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The complex of an admissible bigraded curve

Definition

Fix the normalized bigradings d~i,b~i of Bigraded string types and their contributions to I^{bigr}. Let c~ be an admissible bigraded curve in normal form with respect to the fixed basic set and vertical curves (Basic arcs, admissible curves and the standard normal form), with crossing set cr⁡(c~) and local index (x1,x2):=μbigr(d~x0,c~;x)∈Z2 at each crossing x; here x0 denotes the index of the vertical curve dx0 containing x, so that x∈dx0, and for a crossing of the underlying curve c the local index of the bigrading is interpreted through the identification of the crossings of c~ with those of c (Local indices and bigraded intersection numbers). Put P(x):=Px0[−x1]{x2}∈Cm,L(c~):=⨁x∈cr⁡(c~)P(x), the direct sum of shifted vertex projectives indexed by the crossings, where Px0=Amex0 is the vertex projective of Finite graded A_m-modules, internal shifts and the vertex projectives and the two shifts are the homological shift [−x1] and the internal shift {x2}.

The differential. For crossings x,y which are the two endpoints of an essential segment of c and satisfy y1=x1+1, define the component ∂yx ⁣:P(x)⟶P(y) by the following rules, right multiplication meaning the left Am-linear map Amei→Amej, u↦ua, for a∈eiAmej:

  1. if x0=y0 (in which case x2=y2+1), then ∂yx is the right multiplication by the return (x0∣x0−1∣x0) when x0>0, and is the zero map when x0=0 (the return (0∣1∣0) vanishes);
  2. if x0=y0±1, then ∂yx is the right multiplication by the arrow (x0∣y0), which is (x0∣x0−1) or (x0∣x0+1);
  3. otherwise ∂yx:=0.

Put ∂:=∑x,y∂yx. For a bigraded k-string g~ of c~ the same formulas applied to the crossings and essential segments of g define an object L(g~) with its differential, and the underlying graded module of L(g~) is an abelian subgroup of L(c~).

Claims. (L(c~),∂) is a bounded complex of finitely generated graded projective left Am-modules with a differential that is degree zero for the internal grading, hence an object of Cm=Kb(proj⁡grAm); the object is bounded because there are finitely many crossings; and the deck action translates into the shift rule L(χ(r1,r2)c~)≅L(c~)[−r1]{r2} by the degreewise sign identification described in the proof. These claims are proved below.

Facts & Assumptions

Given: An admissible bigraded curve c~ in normal form with finitely many crossings x=(x0;x1,x2), its essential segments classified by the six types of Figure 12 and the endpoint types of Figures 13-14, and the vertex projectives Pi=Amei with these typed right multiplication maps.

[L1]

The crossing set is finite, each essential segment has two crossings as endpoints, and for every essential segment with endpoints x,y one has y1=x1±1; the segment types 1,1′,2,2′ are the essential ones and the tables of Figures 12-14, in the normalization of Bigraded string types and their contributions to I^{bigr}, record the internal indices at their endpoints: for a segment whose endpoints satisfy y1=x1+1 one has either x0=y0 and x2=y2+1, or x0=y0±1 (Basic arcs, admissible curves and the standard normal form, Local indices and bigraded intersection numbers).

[L2]

For x0>0, (x0∣x0−1∣x0) is the return at x0, of internal degree 1, and (x0∣y0) is either the ascending arrow (x0∣x0+1) of internal degree 0 or the descending arrow (x0∣x0−1) of internal degree 1; the product of two arrow classes is zero whenever it is defined as a path of length two other than a return, the return at vertex 0 is zero, and every path of length at least three vanishes in Am (The 4m+1 path basis).

[L3]

Pi=Amei is a finitely generated graded projective left Am-module, right multiplication by a homogeneous element a∈Am is a degree-zero map Pi{r}→Pj{r−deg⁡a} when a lies in eiAmej, the homological shift and the internal shift act as displayed, and Cm is the homotopy category of bounded complexes of such modules (Finite graded A_m-modules, internal shifts and the vertex projectives).

[L4]

The deck action adds (r1,r2) to the local indices of all crossings: χ(r1,r2) replaces (x1,x2) by (x1+r1,x2+r2) for every crossing x (Local indices and bigraded intersection numbers, The Z^2 cover of the projectivized tangent bundle and bigraded curves).

Proof

technique · direct
1.1L1L2

The composites of two differential components vanish. Let x,y,z be crossings with ∂yx≠0≠∂zy, so that y1=x1+1, z1=y1+1 and the pairs (x,y), (y,z) are endpoints of essential segments; the composite is right multiplication by the concatenation of the two path labels. If either label is a return, its length is at least three, so it vanishes by [L2]. If both labels are arrows and x0≠z0, they form a monotone length-two path, which also vanishes. The remaining case would have x0=z0=y0±1. Both segments would then lie in the same region between these adjacent dividing curves and approach y from the same side of dy0. This contradicts transversality at the crossing y, where the two branches of the embedded curve lie on opposite sides of dy0. Thus no such consecutive arrow return occurs, and every composite is zero. Summing the components gives ∂2=0.

1.2L1L3L4

The shift rule. Under χ(r1,r2) the local index of each crossing x becomes (x1+r1,x2+r2) by [L4], and the crossing data (which crossings are joined by essential segments, and the segment types) are unchanged because the deck action changes only the bigrading, not the underlying curve or its normal form; the source retains the same path entries, while the homological shift [−r1] multiplies the target differential by (−1)r1. Map the summand indexed by x by (−1)r1x1 times the identity. For a nonzero entry x→y one has y1=x1+1, so the target differential followed by the source sign agrees with the target sign followed by the source differential. These invertible sign maps give the claimed chain isomorphism. An identity on every summand would fail for odd r1.

2.1step 1.1L1L3

The differential is degree zero and the terms are finite graded projective. For a component given by right multiplication by a path a∈ex0Amey0 of internal degree δ, an element of underlying degree d has source shifted degree d+x2 and target shifted degree d+δ+y2. The segment tables in [L1] give δ=x2−y2: a return has δ=1, an ascending arrow has δ=0, and a descending arrow has δ=1. Thus both degrees agree. The map is left Am-linear because (bu)a=b(ua), and its image lies in Amey0 because a=ex0aey0; no right-module structure on Amex0 is assumed. Homological degree rises from x1 to y1=x1+1. Each term is a shifted finite graded vertex projective, and there are finitely many crossings; together with step 1.1 this gives a bounded complex of the required bidegree.

3.1step 1.1step 2.1step 1.2∎

Conclusion. (L(c~),∂) is a bounded complex of finite graded projectives with a degree-zero differential, so it defines an object of Cm, and the deck action acts by the shift [−r1]{r2}. No choice principle is used; the verifications are finite checks over the segment types.

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