Real Torus Research Articles

A data aggregation protocol based on TFHE

Torus Fully Homomorphic Encryption (TFHE) is a probabilistic cryptosytem over the real torus which allows one to operate directly on encrypted data without first decrypting them. We present an aggregation protocol based on a variant of TFHE for computing the sum of sensitive data, working only with the corresponding ciphertexts. Our scheme is an ideal choice for a system of smart meters – electronic devices for measuring energy consumption – that demands consumers' privacy. In contrast to some other solutions, our proposal does not require any communication among smart meters and it is quantum-safe.

Harmonic analysis of compact Lie supergroups

We realize the irreducible representations of a compact Lie supergroup G, with a contragredient simple Lie superalgebra, in the space of square integrable (in the sense of Berezin) holomorphic sections on X=GA, A is the real torus in the complexification of G. We give an explicit realization of unitary representations when G=SU(1|1).

Torus fibers and the weight filtration

We show that if ( X , Y ) (X,Y) is a simple normal crossings log Calabi–Yau pair, then there is a real torus of dimension equal to the codimension of the smallest stratum of Y Y which can be used to construct W 2 k − 1 H k ( X ∖ Y ; Q ) W_{2k-1}\mathrm {H}^k(X \setminus Y;\mathbb {Q}) for all k k . We use this to show that a P=W type result holds for pairs ( X , Y ) (X,Y) consisting of a rational surface X X and a nodal anticanonical divisor Y Y .

Invariant Measures for Horospherical Actions and Anosov Groups

AbstractLet $\Gamma $ be a Zariski dense Anosov subgroup of a connected semisimple real algebraic group $G$. For a maximal horospherical subgroup $N$ of $G$, we show that the space of all non-trivial $NM$-invariant ergodic and $A$-quasi-invariant Radon measures on $\Gamma \backslash G$, up to proportionality, is homeomorphic to ${\mathbb {R}}^{\text {rank}\,G-1}$, where $A$ is a maximal real split torus and $M$ is a maximal compact subgroup that normalizes $N$. One of the main ingredients is to establish the $NM$-ergodicity of all Burger–Roblin measures.

Tropical moments of tropical Jacobians

AbstractEach metric graph has canonically associated to it a polarized real torus called its tropical Jacobian. A fundamental real-valued invariant associated to each polarized real torus is its tropical moment. We give an explicit and efficiently computable formula for the tropical moment of a tropical Jacobian in terms of potential theory on the underlying metric graph. We show that there exists a universal linear relation between the tropical moment, a certain capacity called the tau invariant, and the total length of a metric graph. To put our formula in a broader context, we relate our work to the computation of heights attached to principally polarized abelian varieties.

Special curves in modular surfaces

AbstractWe show that geodesics in $\mathbf {H}$ attached to a maximal split torus or a real quadratic torus in $GL_{2, \mathbf {Q}}$ are the only irreducible algebraic curves in $\mathbf {H}$ whose image in $\mathbf {R}^2$ via the j-invariant is contained in an algebraic curve.

Linking of Lagrangian Tori and Embedding Obstructions in Symplectic 4-Manifolds

Abstract We classify weakly exact, rational Lagrangian tori in $T^* \mathbb{T}^2- 0_{\mathbb{T}^2}$ up to Hamiltonian isotopy. This result is related to the classification theory of closed $1$-forms on $\mathbb{T}^n$ and also has applications to symplectic topology. As a 1st corollary, we strengthen a result due independently to Eliashberg–Polterovich and to Giroux describing Lagrangian tori in $T^* \mathbb{T}^2-0_{\mathbb{T}^2}$, which are homologous to the zero section. As a 2nd corollary, we exhibit pairs of disjoint totally real tori $K_1, K_2 \subset T^*\mathbb{T}^2$, each of which is isotopic through totally real tori to the zero section, but such that the union $K_1 \cup K_2$ is not even smoothly isotopic to a Lagrangian. In the 2nd part of the paper, we study linking of Lagrangian tori in $({\mathbb{R}}^4, \omega )$ and in rational symplectic $4$-manifolds. We prove that the linking properties of such tori are determined by purely algebro-topological data, which can often be deduced from enumerative disk counts in the monotone case. We also use this result to describe certain Lagrangian embedding obstructions.

Generic rank of Betti map and unlikely intersections

Let $\mathcal {A} \rightarrow S$ be an abelian scheme over an irreducible variety over $\mathbb {C}$ of relative dimension $g$. For any simply-connected subset $\Delta$ of $S^{\mathrm {an}}$ one can define the Betti map from $\mathcal {A}_{\Delta }$ to $\mathbb {T}^{2g}$, the real torus of dimension $2g$, by identifying each closed fiber of $\mathcal {A}_{\Delta } \rightarrow \Delta$ with $\mathbb {T}^{2g}$ via the Betti homology. Computing the generic rank of the Betti map restricted to a subvariety $X$ of $\mathcal {A}$ is useful to study Diophantine problems, e.g. proving the geometric Bogomolov conjecture over char $0$ and studying the relative Manin–Mumford conjecture. In this paper we give a geometric criterion to detect this rank. As an application we show that it is maximal after taking a large fibered power (if $X$ satisfies some conditions); it is an important step to prove the bound for the number of rational points on curves (Dimitrov et al., Uniformity in Mordell–Lang for Curves, Preprint (2020), arXiv:2001.10276). Another application is to answer a question of André, Corvaja and Zannier and improve a result of Voisin. We also systematically study its link with the relative Manin–Mumford conjecture, reducing the latter to a simpler conjecture. Our tools are functional transcendence and unlikely intersections for mixed Shimura varieties.

Unknottedness of real Lagrangian tori in $$S^2\times S^2$$

We prove the Hamiltonian unknottedness of real Lagrangian tori in the monotone $S^2\times S^2$, namely any real Lagrangian torus in $S^2\times S^2$ is Hamiltonian isotopic to the Clifford torus $\mathbb{T}_{\text{Clif}}$. The proof is based on a neck-stretching argument, Gromov's foliation theorem, and the Cieliebak-Schwingenheuer criterion.

Lagrangian Pairs of Pants

AbstractWe construct a Lagrangian submanifold, inside the cotangent bundle of a real torus, which we call a Lagrangian pair of pants. It is given as the graph of an exact one form on the real blowup of a Lagrangian coamoeba. Lagrangian pairs of pants are the main building blocks in a construction of smooth Lagrangian submanifolds of $( {\mathbb{C}}^*)^n$ that lift tropical subvarieties in $\mathbb R^n$. As an example we explain how to lift tropical curves in $ {\mathbb{R}}^2$ to Lagrangian submanifolds of $( {\mathbb{C}}^*)^2$. We also give several new examples of Lagrangian submanifolds inside toric varieties, some of which are monotone.

Kähler metrics with constant weighted scalar curvature and weighted K‐stability

We introduce a notion of a K\"ahler metric with constant weighted scalar curvature on a compact K\"ahler manifold $X$, depending on a fixed real torus $\mathbb{T}$ in the reduced group of automorphisms of $X$, and two smooth (weight) functions $\mathrm{v}>0$ and $\mathrm{w}$, defined on the momentum image (with respect to a given K\"ahler class $\alpha$ on $X$) of $X$ in the dual Lie algebra of $\mathbb{T}$. A number of natural problems in K\"ahler geometry, such as the existence of extremal K\"ahler metrics and conformally K\"ahler, Einstein--Maxwell metrics, or prescribing the scalar curvature on a compact toric manifold reduce to the search of K\"ahler metrics with constant weighted scalar curvature in a given K\"ahler class $\alpha$, for special choices of the weight functions $\mathrm{v}$ and $\mathrm{w}$. We show that a number of known results obstructing the existence of constant scalar curvature K\"ahler (cscK) metrics can be extended to the weighted setting. In particular, we introduce a functional $\mathcal M_{\mathrm{v}, \mathrm{w}}$ on the space of $\mathbb{T}$-invariant K\"ahler metrics in $\alpha$, extending the Mabuchi energy in the cscK case, and show (following the arguments of Li and Sano--Tipler in the cscK and extremal cases) that if $\alpha$ is Hodge, then constant weighted scalar curvature metrics in $\alpha$ are minima of $\mathcal M_{\mathrm{v},\mathrm{w}}$. Motivated by the recent work of Dervan--Ross and Dyrefelt in the cscK and extremal cases, we define a $(\mathrm{v},\mathrm{w})$-weighted Futaki invariant of a $\mathbb{T}$-compatible smooth K\"ahler test configuration associated to $(X, \alpha, \mathbb{T})$, and show that the boundedness from below of the $(\mathrm{v},\mathrm{w})$-weighted Mabuchi functional $\mathcal M_{\mathrm{v}, \mathrm{w}}$ implies a suitable notion of a $(\mathrm{v},\mathrm{w})$-weighted K-semistability.

Permanental Point Processes on Real Tori, Theta Functions and Monge–Ampère Equations

Inspired by constructions in complex geometry we introduce a thermodynamic framework for Monge–Ampère equations on real tori. We show convergence in law of the associated point processes and explain connections to complex Monge–Ampère equations on abelian varieties and optimal transport.

Arithmetic surfaces and adelic quotient groups

We explicitly calculate an arithmetic adelic quotient group for a locally free sheaf on an arithmetic surface when the fibre over the infinite point of the base is taken into account. The result is stated in the form of a short exact sequence. We relate the last term of this sequence to the projective limit of groups which are finite direct products of copies of the one-dimensional real torus and are connected with the first cohomology groups of locally free sheaves on the arithmetic surface.

Algebraic and o-minimal flows on complex and real tori

We consider the covering map π:Cn→T of a compact complex torus. Given an algebraic variety X⊆Cn we describe the topological closure of π(X) in T. We obtain a similar description when T is a real torus and X⊆Rn is a set definable in an o-minimal structure over the reals.

A Vietoris–Smale mapping theorem for the homotopy of hyperdefinable sets

Results of Smale (Proc Am Math Soc 8(3): 604–604, 1957) and Dugundji (Fundam Math 66:223–235, 1969) allow to compare the homotopy groups of two topological spaces X and Y whenever a map $$f:X\rightarrow Y$$ with strong connectivity conditions on the fibers is given. We can apply similar techniques to compare the homotopy of spaces living in different categories, for instance an abelian variety over an algebraically closed field, and a real torus. More generally, working in o-minimal expansions of fields, we compare the o-minimal homotopy of a definable set X with the homotopy of some of its bounded hyperdefinable quotients X / E. Under suitable assumption, we show that $$\pi _{n}^{{\text {def}}}(X)\cong \pi _{n}(X/E)$$ and $$\dim (X)=\dim _{\mathbb {R}}(X/E)$$ . As a special case, given a definably compact group, we obtain a new proof of Pillay’s group conjecture “ $$\dim (G)=\dim _{\mathbb {R}}(G/G^{00}$$ )” largely independent of the group structure of G. We also obtain different proofs of various comparison results between classical and o-minimal homotopy.

A Parameterization Method for Lagrangian Tori of Exact Symplectic Maps of $R^2r$

We are concerned with analytic exact symplectic maps of ${\mathbb R}^{2r}$ endowed with the standard symplectic form. We study the existence of a real analytic torus of dimension $r$, invariant by the map and carrying quasi-periodic motion with a prefixed Diophantine rotation vector. Therefore, this torus is a Lagrangian manifold. We address the problem by the parameterization method in KAM theory. The main aspect of our approach is that we do not look for the parameterization of the torus as a solution of the corresponding invariance equation. Instead, we consider a set of three equations that, all together, are equivalent to the invariance equation. These equations arise from the geometric and dynamical properties of the map and the torus. Suppose that an approximate solution of these equations is known and that a suitable nondegeneracy (twist) condition is satisfied. Then, this system of equations is solved by a quasi-Newton-like method, provided that the initial error is sufficiently small. By “quasi-Newton-like” we mean that the convergence is almost quadratic but that at each iteration we have to solve a nonlinear equation. Although it is straightforward to build a quasi-Newton method for the selected set of equations, proceeding in this way we improve the convergence condition. The selected definition of error reflects the level at which the error associated with each of these three equations contributes to the total error. The map is not required to be close to integrable or expressed in action-angle variables. Suppose the map is $\varepsilon$-close to an integrable one, and consider the portion of the phase space not filled up by Lagrangian invariant tori of the map. Then, the upper bound for the Lebesgue measure of this set that we may predict from the result is of ${\mathcal O}(\varepsilon^{1/2})$. In light of the classical KAM theory for exact symplectic maps, an upper bound of ${\mathcal O}(\varepsilon^{1/2})$ for this measure is the expected estimate. The result also has some implications for finitely differentiable maps.

An explicit prime geodesic theorem for discrete tori and the hypergeometric functions

The discrete tori are graph analogues of the real tori, which are defined by the Cayley graphs of a finite product of finite cyclic groups. In this paper, using the theory of the heat kernel on the discrete tori established by Chinta, Jorgenson and Karlsson, we derive an explicit prime geodesic theorem for the discrete tori, which is not an asymptotic formula. To describe the formula, we need generalizations of the classical Jacobi polynomials, which are defined by the Lauricella multivariable hypergeometric function of type C.

On the integral cohomology ring of toric orbifolds and singular toric varieties

We examine the integral cohomology rings of certain families of $2n$-dimensional orbifolds $X$ that are equipped with a well-behaved action of the $n$-dimensional real torus. These orbifolds arise from two distinct but closely related combinatorial sources, namely from characteristic pairs $(Q,\lambda)$, where $Q$ is a simple convex $n$-polytope and $\lambda$ a labelling of its facets, and from $n$-dimensional fans $\Sigma$. In the literature, they are referred as toric orbifolds and singular toric varieties respectively. Our first main result provides combinatorial conditions on $(Q,\lambda)$ or on $\Sigma$ which ensure that the integral cohomology groups $H^{\ast}(X)$ of the associated orbifolds are concentrated in even degrees. Our second main result assumes these condition to be true, and expresses the graded ring $H^*(X)$ as a quotient of an algebra of polynomials that satisfy an integrality condition arising from the underlying combinatorial data. Also, we compute several examples.

Geometric bijections between spanning trees and break divisors

The Jacobian group Jac(G) of a finite graph G is a group whose cardinality is the number of spanning trees of G. G also has a tropical Jacobian which has the structure of a real torus; using the notion of break divisors, An et al. obtained a polyhedral decomposition of the tropical Jacobian where vertices and cells correspond to elements of Jac(G) and spanning trees of G, respectively. We give a combinatorial description of bijections coming from this geometric setting. This provides a new geometric method for constructing bijections in combinatorics. We introduce a special class of geometric bijections that we call edge ordering maps, which have good algorithmic properties. Finally, we study the connection between our geometric bijections and the class of bijections introduced by Bernardi; in particular we prove a conjecture of Baker that planar Bernardi bijections are “geometric”. We also give sharpened versions of results by Baker and Wang on Bernardi torsors.

Tropicalization is a non-Archimedean analytic stack quotient

For a complex toric variety $X$ the logarithmic absolute value induces a natural retraction of $X$ onto the set of its non-negative points and this retraction can be identified with a quotient of $X(\mathbb{C})$ by its big real torus. We prove an analogous result in the non-Archimedean world: The Kajiwara-Payne tropicalization map is a non-Archimedean analytic stack quotient of $X^{an}$ by its big affinoid torus. Along the way, we provide foundations for a geometric theory of non-Archimedean analytic stacks, particularly focussing on analytic groupoids and their quotients, the process of analytification, and the underlying topological spaces of analytic stacks.