ScalingStacks

8.1 Mirror symmetry functor on objects [03SS]

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8.1 Mirror symmetry functor on objects

Here we will define dg-category π’žu​n​r​a​m​(Y){\cal C}_{unram}(Y) and the fully faithful embedding FF of this category to π’žβ‘(Y){\cal C}(Y).

In the Appendix we will explain the conventional picture for the mirror symmetry functor in case of complex numbers. There we will use a kind of Fourier-Mukai transform along fibers of the torus fibration. The kernel of this transform is an analog of PoincarΓ© bundle. If one starts with a local system on a Lagrangian section of p∨:Xβˆ¨β†’Yp^{\vee}:X^{\vee}\to Y then the transform makes from it a smooth bundle on XX with the connection which is flat in the anti-holomorphic directions. In other words, one gets a holomorphic bundle on XX.

These considerations cannot be literally repeated in the non-archimedean case, because β€œholomorphic” considerations do not work. One can obtain the same result in the following way. Let (L,ρ)(L,\rho) be an object of the category F​O​(X∨)FO(X^{\vee}) such that r​a​n​k​(ρ)=1rank(\rho)=1 and the projection Lβ†’YL\to Y is one-to-one map. The manifold LL is locally given by the graph of d​f​(m​o​d​(TY𝐙)∨)df\,(mod\,(T_{Y}^{{\bf Z}})^{\vee}), where ff is a smooth function on YY. To such an object we assign a sheaf of rank one π’ͺY{\cal O}_{Y}-modules F⁑(L,ρ)F(L,\rho). For sufficiently small open UβŠ‚YU\subset Y and chosen f∈Cβˆžβ€‹(U)f\in C^{\infty}(U) the sheaf F(L,ρ)|UF(L,\rho)_{|U} is identified with π’ͺY|U{{\cal O}_{Y}}_{|U}. Change f↦f+lf\mapsto f+l, where l∈A​fY​(U)l\in Af_{Y}(U) leads to the change of the trivialization of F(L,ρ)|UF(L,\rho)_{|U} as 1U↦e​x​p​(l)​ 1U1_{U}\mapsto exp(l)\,1_{U} (here 1U∈π’ͺY​(U)1_{U}\in{\cal O}_{Y}(U) is the identity function). If r​a​n​k​(ρ)rank(\rho) is greater than one, we decompose ρ|U\rho_{|U} for small UβŠ‚YU\subset Y into the sum of rank one local systems and then apply the construction. Analogously, if the covering Lβ†’YL\to Y has more than one leaf, we apply the previous construction to each leaf of the covering and then take the direct sum.

We will loosely call FF the mirror symmetry functor on objects. The category π’žu​n​r​a​m​(Y){\cal C}_{unram}(Y) is defined as the dg-category whose class of objects is O​b​(F​O​(X∨))Ob(FO(X^{\vee})), and the spaces of morphisms are

H​o​mπ’žu​n​r​a​m​(Y)​((L1,ρ1),(L2,ρ2)):=H​o​mπ’žβ‘(Y)​(F⁑((L1,ρ1),F⁑(L2,ρ2))CLOSEHom_{{\cal C}_{unram}(Y)}((L_{1},\rho_{1}),(L_{2},\rho_{2})):=Hom_{{\cal C}(Y)}(F((L_{1},\rho_{1}),F(L_{2},\rho_{2}))

The functor FF on morphisms is defined in the obvious way, as the identity map.

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