chain complex of the terms and differentials.TauCeti.ExactStructure.FiniteResolution.termMapIsoandTauCeti.ExactStructure.FiniteResolution.toChainComplexMapIso: the terms and the complex of the image of a resolution under a conflation-exact functor are the images of the terms and of the complex.
Main results #
TauCeti.ExactStructure.FiniteResolution.d_comp_dandTauCeti.ExactStructure.FiniteResolution.d_comp_aug: the differentials square to zero and the augmentation kills the first differential.TauCeti.ExactStructure.FiniteResolution.isDeflation_aug: the augmentation is a deflation.TauCeti.ExactStructure.FiniteResolution.prop_term_of_le_lengthandTauCeti.ExactStructure.FiniteResolution.isZero_term_of_length_lt: the terms up to the length satisfyP, and the terms beyond it are zero.TauCeti.ExactStructure.FiniteResolution.aug_mapandTauCeti.ExactStructure.FiniteResolution.d_map: the augmentation and the differentials of an image resolution are the images of the augmentation and the differentials.
Implementation notes #
TauCeti.ExactStructure.FiniteResolution.term is exposed, for the same reason as
TauCeti.ExactStructure.FiniteResolution.syzygy: it is the type index of the augmentation, of the
differential, and of every family of morphisms between the complexes of two resolutions, and the
recursive constructions of those families only typecheck when (step … r).term (n + 1) reduces to
r.term n. It is moreover reducible, so that simp and rw unify a morphism typed with
(step … r).term (n + 1) against one typed with r.term n; without this every lemma mixing the
two would need an explicit rfl step. The price is that the equation lemmas of term are not simp
lemmas: simp indexes left-hand sides at reducible transparency, so (base hX).term 0 is unfolded
before it could be matched and the lemmas would never fire. They are stated for rw and term-mode
use. The augmentation and the differential are sealed behind their equations. toChainComplex is
an abbreviation, exactly as Mathlib's ChainComplex.of is, so that its terms are the terms of the
resolution definitionally; simp therefore computes its differentials through
ChainComplex.of_d, and toChainComplex_d is the rw form of that lemma.
References #
- Theo Bühler, Exact categories, Expositiones Mathematicae 28 (2010), 1--69, Section 12, for resolutions in a Quillen exact category as acyclic complexes.
The n-th term of the chain complex of a finite resolution: the resolving term Qₙ for
n below the length, the last syzygy Kₙ in degree n equal to the length, and a zero object
beyond.
Equations
- (TauCeti.ExactStructure.FiniteResolution.base hX).term 0 = x✝
- (TauCeti.ExactStructure.FiniteResolution.base hX).term n.succ = 0
- (TauCeti.ExactStructure.FiniteResolution.step hQ i p zero hp r).term 0 = Q
- (TauCeti.ExactStructure.FiniteResolution.step hQ i p zero hp r).term n.succ = r.term n
Instances For
Every term of index at most the length satisfies P.
Every term of index beyond the length is a zero object.
The augmentation of the complex of a resolution: the deflation Q₀ ↠ X of its first
conflation, or the identity of X for the empty chain.
Equations
Instances For
The augmentation is a deflation.
The differential Qₙ₊₁ ⟶ Qₙ of the complex of a resolution: the composite of the
deflation Qₙ₊₁ ↠ Kₙ₊₁ with the inflation Kₙ₊₁ ↪ Qₙ.
Equations
- (TauCeti.ExactStructure.FiniteResolution.base hX).d x✝ = 0
- (TauCeti.ExactStructure.FiniteResolution.step hQ i p zero hp r).d 0 = CategoryTheory.CategoryStruct.comp r.aug i
- (TauCeti.ExactStructure.FiniteResolution.step hQ i p zero hp r).d n.succ = r.d n
Instances For
The augmentation kills the first differential.
The augmentation kills the first differential.
The differentials square to zero.
The differentials square to zero.
The chain complex of a finite resolution: the resolving terms with the spliced
differentials, Kₙ in degree n equal to the length, and zero beyond.
Equations
- r.toChainComplex = ChainComplex.of r.term r.d ⋯
Instances For
The differential of the complex in consecutive degrees. Not a simp lemma, since simp proves
it from ChainComplex.of_d through the abbreviation; it is the form rw can use, which does not
see ChainComplex.of.d through the abbreviation, and unifies with offset degrees such as
r.toChainComplex.d (n + 2) (n + 1).
The terms of the image of a resolution under a conflation-exact functor F are the images of
its terms: the identity in degrees up to the length, and the isomorphism 0 ≅ F 0 beyond it.
Equations
- One or more equations did not get rendered due to their size.
- TauCeti.ExactStructure.FiniteResolution.termMapIso hF hPP' (TauCeti.ExactStructure.FiniteResolution.base hX) 0 = CategoryTheory.Iso.refl (F.obj x✝)
- TauCeti.ExactStructure.FiniteResolution.termMapIso hF hPP' (TauCeti.ExactStructure.FiniteResolution.base hX) n.succ = F.mapZeroObject.symm
- TauCeti.ExactStructure.FiniteResolution.termMapIso hF hPP' (TauCeti.ExactStructure.FiniteResolution.step hQ i p zero hp r) 0 = CategoryTheory.Iso.refl (F.obj Q)
Instances For
The augmentation of the image of a resolution is the image of its augmentation.
The differentials of the image of a resolution are the images of its differentials.
The complex of the image of a resolution under a conflation-exact functor F is the image
of its complex under F.