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Register a new userVariation of Hodge structure and enumerating tilings of surfaces by triangles and squares
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Let $S$ be a connected closed oriented surface of genus $g$. Given a triangulation (resp. quadrangulation) of $S$, define the index of each of its vertices to be the number of edges originating from this vertex minus $6$ (resp. minus $4$). Call the set of integers recording the non-zero indices the profile of the triangulation (resp. quadrangulation). If $kappa$ is a profile for triangulations (resp. quadrangulations) of $S$, for any $min mathbb{Z}_{>0}$, denote by $mathscr{T}(kappa,m)$ (resp. $mathscr{Q}(kappa,m)$) the set of (equivalence classes of) triangulations (resp. quadrangulations) with profile $kappa$ which contain at most $m$ triangles (resp. squares). In this paper, we will show that if $kappa$ is a profile for triangulations (resp. for quadrangulations) of $S$ such that none of the indices in $kappa$ is divisible by $6$ (resp. by $4$), then $mathscr{T}(kappa,m)sim c_3(kappa)m^{2g+|kappa|-2}$ (resp. $mathscr{Q}(kappa,m) sim c_4(kappa)m^{2g+|kappa|-2}$), where $c_3(kappa) in mathbb{Q}cdot(sqrt{3}pi)^{2g+|kappa|-2}$ and $c_4(kappa)in mathbb{Q}cdotpi^{2g+|kappa|-2}$. The key ingredient of the proof is a result of J. Kollar on the link between the curvature of the Hogde metric on vector subbundles of a variation of Hodge structure over algebraic varieties, and Chern classes of their extensions. By the same method, we also obtain the rationality (up to some power of $pi$) of the Masur-Veech volume of arithmetic affine submanifolds of translation surfaces that are transverse to the kernel foliation.
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The main problem addressed in the paper is the Torelli problem for n-dimensional varieties of general type, more specifically for varieties with ample canonical bundle. It asks under which geometrical condition for a variety the period map for the Hodge structure of weight n is a local embedding. We define a line bundle to be almost very ample iff the associated linear system is base point free and yields an injective morphism. We define instead a line bundle to be quasi very ample if it yields a birational morphism which is a local embedding on the complement of a finite set. Our main result is the existence of infinitely many families of surfaces of general type, with quasi very ample canonical bundle, each yielding an irreducible connected component of the moduli space, such that the period map has everywhere positive dimensional fibres. These surfaces are surfaces isogenous to a product, i.e., quotients of a product of curves by the free action of a finite group G. In the paper we also give some sufficient conditions in order that global double Torelli holds for these surfaces, i.e., the isomorphism type of the surface is reconstructed from the fundamental group plus the Hodge structure on the cohomology algebra. We do this via some useful lemmas on the action of an abelian group on the cohomology of an algebraic curve. We also establish a birational description of the moduli space of curves of genus 3 with a non trivial 3-torsion divisor.
In this paper we describe the space of maximal components of the character variety of surface group representations into PSp(4,R) and Sp(4,R). For every rank 2 real Lie group of Hermitian type, we construct a mapping class group invariant complex structure on the maximal components. For the groups PSp(4,R) and Sp(4,R), we give a mapping class group invariant parameterization of each maximal component as an explicit holomorphic fiber bundle over Teichmuller space. Special attention is put on the connected components which are singular, we give a precise local description of the singularities and their geometric interpretation. We also describe the quotient of the maximal components for PSp(4,R) and Sp(4,R) by the action of the mapping class group as a holomorphic submersion over the moduli space of curves. These results are proven in two steps, first we use Higgs bundles to give a non-mapping class group equivariant parameterization, then we prove an analogue of Labouries conjecture for maximal PSp(4,R) representations.
We analyse the possible ways of gluing twisted products of circles with asymptotically cylindrical Calabi-Yau manifolds to produce manifolds with holonomy G_2, thus generalising the twisted connected sum construction of Kovalev and Corti, Haskins, Nordstrom, Pacini. We then express the extended nu-invariant of Crowley, Goette, and Nordstrom in terms of fixpoint and gluing contributions, which include different types of (generalised) Dedekind sums. Surprisingly, the calculations involve some non-trivial number-theoretical arguments connected with special values of the Dedekind eta-function and the theory of complex multiplication. One consequence of our computations is that there exist compact G_2-manifolds that are not G_2-nullbordant.
Supersymmetric D-brane bound states on a Calabi-Yau threefold $X$ are counted by generalized Donaldsdon-Thomas invariants $Omega_Z(gamma)$, depending on a Chern character (or electromagnetic charge) $gammain H^*(X)$ and a stability condition (or central charge) $Z$. Attractor invariants $Omega_*(gamma)$ are special instances of DT invariants, where $Z$ is the attractor stability condition $Z_gamma$ (a generic perturbation of self-stability), from which DT invariants for any other stability condition can be deduced. While difficult to compute in general, these invariants become tractable when $X$ is a crepant resolution of a singular toric Calabi-Yau threefold associated to a brane tiling, and hence to a quiver with potential. We survey some known results and conjectures about framed and unframed refined DT invariants in this context, and compute attractor invariants explicitly for a variety of toric Calabi-Yau threefolds, in particular when $X$ is the total space of the canonical bundle of a smooth projective surface, or when $X$ is a crepant resolution of $C^3/G$. We check that in all these cases, $Omega_*(gamma)=0$ unless $gamma$ is the dimension vector of a simple representation or belongs to the kernel of the skew-symmetrized Euler form. Based on computations in small dimensions, we predict the values of all attractor invariants, thus potentially solving the problem of counting DT invariants of these threefolds in all stability chambers. We also compute the non-commutative refined DT invariants and verify that they agree with the counting of molten crystals in the unrefined limit.
The Hilberts 17th problem asks that whether every nonnegative polynomial can be a sum of squares of rational functions. It has been answered affirmatively by Artin. However, as to the question whether a given nonnegative polynomial is a sum of squares of polynomials is still a central question in real algebraic geometry. In this paper, we solve this question completely for the nonnegative polynomials associated with isoparametric polynomials (initiated by E. Cartan) which define the focal submanifolds of the corresponding isoparametric hypersurfaces.
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