Integrability of higher pentagram maps

We define higher pentagram maps on polygons in $\mathbb{P }^d$ for any dimension $d$ , which extend R. Schwartz’s definition of the 2D pentagram map. We prove their integrability by presenting Lax representations with a spectral parameter for scale invariant maps. The corresponding continuous limit of the pentagram map in dimension $d$ is shown to be the $(2,d+1)$ -equation of the KdV hierarchy, generalizing the Boussinesq equation in 2D. We also study in detail the 3D case, where we prove integrability for both closed and twisted polygons and describe the spectral curve, first integrals, the corresponding tori and the motion along them, as well as an invariant symplectic structure.     

Relaxed Hyperelastic Curves on a Non-Degenerate Surface

We study the variational problem belonging to a relaxed hyperelastic curve for non-null curve on a non-degenerate surface in Minkowski three-space \({E_{1}^{3}}\) . Firstly, we derive the intrinsic equations for a relaxed hyperelastic curve and we give the necessary condition for being relaxed hyperelastic curve of any non-null geodesic on the surface in \({E_{1}^{3}}\) . Then, we examine this formulation on non-null geodesics of pseudo-plane, pseudo-sphere \({S_{1}^{2}(r) }\) , hyperbolic space \({H_{0}^{2}(r)}\) and pseudo-cylinder \({C_{1}^{2}(r)}\) .     

On Maximally Inflected Hyperbolic Curves

In this note we study the distribution of real inflection points among the ovals of a real non-singular hyperbolic curve of even degree. Using Hilbert’s method we show that for any integers \(d\) and \(r\) such that \(4\le r \le 2d^2-2d\) , there is a non-singular hyperbolic curve of degree \(2d\) in \({\mathbb R}^2\) with exactly \(r\) line segments in the boundary of its convex hull. We also give a complete classification of possible distributions of inflection points among the ovals of a maximally inflected non-singular hyperbolic curve of degree \(6\) .     

Characterization of the Fermat curve as the most symmetric nonsingular algebraic plane curve

A projective nonsingular plane algebraic curve of degree \(d\ge 4\) is called maximally symmetric if it attains the maximum order of the automorphism groups for complex nonsingular plane algebraic curves of degree \(d\) . For \(d\le 7\) , all such curves are known. Up to projectivities, they are the Fermat curve for \(d=5,7\) ; see Kaneta et al. (RIMS Kokyuroku 1109:182–191, 1999) and Kaneta et al. (Geom. Dedic. 85:317–334, 2001), the Klein quartic for \(d=4\) , see Hartshorne (Algebraic Geometry. Springer, New York, 1977), and the Wiman sextic for \(d=6\) ; see Doi et al. (Osaka J. Math. 37:667–687, 2000). In this paper we work on projective plane curves defined over an algebraically closed field of characteristic zero, and we extend this result to every \(d\ge 8\) showing that the Fermat curve is the unique maximally symmetric nonsingular curve of degree \(d\) with \(d\ge 8\) , up to projectivity. For \(d=11,13,17,19\) , this characterization of the Fermat curve has already been obtained; see Kaneta et al. (Geom. Dedic. 85:317–334, 2001).     

On the Durrmeyer-type modification of some discrete approximation operators

In [10], for continuous functionsf from the domain of certain discrete operatorsL _n the inequalities are proved concerning the modulus of continuity ofL _nf. Here we present analogues of the results obtained for the Durrmeyer-type modification $\tilde L_n $ ofL _n. Moreover, we give the estimates of the rate of convergence of $\tilde L_n f$ in Hölder-type norms     

On torsors under elliptic curves and Serre’s pro-algebraic structures

Let ${\mathcal {O}}_K$ be a complete discrete valuation ring with algebraically closed residue field of positive characteristic and field of fractions $K$ . Let $X_K$ be a torsor under an elliptic curve $A_K$ over $K$ , $X$ the proper minimal regular model of $X_K$ over $S:=\hbox {Spec}({\mathcal {O}}_K)$ , and $J$ the identity component of the Néron model of $\mathrm{Pic}_{X_K/K}^{0}$ . We study the canonical morphism $q:\mathrm{Pic}^{0}_{X/S}\rightarrow J$ which extends the natural isomorphism on generic fibres. We show that $q$ is pro-algebraic in nature with a construction that recalls Serre’s work on local class field theory. Furthermore, we interpret our results in relation to Shafarevich’s duality theory for torsors under abelian varieties.     

Asymptotic behavior of the isoperimetric deficit for expanding convex plane curves

We consider the expansion of a convex closed plane curve C ₀ along its outward normal direction with speed G(1/k), where k is the curvature and \({G \left(z \right) :\left(0, \infty \right) \rightarrow \left( 0, \infty \right)}\) is a strictly increasing function. We show that if \({{\rm lim}_{z \rightarrow \infty} G \left(z \right) = \infty}\) , then the isoperimetric deficit \({D \left(t \right) : = L^{2}\left(t \right) -4 \pi A \left(t \right)}\) of the flow converges to zero. On the other hand, if \({{\rm lim}_{z \rightarrow \infty}G \left(z \right) = \lambda \in (0,\infty)}\) , then for any number d ≥ 0 and \({\varepsilon > 0}\) , one can choose an initial curve C ₀ so that its isoperimetric deficit \({D \left(t \right)}\) satisfies \({\left \vert D \left(t \right) -d \right \vert < \varepsilon}\) for all \({t \in [0, \infty)}\) . Hence, without rescaling, the expanding curve C _t will not become circular. It is close to some expanding curve P _t , where each P _t is parallel to P ₀. The asymptotic speed of P _t is given by the constant \({\lambda}\) .     

Some new classes of Kadison-Singer lattices in Hilbert spaces

Let N be a maximal and discrete nest on a separable Hilbert space H,E the projection from H onto the subspace[C]spanned by a particular separating vector for N′and Q the projection from K=H⊕H onto the closed subspace{(,):∈H}.Let L be the closed lattice in the strong operator topology generated by the projections(E 00 0),{(E 00 0):E∈N}and Q.We show that L is a Kadison-Singer lattice with trivial commutant,i.e.,L′=CI.Furthermore,we similarly construct some Kadison-Singer lattices in the matrix algebras M2n(C)and M2n.1(C).     

The Brill–Noether curve and Prym-Tyurin varieties

We prove that the Jacobian of a general curve C of genus $g=2a+1$ , with $a\ge 2$ , can be realized as a Prym-Tyurin variety for the Brill–Noether curve $W^{1}_{a+2}(C)$ . As consequence of this result we are able to compute the class of the sum of secant divisors of the curve C, embedded with a complete linear series $g^{a-1}_{3a-2}$ .     

The Algebra of {{\mathbf{SL}}_3}\left( \mathbb{C} \right) Conformal Blocks

We construct and study a family of toric degenerations of the Cox ring of the moduli of quasi-parabolic principal SL₃( $ \mathbb{C} $ ) bundles on a smooth, marked curve (C, $ \vec{p} $ ): Elements of this algebra have a well known interpretation as conformal blocks, from the Wess-Zumino-Witten model of conformal field theory. For the genus 0; 1 cases we find the level of conformal blocks necessary to generate the algebra. In the genus 0 case we also find bounds on the degrees of relations required to present the algebra. As a consequence we obtain a toric degeneration for the projective coordinate ring of an effective divisor on the moduli $ {{\mathcal{M}}_{{C,\vec{p}}}}\left( {\mathrm{S}{{\mathrm{L}}_3}\left( \mathbb{C} \right)} \right) $ of quasi-parabolic principal SL₃( $ \mathbb{C} $ ) bundles on (C, $ \vec{p} $ ). Along the way we recover positive polyhedral rules for counting conformal blocks.     

Existence and stability of ground states for fully discrete approximations of the nonlinear Schrödinger equation

In this paper we study the long time behavior of a discrete approximation in time and space of the cubic nonlinear Schrödinger equation on the real line. More precisely, we consider a symplectic time splitting integrator applied to a discrete nonlinear Schrödinger equation with additional Dirichlet boundary conditions on a large interval. We give conditions ensuring the existence of a numerical ground state which is close in energy norm to the continuous ground state. Such result is valid under a CFL condition of the form $\tau h^{-2}\le C$ where $\tau $ and $h$ denote the time and space step size respectively. Furthermore we prove that if the initial datum is symmetric and close to the continuous ground state $\eta $ then the associated numerical solution remains close to the orbit of $\eta ,\Gamma =\cup _\alpha \{e^{i\alpha }\eta \}$ , for very long times.     

Separation and relaxation for cones of quadratic forms

Let ${P \subseteq \mathfrak{R}_{n}}$ be a pointed, polyhedral cone. In this paper, we study the cone ${\mathcal{C} = {\rm cone}\{xx^T : x \in P\}}$ of quadratic forms. Understanding the structure of ${\mathcal{C}}$ is important for globally solving NP-hard quadratic programs over P. We establish key characteristics of ${\mathcal{C}}$ and construct a separation algorithm for ${\mathcal{C}}$ provided one can optimize with respect to a related cone over the boundary of P. This algorithm leads to a nonlinear representation of ${\mathcal{C}}$ and a class of tractable relaxations for ${\mathcal{C}}$ , each of which improves a standard polyhedral-semidefinite relaxation of ${\mathcal{C}}$ . The relaxation technique can further be applied recursively to obtain a hierarchy of relaxations, and for constant recursive depth, the hierarchy is tractable. We apply this theory to two important cases: P is the nonnegative orthant, in which case ${\mathcal{C}}$ is the cone of completely positive matrices; and P is the homogenized cone of the “box” [0, 1] ⁿ . Through various results and examples, we demonstrate the strength of the theory for these cases. For example, we achieve for the first time a separation algorithm for 5 × 5 completely positive matrices.     

On the roots of the equation \zeta (s)=a

Given any complex number $a$ , we prove that there are infinitely many simple roots of the equation $\zeta (s)=a$ with arbitrarily large imaginary part. Besides, we give a heuristic interpretation of a certain regularity of the graph of the curve $t\mapsto \zeta ({1\over 2}+it)$ . Moreover, we show that the curve $\mathbb {R}\ni t\mapsto (\zeta ({1\over 2}+it),\zeta '({1\over 2}+it))$ is not dense in $\mathbb {C}^2$ .     

Lower large deviations for the maximal flow through a domain of {\mathbb{R}^d} in first passage percolation

We consider the standard first passage percolation model in the rescaled graph ${\mathbb{Z}^d/n}$ for d??? 2, and a domain ?? of boundary ?? in ${\mathbb{R}^d}$ . Let ??¹ and ??² be two disjoint open subsets of ??, representing the parts of ?? through which some water can enter and escape from ??. We investigate the asymptotic behaviour of the flow ${\phi_n}$ through a discrete version ?? _n of ?? between the corresponding discrete sets ${\Gamma^{1}_{n}}$ and ${\Gamma^{2}_{n}}$ . We prove that under some conditions on the regularity of the domain and on the law of the capacity of the edges, the lower large deviations of ${\phi_n/ n^{d-1}}$ below a certain constant are of surface order.     

Invariant theory for the elliptic normal quintic, I. Twists of X(5)

A genus one curve of degree 5 is defined by the $4 \times 4$ Pfaffians of a $5 \times 5$ alternating matrix of linear forms on $\mathbb{P }^4$ . We describe a general method for investigating the invariant theory of such models. We use it to explain how we found our algorithm for computing the invariants and to extend our method for computing equations for visible elements of order 5 in the Tate-Shafarevich group of an elliptic curve. As a special case of the latter we find a formula for the family of elliptic curves 5-congruent to a given elliptic curve in the case the 5-congruence does not respect the Weil pairing. We also give an algorithm for doubling elements in the $5$ -Selmer group of an elliptic curve, and make a conjecture about the matrices representing the invariant differential on a genus one normal curve of arbitrary degree.     

On the peripheral point spectrum and the asymptotic behavior of irreducible semigroups of Harris operators

Given a positive, irreducible and bounded $C_0$ -semigroup on a Banach lattice with order continuous norm, we prove that the peripheral point spectrum of its generator is trivial whenever one of its operators dominates a non-trivial compact or kernel operator. For a discrete semigroup, i.e. for powers of a single operator $T$ , we show that the point spectrum of some power $T^k$ intersects the unit circle at most in $1$ . As a consequence, we obtain a sufficient condition for strong convergence of the $C_0$ -semigroup and for a subsequence of the powers of $T$ , respectively.     

Admissible closures of polynomial time computable arithmetic

We propose two admissible closures ${\mathbb{A}({\sf PTCA})}$ and ${\mathbb{A}({\sf PHCA})}$ of Ferreira??s system PTCA of polynomial time computable arithmetic and of full bounded arithmetic (or polynomial hierarchy computable arithmetic) PHCA. The main results obtained are: (i) ${\mathbb{A}({\sf PTCA})}$ is conservative over PTCA with respect to ${\forall\exists\Sigma^b_1}$ sentences, and (ii) ${\mathbb{A}({\sf PHCA})}$ is conservative over full bounded arithmetic PHCA for ${\forall\exists\Sigma^b_{\infty}}$ sentences. This yields that (i) the ${\Sigma^b_1}$ definable functions of ${\mathbb{A}({\sf PTCA})}$ are the polytime functions, and (ii) the ${\Sigma^b_{\infty}}$ definable functions of ${\mathbb{A}({\sf PHCA})}$ are the functions in the polynomial time hierarchy.     

Calabi Product Lagrangian Immersions in Complex Projective Space and Complex Hyperbolic Space

Starting from two Lagrangian immersions and a Legendre curve ${\tilde{\gamma}(t)}$ in ${\mathbb{S}^3(1)}$ $({\rm or\,in}\,{\mathbb{H}_1^3(-1)})$ , it is possible to construct a new Lagrangian immersion in ${\mathbb{CP}^n(4)}$ $({\rm or\,in}\,{\mathbb{CH}^n(-4)})$ , which is called a warped product Lagrangian immersion. When ${\tilde{\gamma}(t)=(r_1e^{i(\frac{r_2}{r_1}at)}, \;r_2e^{i(- \frac{r_1}{r_2}at)})}$ $({\rm or}\,{\tilde{\gamma}(t)=(r_1e^{i(\frac{r_2}{r_1}at)}, \;r_2e^{i( \frac{r_1}{r_2}at)})})$ , where r ₁, r ₂, and a are positive constants with ${r_1^2+r_2^2=1}$ $({\rm or}\,{-r_1^2+r_2^2=-1})$ , we call the new Lagrangian immersion a Calabi product Lagrangian immersion. In this paper, we study the inverse problem: how to determine from the properties of the second fundamental form whether a given Lagrangian immersion of ${\mathbb{CP}^n(4)}$ or ${\mathbb{CH}^n(-4)}$ is a Calabi product Lagrangian immersion. When the Calabi product is minimal, or is Hamiltonian minimal, or has parallel second fundamental form, we give some further characterizations.