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TauCeti.Algebra.Homology.Curved.Duplex

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.

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Main definitions #

Main results #

References #

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.

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    The square of the differential on the odd component is the curvature.

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    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.

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      theorem TauCeti.CurvedDuplex.Hom.ext {C : Type u} {inst✝ : CategoryTheory.Category.{v, u} C} {inst✝¹ : CategoryTheory.Preadditive C} {R : Type w'} {inst✝² : Semiring R} {inst✝³ : CategoryTheory.Linear R C} {w : R} {X Y : CurvedDuplex C w} {x y : X.Hom Y} (f₀ : x.f₀ = y.f₀) (f₁ : x.f₁ = y.f₁) :
      x = y
      theorem TauCeti.CurvedDuplex.Hom.ext_iff {C : Type u} {inst✝ : CategoryTheory.Category.{v, u} C} {inst✝¹ : CategoryTheory.Preadditive C} {R : Type w'} {inst✝² : Semiring R} {inst✝³ : CategoryTheory.Linear R C} {w : R} {X Y : CurvedDuplex C w} {x y : X.Hom Y} :
      x = y ↔ x.f₀ = y.f₀ ∧ x.f₁ = y.f₁

      The identity morphism of a curved duplex.

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        def TauCeti.CurvedDuplex.Hom.comp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] {w : R} {X Y Z : CurvedDuplex C w} (f : X.Hom Y) (g : Y.Hom Z) :
        X.Hom Z

        The composition of morphisms of curved duplexes.

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          theorem TauCeti.CurvedDuplex.hom_ext {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] {w : R} {X Y : CurvedDuplex C w} {f g : X ⟶ Y} (h₀ : f.f₀ = g.f₀) (h₁ : f.f₁ = g.f₁) :
          f = g

          A constructor for morphisms of curved duplexes when the commutativity conditions are not obvious.

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            theorem TauCeti.CurvedDuplex.smul_f₀ {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] {w : R} {X Y : CurvedDuplex C w} (a : R) (f : X ⟶ Y) :
            (a • f).f₀ = a • f.f₀
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            theorem TauCeti.CurvedDuplex.smul_f₁ {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] {w : R} {X Y : CurvedDuplex C w} (a : R) (f : X ⟶ Y) :
            (a • f).f₁ = a • f.f₁
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            Evaluation functors #

            The functor sending a curved duplex to its even component.

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              theorem TauCeti.CurvedDuplex.eval₀_map (C : Type u) [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] (w : R) {X✝ Y✝ : CurvedDuplex C w} (f : X✝ ⟶ Y✝) :
              (eval₀ C w).map f = f.f₀

              The functor sending a curved duplex to its odd component.

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                theorem TauCeti.CurvedDuplex.eval₁_map (C : Type u) [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] (w : R) {X✝ Y✝ : CurvedDuplex C w} (f : X✝ ⟶ Y✝) :
                (eval₁ C w).map f = f.f₁

                A constructor for isomorphisms of curved duplexes from isomorphisms of their components commuting with the differentials.

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                  theorem TauCeti.CurvedDuplex.isoMk_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] {w : R} {X Y : CurvedDuplex C w} (e₀ : X.X₀ ≅ Y.X₀) (e₁ : X.X₁ ≅ Y.X₁) (comm₀ : CategoryTheory.CategoryStruct.comp e₀.hom Y.d₀ = CategoryTheory.CategoryStruct.comp X.d₀ e₁.hom) (comm₁ : CategoryTheory.CategoryStruct.comp e₁.hom Y.d₁ = CategoryTheory.CategoryStruct.comp X.d₁ e₀.hom) :
                  (isoMk e₀ e₁ comm₀ comm₁).hom = homMk e₀.hom e₁.hom comm₀ comm₁

                  A morphism of curved duplexes is an isomorphism exactly when both of its components are.

                  The parity shift #

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                  The parity shift of curved duplexes: it swaps the even and odd components and negates both differentials, (X₁ --(-d₁)--> X₀ --(-d₀)--> X₁).

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                    Applying the parity shift twice gives back the original duplex: the components are the same, and the differentials are negated twice.

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                      The parity shift is a self-equivalence of the category of curved duplexes.

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                        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.

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                          theorem TauCeti.CurvedDuplex.nullHomotopicMap_add {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] {w : R} {X Y : CurvedDuplex C w} (h₀ h₀' : X.X₀ ⟶ Y.X₁) (h₁ h₁' : X.X₁ ⟶ Y.X₀) :
                          nullHomotopicMap (h₀ + h₀') (h₁ + h₁') = nullHomotopicMap h₀ h₁ + nullHomotopicMap h₀' h₁'

                          The two-sided ideal of null-homotopic morphisms of curved duplexes: those of the form d h + h d for an odd map h.

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                            theorem TauCeti.CurvedDuplex.mem_nullHomotopic_iff {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.Preadditive C] {R : Type w'} [Semiring R] [CategoryTheory.Linear R C] {w : R} {X Y : CurvedDuplex C w} {f : X ⟶ Y} :
                            f ∈ (nullHomotopic C w).hom X Y ↔ ∃ (h₀ : X.X₀ ⟶ Y.X₁) (h₁ : X.X₁ ⟶ Y.X₀), nullHomotopicMap h₀ h₁ = f

                            The null-homotopic morphisms are exactly those whose parity shift is null-homotopic.

                            Elementary disks #

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                            The elementary disk A --𝟙--> A --w•𝟙--> A on an object A. It is exposed so that its components unfold to A.

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                              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.

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                                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 #

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                                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.

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                                  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.

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                                    On the image of a curved duplex, the parity shift of the homotopy category is the image of its parity shift.

                                    The disk on A is contractible, hence a zero object of the homotopy category.