Mirror Symmetry for Honeycombs
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TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 373, Number 1, January 2020, Pages 71–107 https://doi.org/10.1090/tran/7909 Article electronically published on September 10, 2019 MIRROR SYMMETRY FOR HONEYCOMBS BENJAMIN GAMMAGE AND DAVID NADLER Abstract. We prove a homological mirror symmetry equivalence between the A-brane category of the pair of pants, computed as a wrapped microlocal sheaf category, and the B-brane category of its mirror LG model, understood as a category of matrix factorizations. The equivalence improves upon prior results in two ways: it intertwines evident affine Weyl group symmetries on both sides, and it exhibits the relation of wrapped microlocal sheaves along different types of Lagrangian skeleta for the same hypersurface. The equivalence proceeds through the construction of a combinatorial realization of the A-model via arboreal singularities. The constructions here represent the start of a program to generalize to higher dimensions many of the structures which have appeared in topological approaches to Fukaya categories of surfaces. Contents 1. Introduction 71 2. Combinatorial A-model 78 3. Mirror symmetry 95 4. Symplectic geometry 100 Acknowledgments 106 References 106 1. Introduction This paper fits into the framework of homological mirror symmetry, as introduced in [23] and expanded in [19, 20, 22]. The formulation of interest to us relates the A-model of a hypersurface X in a toric variety to the mirror Landau-Ginzburg B- model of a toric variety X∨ equipped with superpotential W ∨ ∈O(X∨). Following Mikhalkin [27], a distinguished “atomic” case is when the hypersurface is the pair of pants ∗ n ∼ ∗ 1 n Pn−1 = {z1 + ···+ zn +1=0}⊂(C ) = T (S ) n+1 with mirror Landau-Ginzburg model (A ,z1 ···zn+1). In this paper, we will also be interested in the universal abelian cover Pn−1 of the pair of pants, which fits in Received by the editors December 7, 2018, and, in revised form, April 12, 2019. 2010 Mathematics Subject Classification. Primary 14J33, 53D37. Key words and phrases. Mirror symmetry, microlocal sheaves. The first author is grateful to the NSF for the support of a Graduate Research Fellowship. The second author is grateful for the support of grant DMS-1502178. c 2019 American Mathematical Society 71 Licensed to Univ of Calif, Berkeley. Prepared on Fri May 1 11:16:12 EDT 2020 for download from IP 128.32.10.230. License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 72 BENJAMIN GAMMAGE AND DAVID NADLER the Cartesian diagram / ∗ n Pn−1 T R / ∗ 1 n Pn−1 T (S ) ∗ n ∗ 1 n as the pullback of Pn−1 along the universal covering map T R → T (S ) ; it has mirror a torus-equivariant version of the Landau-Ginzburg model (An+1, z1 ···zn+1). This paper expands upon prior mirror symmetry equivalences for pairs of pants found in [1, 36, 39]; however, it differs from those in its understanding of the A- model. The traditional mathematical realization of the A-model is the Fukaya category, with objects decorated Lagrangian submanifolds, morphisms their deco- rated intersections, and structure constants defined by integrals over moduli spaces of pseudoholomorphic polygons. There is increasing evidence (for example, [3, 9, 14–16, 24, 30, 33, 38, 41]) that the Fukaya category of a Weinstein manifold is equivalent to microlocal sheaves (as developed by Kashiwara-Schapira [21]) along a Lagrangian skeleton. In this paper, we follow [32] and study the A-model of the pair of pants in its guise as wrapped microlocal sheaves. A calculation of microlocal sheaves on a skeleton for the pair of pants was per- formed already in [32]; our calculation here involves a different skeleton, which is of independent interest. The skeleton we study is more symmetrical, having an action of the symmetric group Σn+1 rather than just Σn, but more importantly, the skeleton here is of a different “flavor” compared to the one constructed there. The calculations from [32] are well-adapted to considerations of mirror symmetry which relate a hypersurface in (C×)n to a toric degeneration and were used in [17] for this purpose. The skeleton we study in this paper is more adapted to mirror symmetry equivalences which relate a hypersurface in (C×)n to a Landau-Ginzburg model. The first sort of skeleton can be considered as a “degeneration” of the sec- ond; indeed, the relation between these two flavors of skeleton is very interesting and will be studied further in future work. The skeleton from this paper is very well-suited to a combinatorial perspective, since singularities are all arboreal in the sense of [28]. The form of our calculations should be understood as a paradigm for extending the substantial literature devoted to understanding Fukaya categories of Riemann surfaces through topological skeleta and ribbon graphs (for example, [2, 6, 18, 35, 40]) to higher-dimensional examples. Moreover, the type of skeleton described here has close relations to the dimer models which have appeared in earlier mirror symmetry contexts (e.g., [10, 11]). In future work, we hope to explore further the relation between skeleta and dimer models, along with generalizations to higher dimensions. This correspondence was noticed (in a slightly different form) in [11], and in Section 4.2 we make use of the Lefschetz fibrations described in that paper. 1.1. Symplectic geometry. 1.1.1. Cotangent bundles. Fix a characteristic zero coefficient field k, and let L ⊂ T ∗X be a closed conic Lagrangian submanifold of a cotangent bundle. There are ♦ ∗ conic sheaves of dg categories μShL and μShL on T X, localized along L, which ⊂ ∗ ♦ to a conic open set Ω T X assign, respectively, the dg category μShL(Ω) of Licensed to Univ of Calif, Berkeley. Prepared on Fri May 1 11:16:12 EDT 2020 for download from IP 128.32.10.230. License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use MIRROR SYMMETRY FOR HONEYCOMBS 73 unbounded-rank microlocal sheaves and the dg category μShL(Ω) of traditional microlocal sheaves along L ∩ Ω. wr Definition 1.1.2. The category μShL (Ω) of wrapped microlocal sheaves along ∩ ♦ c ♦ L Ω is the category μShL(Ω) of compact objects inside μShL(Ω). → wr Proposition 1.1.3 ([32, Proposition 3.16]). The assignment Ω μShL (Ω) forms wr ∗ a cosheaf μShL of dg categories on T X, localized on L. If x is a smooth point of L and Ω is a contractible conic neighborhood of x,then ♦ the stalk of the sheaf μShL is equivalent to the dg category Modk of (unbounded- w rank) k-modules, while the stalk of μShL and the costalk of μShL at x are both equivalent to the dg category Perfk of perfect k-modules. If x is a singular point, the local calculation is more complicated, but this cal- culation has already been performed in [28] for a certain class of Legendrian sin- gularities termed arboreal. In this paper we will only be concerned with the An L arboreal singularity An , a certain singular Legendrian in the projectivized cotan- ∞ n n gent bundle T (R ) which is homeomorphic to Cone(skn−2Δ ), the cone on the (n − 2)-skeleton of an n-simplex. Proposition 1.1.4 ([28]). Let L be a conic Lagrangian in T ∗Rn which is locally ∈ L equivalent, near a point x L, to the cone on An . Then to a neighborhood of x, wr the sheaf μShL and the cosheaf μShL each assign the category An -Perfk of perfect modules over the An quiver. 1.1.5. Weinstein manifolds. Let W be a Weinstein manifold. The Weinstein struc- ture of W endows it with a Lagrangian skeleton Λ, onto which W deformation retracts. Since our definition of wrapped microlocal sheaf categories applies only in the setting of cotangent bundles, in order to apply it here we have to relate the geometry of our Weinstein manifold to the geometry of a cotangent bundle. Let Λ be the skeleton of a Weinstein manifold W and let U be an open neighborhood of Λ which is conic for the flow of the Liouville vector field of W , and suppose that there exist a manifold X and a closed conic Lagrangian L ⊂ T ∗X such that U is exact symplectomorphic to a neighborhood Ω of L by a symplectomorphism taking Λ to L. w Definition 1.1.6. In the situation described above, the category μShL (Ω) is the microlocal A-model category associated to the Weinstein manifold W .(Tomake explicit the dependence on Weinstein structure, we will sometimes call this category the wrapped microlocal A-model category of W associated to Λ.) In practice, the symplectomorphism relating Λ to a conic Lagrangian in a cotan- gent bundle might not exist. However, such symplectomorphisms always exist lo- cally, so that we can obtain a cosheaf of categories on the skeleton by defining these categories locally, gluing them together, and checking that the resulting cosheaf didn’t depend on choices. A more detailed explanation of our expectations can be found in Conjecture 4.1.8. One skeleton for the pair of pants Pn−1 was described in [32], where it was used to prove a mirror symmetry equivalence. In this paper, we study a more symmetric skeleton of the pair of pants, which we can describe using the geometry of the permutohedron. Licensed to Univ of Calif, Berkeley. Prepared on Fri May 1 11:16:12 EDT 2020 for download from IP 128.32.10.230. License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 74 BENJAMIN GAMMAGE AND DAVID NADLER n+1 Let Vn be the quotient of R by the span of the vector λ1 + ···+ λn+1, where n+1 {λi} is the standard coordinate basis of R .