ScalingStacks

9.2 Sectors in the space of Dolbeault forms [03T9]

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9.2 Sectors in the space of Dolbeault forms

Let Ei=F(Li,ρi),i=1,2E_{i}=F(L_{i},\rho_{i}),i=1,2 be holomorphic vector bundles as above. There is an analog of the dg-category π’žβ‘(Y){\cal C}(Y) in the case of complex numbers. We will denote it by π’œβ‘(Y){\cal A}(Y). Objects of π’œβ‘(Y){\cal A}(Y) are holomorphic vector bundles on XX of the type E=F⁑(L,ρ)E=F(L,\rho). Morphisms are sections of soft sheaves on YY. Namely, we define the sheaf H​o​mΒ―π’œβ‘(Y)​(E1,E2)\underline{Hom}_{{\cal A}(Y)}(E_{1},E_{2}) on YY as the direct image pβˆ—β€‹(H​o​mΒ―D​o​l​b​(E1,E2))p_{\ast}(\underline{Hom}_{Dolb}(E_{1},E_{2})) (in the self-explained notation). Then H​o​mπ’œβ‘(Y)​(E1,E2)Hom_{{\cal A}(Y)}(E_{1},E_{2}) are global sections of this sheaf. This sheaf corresponds to the sheaf
H​o​mΒ―π’žβ‘(Y)​(E1,E2)\underline{Hom}_{{\cal C}(Y)}(E_{1},E_{2}) in the non-archimedean geometry. Let us choose an open affine chart UβŠ‚Y,U≃𝐑nU\subset Y,U\simeq{{\bf R}}^{n}. Then Γ⁑(U,H​o​mΒ―π’œβ‘(Y)​(E1,E2))\Gamma(U,\underline{Hom}_{{\cal A}(Y)}(E_{1},E_{2})) contains a subsheaf of finite Fourier sums with respect to the natural action of the torus
TnT^{n} on Γ⁑(UΓ—Tn,H​o​mΒ―D​o​l​b​(E1,E2))\Gamma(U\times T^{n},\underline{Hom}_{Dolb}(E_{1},E_{2})). Thus we have the sheaf H​o​mΒ―π’œβ‘(Y)a​l​g​(E1,E2)\underline{Hom}_{{\cal A}(Y)}^{alg}(E_{1},E_{2}) which is an analog of the sheaf H​o​mΒ―π’žβ‘(Y)a​l​g​(E1,E2)\underline{Hom}_{{\cal C}(Y)}^{alg}(E_{1},E_{2}) considered in the non-archimedean case. Notice that there exists a natural homomorphism of sheaves j:pβˆ—β€‹(Ω¯Yβˆ—)→Ω¯X0,βˆ—j:p^{\ast}(\underline{\Omega}^{\ast}_{Y})\to\underline{\Omega}^{0,\ast}_{X}. The image of jj consists of Dolbeault forms on XX which have coefficients locally constant along fibers of pp. In local coordinates jj is given by the formula fi1,…,in​(y)​d​yi1βˆ§β€¦βˆ§d​yin↦fi1,…,in​(y)​d​zΒ―i1βˆ§β€¦βˆ§d​zΒ―inf_{i_{1},...,i_{n}}(y)dy_{i_{1}}\wedge...\wedge dy_{i_{n}}\mapsto f_{i_{1},...,i_{n}}(y)d\overline{z}_{i_{1}}\wedge...\wedge d\overline{z}_{i_{n}}, where zk=ykβˆ’βˆ’1​xk,1≀k≀nz_{k}=y_{k}-\sqrt{-1}x_{k},1\leq k\leq n. It is easy to see that jj is compatible with the structure of dg-algebras on de Rham and Dolbeault forms. Thus for a pair of holomorphic vector bundles E1E_{1} and E2E_{2} on XX we have a canonical structure of dg-module over Ω¯Yβˆ—\underline{\Omega}^{\ast}_{Y} on the sheaf H​o​mΒ―π’œβ‘(Y)​(E1,E2)\underline{Hom}_{{\cal A}(Y)}(E_{1},E_{2}). In the case when Ei=F(Li,ρi),i=1,2E_{i}=F(L_{i},\rho_{i}),i=1,2 the subsheaf H​o​mΒ―π’œβ‘(Y)a​l​g​(E1,E2)\underline{Hom}_{{\cal A}(Y)}^{alg}(E_{1},E_{2}) is also a sheaf of dg-modules over Ω¯Yβˆ—\underline{\Omega}^{\ast}_{Y}.

As in the non-archimedean case there is a canonical decomposition of the stalk H​o​mΒ―π’œβ‘(Y)a​l​g​(E1,E2)y,y∈Y\underline{Hom}_{{\cal A}(Y)}^{alg}(E_{1},E_{2})_{y},y\in Y into the direct sum of dg-modules of finite rank over Ω¯Yβˆ—\underline{\Omega}^{\ast}_{Y}. Summands are labeled by the homotopy classes [Ξ³]∈P⁑(L1,L2,y)[\gamma]\in P(L_{1},L_{2},y) and called sectors. We will denote them by H​o​mΒ―π’œβ‘(Y)a​l​g,[Ξ³]​(E1,E2)y\underline{Hom}_{{\cal A}(Y)}^{alg,[\gamma]}(E_{1},E_{2})_{y}. Informally, sectors correspond to β€œFourier components” of Dolbeault forms in H​o​mΒ―π’œβ‘(Y)a​l​g​(E1,E2)y\underline{Hom}_{{\cal A}(Y)}^{alg}(E_{1},E_{2})_{y} in the direction of torus fibers. Let us describe them more explicitly. For simplicity we will assume that ρi,i=1,2\rho_{i},i=1,2 are rank one trivial local systems, and Li,i=1,2L_{i},i=1,2 intersect with each fiber of pp at exactly one point. Then near pβˆ’1​(y)p^{-1}(y) we can write Li=graph(dfi)(mod(TY𝐙)∨),i=1,2L_{i}=graph(df_{i})\,(mod(T_{Y}^{{\bf Z}})^{\vee}),i=1,2, where fif_{i} are germs at yy of smooth functions on YY. ΒΏFrom the description of the PoincarΓ© bundle PP we deduce that H​o​mΒ―π’œβ‘(Y)a​l​g​(E1,E2)y\underline{Hom}_{{\cal A}(Y)}^{alg}(E_{1},E_{2})_{y} is canonically identified with the space of germs of βˆ‚Β―\overline{\partial}-forms near Tn=pβˆ’1​(y)T^{n}=p^{-1}(y), endowed with the twisted differential βˆ‚Β―β€²β€‹Ξ±=βˆ‚Β―β€‹Ξ±+iΞ΅β€‹βˆ‘βˆ‚f/βˆ‚yi​d​zΒ―i∧α\overline{\partial}^{\prime}\alpha=\overline{\partial}\alpha+{i\over{\varepsilon}}\sum\partial f/\partial y_{i}d\overline{z}_{i}\wedge\alpha, where f=f1βˆ’f2f=f_{1}-f_{2}. Then the sector corresponding to a path Ξ³\gamma consists of Dolbeault forms Ξ±=βˆ‘i1,…,ine​x​p​(i⁑⟨m,x⟩/Ξ΅)​fi1​…​in​(y)​d​zΒ―1βˆ§β€¦βˆ§d​zΒ―n\alpha=\sum_{i_{1},...,i_{n}}exp(i\langle m,x\rangle/\varepsilon)f_{i_{1}...i_{n}}(y)d\overline{z}_{1}\wedge...\wedge d\overline{z}_{n}. Here vector m=m⁑(Ξ³)m=m(\gamma) is the homotopy class of the loop in Tn=pβˆ’1​(y)T^{n}=p^{-1}(y) which is the composition of three paths:

1) the path [0,1]β†’Tn,t↦t​(d​f1)y​m​o​d​(TY𝐙)∨[0,1]\to T^{n},t\mapsto t(df_{1})_{y}\,mod\,(T_{Y}^{{\bf Z}})^{\vee};

2) the path Ξ³\gamma;

3) the path [0,1]β†’Tn,t↦(1βˆ’t)​(d​f2)y​m​o​d​(TY𝐙)∨[0,1]\to T^{n},t\mapsto(1-t)(df_{2})_{y}\,mod\,(T_{Y}^{{\bf Z}})^{\vee}.

A choice of sector corresponds to the choice of monomial z1i1​…​zninz_{1}^{i_{1}}...z_{n}^{i_{n}} in the non-archimedean case. Homotopy classes of paths in non-archimedean approach correspond the summands of Fourier series. Locally each sector can be identified with the de Rham complex on YY. Namely, to a form Ξ±=βˆ‘i1,…,infi1​…​in​(y)​e​x​p​(⟨m,x⟩)​d​zΒ―i1βˆ§β€¦βˆ§d​zΒ―in\alpha=\sum_{i_{1},...,i_{n}}f_{i_{1}...i_{n}}(y)exp(\langle m,x\rangle)d\overline{z}_{i_{1}}\wedge...\wedge d\overline{z}_{i_{n}} we assign the form Ξ±m=βˆ‘i1,…,infi1​…​in​(y)​e​x​p​(1Ξ΅β€‹βŸ¨m,x⟩)​d​yi1βˆ§β€¦β€‹d​yin\alpha_{m}=\sum_{i_{1},...,i_{n}}f_{i_{1}...i_{n}}(y)exp({1\over{\varepsilon}}\langle m,x\rangle)dy_{i_{1}}\wedge...dy_{i_{n}}, where m=m⁑(Ξ³)m=m(\gamma) defines the sector. It is easy to see that the differential βˆ‚Β―β€²\overline{\partial}^{\prime} on Dolbeault forms on XX corresponds to the de Rham differential dd on Ξ©βˆ—β€‹(Y)\Omega^{\ast}(Y). In this way we obtain an isomorphism of complexes H​o​mΒ―π’œβ‘(Y)a​l​g,[Ξ³]​(E1,E2)y≃Ω¯Y,yβˆ—βŠ—π‚\underline{Hom}_{{\cal A}(Y)}^{alg,[\gamma]}(E_{1},E_{2})_{y}\simeq\underline{\Omega}^{\ast}_{Y,y}\otimes{{\bf C}}.

Remark 21

When Ξ΅\varepsilon is not a fixed number, but a parameter Ξ΅β†’0\varepsilon\to 0, the coefficients fi1​…​in​(y)f_{i_{1}...i_{n}}(y) are asymptotic series in Ξ΅\varepsilon of the type fi1​…​in(y,Ξ΅)=βˆ‘jβ‰₯1exp(βˆ’Ξ»j/Ξ΅)fj,i1​…​inf_{i_{1}...i_{n}}(y,\varepsilon)=\sum_{j\geq 1}exp(-\lambda_{j}/\varepsilon)f_{j,i_{1}...i_{n}} where Ξ»jβˆˆπ‘\lambda_{j}\in{{\bf R}} , Ξ»1<…<Ξ»j<…\lambda_{1}<...<\lambda_{j}<..., and Ξ»jβ†’+∞\lambda_{j}\to+\infty.

The set of exponents appearing in the expansion of Ξ±m\alpha_{m} at yy corresponds to the spectrum S​py​(Ξ±)Sp_{y}(\alpha) considered in the non-archimedean case.

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