Curved duplexes and their homotopy category #
Let C be an R-linear category and w : R. A curved duplex of curvature w in C is a
pair of objects with maps in both directions
X₀ --d₀--> X₁ --d₁--> X₀
whose two composites are both w • 𝟙. Since w acts through the linear structure, it commutes
with every morphism of C; this is the central, parity-graded case of a curved module, in which
the square of the differential is multiplication by the curvature. For C = ModuleCat S over a
commutative ring S, a curved duplex whose two components are finitely generated projective is a
matrix factorization of w. For w = 0 the two equations say that both composites vanish: a
curved duplex of curvature zero is a genuine two-periodic complex. For w ≠ 0 a curved duplex
has no homology in general, so homotopy is its primitive notion of equivalence.
This file sets up
- the category
CurvedDuplex C wof curved duplexes and even closed morphisms (pairs of maps commuting with both differentials), which is preadditive andR-linear, with the two evaluation functors toC; - the parity shift
CurvedDuplex.parityShift, which swaps the two components and negates both differentials, and the natural isomorphism exhibiting it as an involution; - the null-homotopic morphisms
d h + h dfor odd mapsh = (h₀, h₁), which form a two-sided idealCurvedDuplex.nullHomotopicbecause source and target have the same curvature; - the homotopy category
CurvedDuplex.HomotopyCategory C w, the quotient by that ideal, which is preadditive andR-linear, together with the parity shift it inherits; - the elementary disk
A --𝟙--> A --w•𝟙--> Aon an objectA, whose morphisms to a duplex are the morphisms fromAto its even component, and which is contractible and therefore zero in the homotopy category.
Main definitions #
TauCeti.CurvedDuplex C w: curved duplexes of curvaturewin the linear categoryC.TauCeti.CurvedDuplex.parityShift: the parity shift, andTauCeti.CurvedDuplex.parityShiftEquivalencethe resulting self-equivalence.TauCeti.CurvedDuplex.nullHomotopicMap: the morphismd h + h dof an odd map(h₀, h₁).TauCeti.CurvedDuplex.nullHomotopic: the ideal of null-homotopic morphisms.TauCeti.CurvedDuplex.HomotopyCategory: the homotopy category of curved duplexes.TauCeti.CurvedDuplex.disk: the elementary disk on an object.
Main results #
TauCeti.CurvedDuplex.quotientFunctor_map_eq_quotientFunctor_map_iff: two morphisms become equal in the homotopy category exactly when their difference isd h + h dfor an odd maph.TauCeti.CurvedDuplex.diskHomEquiv: morphisms out of the disk onAcorrespond to morphisms fromAto the even component.TauCeti.CurvedDuplex.isZero_quotientFunctor_obj_disk: the disk is zero in the homotopy category.
References #
- D. Eisenbud, Homological algebra on a complete intersection, with an application to group representations, Trans. Amer. Math. Soc. 260 (1980), 35–64, Section 5.
- I. Frenkel, M. Khovanov, O. Schiffmann, Homological realization of Nakajima varieties and Weyl group actions, Compos. Math. 141 (2005), 1479–1503, Sections 2–3 (curved complexes and duplexes and their homotopy categories).
- The category structure follows Joël Riou's
CategoryTheory.ShortComplexinMathlib.Algebra.Homology.ShortComplex.Basic.
A curved duplex of curvature w in an R-linear category C: objects X₀ and X₁
with maps d₀ : X₀ ⟶ X₁ and d₁ : X₁ ⟶ X₀ whose composites d₀ ≫ d₁ and d₁ ≫ d₀ are both
w • 𝟙. At w = 0 this is a two-periodic complex.
- X₀ : C
The even component.
- X₁ : C
The odd component.
The differential from the even to the odd component.
The differential from the odd to the even component.
- d₀_comp_d₁ : CategoryTheory.CategoryStruct.comp self.d₀ self.d₁ = w • CategoryTheory.CategoryStruct.id self.X₀
The square of the differential on the even component is the curvature.
- d₁_comp_d₀ : CategoryTheory.CategoryStruct.comp self.d₁ self.d₀ = w • CategoryTheory.CategoryStruct.id self.X₁
The square of the differential on the odd component is the curvature.
Instances For
The square of the differential on the odd component is the curvature.
The square of the differential on the even component is the curvature.
A morphism of curved duplexes: an even closed map, that is a pair of maps between the components commuting with both differentials.
The component on the even objects.
The component on the odd objects.
- comm₀ : CategoryTheory.CategoryStruct.comp self.f₀ Y.d₀ = CategoryTheory.CategoryStruct.comp X.d₀ self.f₁
The morphism commutes with the even differentials.
- comm₁ : CategoryTheory.CategoryStruct.comp self.f₁ Y.d₁ = CategoryTheory.CategoryStruct.comp X.d₁ self.f₀
The morphism commutes with the odd differentials.
Instances For
The morphism commutes with the odd differentials.
The morphism commutes with the even differentials.
The identity morphism of a curved duplex.
Equations
- TauCeti.CurvedDuplex.Hom.id X = { f₀ := CategoryTheory.CategoryStruct.id X.X₀, f₁ := CategoryTheory.CategoryStruct.id X.X₁, comm₀ := ⋯, comm₁ := ⋯ }
Instances For
The composition of morphisms of curved duplexes.
Equations
- f.comp g = { f₀ := CategoryTheory.CategoryStruct.comp f.f₀ g.f₀, f₁ := CategoryTheory.CategoryStruct.comp f.f₁ g.f₁, comm₀ := ⋯, comm₁ := ⋯ }
Instances For
Equations
- One or more equations did not get rendered due to their size.
A constructor for morphisms of curved duplexes when the commutativity conditions are not obvious.
Equations
- TauCeti.CurvedDuplex.homMk f₀ f₁ comm₀ comm₁ = { f₀ := f₀, f₁ := f₁, comm₀ := comm₀, comm₁ := comm₁ }
Instances For
Equations
- One or more equations did not get rendered due to their size.
Equations
- TauCeti.CurvedDuplex.instPreadditive = { homGroup := inferInstance, add_comp := ⋯, comp_add := ⋯ }
Equations
- TauCeti.CurvedDuplex.instModuleHom = { toSMul := TauCeti.CurvedDuplex.instSMulHom, mul_smul := ⋯, one_smul := ⋯, smul_zero := ⋯, smul_add := ⋯, add_smul := ⋯, zero_smul := ⋯ }
Equations
- TauCeti.CurvedDuplex.instLinear = { homModule := inferInstance, smul_comp := ⋯, comp_smul := ⋯ }
Evaluation functors #
The functor sending a curved duplex to its even component.
Equations
- TauCeti.CurvedDuplex.eval₀ C w = { obj := fun (X : TauCeti.CurvedDuplex C w) => X.X₀, map := fun {X Y : TauCeti.CurvedDuplex C w} (f : X ⟶ Y) => f.f₀, map_id := ⋯, map_comp := ⋯ }
Instances For
The functor sending a curved duplex to its odd component.
Equations
- TauCeti.CurvedDuplex.eval₁ C w = { obj := fun (X : TauCeti.CurvedDuplex C w) => X.X₁, map := fun {X Y : TauCeti.CurvedDuplex C w} (f : X ⟶ Y) => f.f₁, map_id := ⋯, map_comp := ⋯ }
Instances For
A constructor for isomorphisms of curved duplexes from isomorphisms of their components commuting with the differentials.
Equations
- One or more equations did not get rendered due to their size.
Instances For
A morphism of curved duplexes is an isomorphism exactly when both of its components are.
The parity shift #
The parity shift of curved duplexes: it swaps the even and odd components and negates
both differentials, (X₁ --(-d₁)--> X₀ --(-d₀)--> X₁).
Equations
- One or more equations did not get rendered due to their size.
Instances For
Applying the parity shift twice gives back the original duplex: the components are the same, and the differentials are negated twice.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The parity shift is a self-equivalence of the category of curved duplexes.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Null-homotopic morphisms #
The null-homotopic morphism d h + h d attached to an odd map h = (h₀, h₁) from X to
Y. It commutes with the differentials because X and Y have the same curvature.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The two-sided ideal of null-homotopic morphisms of curved duplexes: those of the form
d h + h d for an odd map h.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The null-homotopic morphisms are exactly those whose parity shift is null-homotopic.
Elementary disks #
The elementary disk A --𝟙--> A --w•𝟙--> A on an object A. It is exposed so that its
components unfold to A.
Equations
- TauCeti.CurvedDuplex.disk w A = { X₀ := A, X₁ := A, d₀ := CategoryTheory.CategoryStruct.id A, d₁ := w • CategoryTheory.CategoryStruct.id A, d₀_comp_d₁ := ⋯, d₁_comp_d₀ := ⋯ }
Instances For
A morphism out of the disk on A is determined by its even component, which is an arbitrary
morphism A ⟶ X₀; this correspondence is R-linear.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The identity of the disk on A is null-homotopic: it is d h + h d for the odd map which is
the identity from the odd to the even component.
The homotopy category #
The homotopy category of curved duplexes of curvature w: the quotient of the category
of curved duplexes by the ideal of null-homotopic morphisms. It is preadditive and R-linear.
Equations
Instances For
Two morphisms of curved duplexes become equal in the homotopy category exactly when they are
homotopic, that is when their difference is d h + h d for an odd map h.
The parity shift of the homotopy category of curved duplexes, a self-equivalence induced by the parity shift of curved duplexes.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The parity shift of the homotopy category is induced by the parity shift of curved duplexes.
On the image of a curved duplex, the parity shift of the homotopy category is the image of its parity shift.
On the image of a morphism of curved duplexes, the parity shift of the homotopy category is
the image of its parity shift, up to the identification of objects
HomotopyCategory.parityShiftEquivalence_functor_obj_quotientFunctor_obj.
The disk on A is contractible, hence a zero object of the homotopy category.