1. Algebraic Stories from One and from the Other Pockets
   Ralf Fröberg, Samuel Lundqvist, Alessandro Oneto, Boris Shapiro
   Arnold Mathematical Journal (2018) 4:137–160
   Received: 7 January 2018 / Revised: 31 May 2018 / Accepted: 18 July 2018 / Published online: 31 July 2018
   Abstract
   
   In what follows, we present a large number of questions which were posed on the problem solving seminar in algebra at Stockholm University during the period Fall 2014 – Spring 2017 along with a number of results related to these problems. Many of the results were obtained by participants of the latter seminar.
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2. Open Problems on Configuration Spaces of Tensegrities
   Oleg Karpenkov
   Arnold Mathematical Journal (2018) 4:19–25
   Received: 7 January Received: 8 June 2017 / Revised: 13 January 2018 / Accepted: 22 January 2018
   Abstract
   
   In this small paper we bring together some open problems related to the study of the configuration spaces of tensegrities, i.e. graphs with stresses on edges. These problems were announced in Doray et al. (Discrete Comput Geom 43:436-466, 2010), Karpenkov et al. (ARS Math Contemp 6:305-322, 2013), Karpenkov (The combinatorial geometry of stresses in frameworks. arXiv:1512.02563 [math.MG]), and Karpenkov (Geometric Conditions of Rigidity in Nongeneric settings, 2016 (by F. Doray, J. Schepers, B. Servatius, and the author), for more details we refer to the mentioned articles.
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3. Modular Periodicity of the Euler Numbers and a Sequence by Arnold
   Sanjay Ramassamy
   Arnold Math J. (2018) 3:519–524
   Received: 19 November 2017 / Accepted: 11 January 2018 / Published online: 22 January 2018
   Abstract
   
   For any positive integer $q$, the sequence of the Euler up/down numbers reduced modulo $q$ was proved to be ultimately periodic by Knuth and Buckholtz. Based on computer simulations, we state for each value of $q$ precise conjectures for the minimal period and for the position at which the sequence starts being periodic. When $q$ is a power of $2$, a sequence defined by Arnold appears, and we formulate a conjecture for a simple computation of this sequence.
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4. The Number $ \pi$ and a Summation by $ SL(2,{\mathbb{Z}})$
   Nikita Kalinin, Mikhail Shkolnikov
   Arnold Math J. (2018) 3:511–517
   Received: 7 October 2016 / Revised: 30 June 2017 / Accepted: 8 October 2017
   Abstract
   
   The sum (resp. the sum of squares) of the defects in the triangle inequalities for the area one lattice parallelograms in the first quadrant has a surprisingly simple expression. Namely, let $ f(a,b,c,d)=\sqrt{a^2+b^2}+\sqrt{c^2+d^2}-\sqrt{(a+c)^2+(b+d)^2}$. Then,

   \begin{align*}\tag{Ж} \sum f(a,b,c,d)^2 = 2-\pi/2,\label{eqspspi} \end{align*}

   \begin{align*}\tag{ж} \sum f(a,b,c,d) = 2,\label{eqspstwo} \end{align*}

   where the sum runs by all $ a,b,c,d\in{\mathbb{Z}}_{\geq 0}$ such that $ ad-bc=1$. We present a proof of these formulae and list several directions for the future studies.

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5. Open Problems and Conjectures Related to the Theory of Mathematical Quasicrystals
   Faustin Adiceam, David Damanik, Franz Gähler, Uwe Grimm, Alan Haynes, Antoine Julien, Andrés Navas, Lorenzo Sadun, Barak Weiss
   Arnold Math J. (2016) 2:4, 579–592
   Received: 15 January 2016 / Revised: 21 May 2016 / Accepted: 11 June 2016 / Published Online: 11 July 2016
   Abstract
   
   This list of problems arose as a collaborative effort among the participants of the Arbeitsgemeinschaft on Mathematical Quasicrystals, which was held at the Mathematisches Forschungsinstitut Oberwolfach in October 2015. The purpose of our meeting was to bring together researchers from a variety of disciplines, with a common goal of understanding different viewpoints and approaches surrounding the theory of mathematical quasicrystals. The problems below reflect this goal and this diversity and we hope that they will motivate further cross-disciplinary research and lead to new advances in our overall vision of this rapidly developing field.
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6. Betti Posets and the Stanley Depth
   L. Katthän
   Arnold Math J. (2016) 2:2, 267–276
   Received: 9 October 2015 / Revised: 19 December 2015 / Accepted: 4 February 2016 / Published online: 15 January 2016
   Abstract
   
   Let $S$ be a polynomial ring and let $I \subseteq S$ be a monomial ideal. In this short note, we propose the conjecture that the Betti poset of $I$ determines the Stanley projective dimension of $S/I$ or $I$. Our main result is that this conjecture implies the Stanley conjecture for $I$, and it also implies that ${{\mathrm{sdepth}}}S/I \ge {{\mathrm{depth}}}S/I - 1$. Recently, Duval et al. (A non-partitionable Cohen-Macaulay simplicial complex, arXiv:1504.04279, 2015), found a counterexample to the Stanley conjecture, and their counterexample satisfies ${{\mathrm{sdepth}}}S/I = {{\mathrm{depth}}}S/I - 1$. So if our conjecture is true, then the conclusion is best possible.
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7. Volumes of Strata of Abelian Differentials and Siegel-Veech Constants in Large Genera
   A. Eskin, A. Zorich
   Arnold Math J. (2015) 1:4, 481–488
   Received: 19 July 2015 / Revised: 16 September 2015 / Accepted: 20 October 2015 / Published online: 05 November 2015
   Abstract
   
   We state conjectures on the asymptotic behavior of the volumes of moduli spaces of Abelian differentials and their Siegel-Veech constants as genus tends to infinity. We provide certain numerical evidence, describe recent advances and the state of the art towards proving these conjectures.
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8. Disconjugacy and the Secant Conjecture
   A. Eremenko
   Arnold Math J. (2015) 1:3, 339–342
   Received: 5 July 2015 / Accepted: 28 July 2015 / Published online: 4 August 2015
   Abstract
   
   We discuss the so-called secant conjecture in real algebraic geometry, and show that it follows from another interesting conjecture, about disconjugacy of vector spaces of real polynomials in one variable.
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9. A Few Problems on Monodromy and Discriminants
   V. A. Vassiliev
   Arnold Math J. (2015) 1:2, 201–209
   Received: 15 February 2015 / Accepted: 31 March 2015 / Published online: 16 April 2015
   Abstract
   
   The article contains several problems concerning local monodromy groups of singularities, Lyashko-Looijenga maps, integral geometry, and topology of spaces of real algebraic manifolds.
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10. Problems Around Polynomials: The Good, The Bad and The Ugly...
    Boris Shapiro
    Arnold Math J. (2015) 1:1, 91–99
    Received: 7 November 2014 / Accepted: 16 March 2015 / Published online: 25 March 2015
    Abstract
    
    The Russian style of formulating mathematical problems means that nobody will be able to simplify your formulation as opposed to the French style which means that nobody will be able to generalize it, - Vladimir Arnold.
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11. Space of Smooth 1-Knots in a 4-Manifold: Is Its Algebraic Topology Sensitive to Smooth Structures?
    Oleg Viro
    Arnold Math J. (2015) 1:1, 83–89
    Received: 12 December 2014 / Accepted: 12 February 2015
    Abstract
    
    We discuss a possibility to get an invariant of a smooth structure on a closed simply connected 4-manifold from homotopy invariants of the space of loops smoothly embedded into the manifold.
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12. Classification of Finite Metric Spaces and Combinatorics of Convex Polytopes
    A. M. Vershik
    Arnold Math J. (2015) 1:1, 75–81
    Received: 8 November 2014 / Accepted: 31 December 2014
    Abstract
    
    We describe the canonical correspondence between finite metric spaces and symmetric convex polytopes, and formulate the problem about classification of the metric spaces in terms of combinatorial structure of those polytopes.
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13. Periods of Pseudo-Integrable Billiards
    Vladimir Dragović, Milena Radnović
    Arnold Math J. (2015) 1:1, 69–73
    Received: 10 November 2014 / Accepted: 26 December 2014
    Abstract
    
    Consider billiard desks composed of two concentric half-circles connected with two edges. We examine billiard trajectories having a fixed circle concentric with the boundary semicircles as the caustic, such that the rotation numbers with respect to the half-circles are ρ₁ and ρ₂ respectively. Are such billiard trajectories periodic, and what are all possible periods for given ρ₁ and ρ₂?
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14. A Baker's Dozen of Problems
    Serge Tabachnikov
    Arnold Math J. (2015) 1:1, 59–67
    Received: 16 September 2014 / Revised: 23 September 2014 / Accepted: 14 October 2014
    Abstract
    
    This article is a collection of open problems, with brief historical and bibliographical comments, somewhat in the spirit of the problem with which V. Arnold opened his famous seminar every semester and that were recently collected and published in a book form.
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1. Counting Borel Orbits in Symmetric Spaces of Types $BI$ and $CII$
   Mahir Bilen Can, Özlem Uǧurlu
   Arnold Math J. (2018) 4:213–250
   Received: 1 February 2018 / Revised: 29 August 2018 / Accepted: 4 September 2018 / Published online: 10 September 2018
   Abstract
   
   This is a continuation of our combinatorial program on the enumeration of Borel orbits in symmetric spaces of classical types. Here, we determine the generating series the numbers of Borel orbits in $\mathbf{SO}_{2n+1}/\mathbf{S}(\mathbf{O}_{2p}\times\mathbf{O}_{2q+1})$ (type $BI$) and in $\mathbf{Sp}_n /\mathbf{Sp}_p\times\mathbf{Sp}_q$ (type $CII$). In addition, we explore relations to lattice path enumeration.
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2. Solvability of Equations by Quadratures and Newton's Theorem
   Askold Khovanskii
   Arnold Math J. (2018) 4:193–211
   Received: 25 July 2018 / Revised: 21 August 2018 / Accepted: 27 August 2018 / Published online: 14 September 2018
   Abstract
   
   Picard–Vessiot theorem (1910) provides a necessary and sufficient condition for solvability of linear differential equations of order $n$ by quadratures in terms of its Galois group. It is based on the differential Galois theory and is rather involved. Liouville in 1839 found an elementary criterium for such solvability for $n = 2$. Ritt simplified Liouville's theorem (1948). In 1973 Rosenlicht proved a similar criterium for arbitrary $n$. Rosenlicht work relies on the valuation theory and is not elementary. In these notes we show that the elementary Liouville–Ritt method based on developing solutions in Puiseux series as functions of a parameter works smoothly for arbitrary $n$ and proves the same criterium.
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3. Renormalization for Unimodal Maps with Non-integer Exponents
   Igors Gorbovickis, Michael Yampolsky
   Arnold Math J. (2018) 4:179–191
   Received: 19 November 2017 / Revsed: 8 August 2018 / Accepted: 12 August 2018 / Published online: 23 August 2018
   Abstract
   
   We define an analytic setting for renormalization of unimodal maps with an arbitrary critical exponent. We prove the global hyperbolicity of renormalization conjecture for unimodal maps of bounded type with a critical exponent which is sufficiently close to an even integer. Furthermore, we prove the global $C^{1+\beta}$-rigidity conjecture for such maps, giving the first example of a smooth rigidity theorem for unimodal maps whose critical exponent is not an even integer.
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4. Conway River and Arnold Sail
   K. Spalding, A. P. Veselov
   Arnold Math J. (2018) 4:169–177
   Received: 21 February 2018 / Revised: 21 June 2018 / Accepted: 5 July 2018 / Published online: 18 July 2018
   Abstract
   
   We establish a simple relation between two geometric constructions in number theory: the Conway river of a real indefinite binary quadratic form and the Arnold sail of the corresponding pair of lines.
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5. Upper Bounds on Betti Numbers of Tropical Prevarieties
   Dima Grigoriev, Nicolai Vorobjov
   Received: 5 October 2017 / Revised: 9 March 2018 / Accepted: 12 March 2018
   Abstract
   
   We prove upper bounds on the sum of Betti numbers of tropical prevarieties in dense and sparse settings. In the dense setting the bound is in terms of the volume of Minkowski sum of Newton polytopes of defining tropical polynomials, or, alternatively, via the maximal degree of these polynomials. In sparse setting, the bound involves the number of the monomials.
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6. Trace Test
   Anton Leykin, Jose Israel Rodriguez, Frank Sottile
   Received: 16 December 2016 / Revised: 6 February 2018 / Accepted: 10 March 2018
   Abstract
   
   The trace test in numerical algebraic geometry verifies the completeness of a witness set of an irreducible variety in affine or projective space. We give a brief derivation of the trace test and then consider it for subvarieties of products of projective spaces using multihomogeneous witness sets. We show how a dimension reduction leads to a practical trace test in this case involving a curve in a low-dimensional affine space.
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7. Cyclohedron and Kantorovich–Rubinstein Polytopes
   Filip D. Jevtić, Marija Jelić, Rade T. Živaljević
   Received: 14 July 2017 / Revised: 13 December 2017 / Accepted: 10 March 2018/ Published online: 9 April 2018
   Abstract
   
   We show that the cyclohedron (Bott–Taubes polytope) $ W_n$ arises as the polar dual of a Kantorovich–Rubinstein polytope $ KR(\rho)$, where $ \rho$ is an explicitly described quasi-metric (asymmetric distance function) satisfying strict triangle inequality. From a broader perspective, this phenomenon illustrates the relationship between a nestohedron $ \Delta_{{\widehat{\mathcal{F}}}}$ (associated to a building set $ {\widehat{\mathcal{F}}}$) and its non-simple deformation $ \Delta_{\mathcal{F}}$, where $ \mathcal{F}$ is an irredundant or tight basis of $ {\widehat{\mathcal{F}}}$. Among the consequences are a new proof of a recent result of Gordon and Petrov (Arnold Math. J. 3(2):205–218, 2017gp) about $ f$-vectors of generic Kantorovich–Rubinstein polytopes and an extension of a theorem of Gelfand, Graev, and Postnikov, about triangulations of the type A, positive root polytopes.
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8. Affine Hecke Algebras via DAHA
   Ivan Cherednik
   Received: 12 October 2017 / Revised: 22 January 2018 / Accepted: 9 March 2018
   Abstract
   
   A method is suggested for obtaining the Plancherel measure for Affine Hecke Algebras as a limit of integral-type formulas for inner products in the polynomial and related modules of Double Affine Hecke Algebras. The analytic continuation necessary here is a generalization of "picking up residues" due to Arthur, Heckman, Opdam and others, which can be traced back to Hermann Weyl. Generally, it is a finite sum of integrals over double affine residual subtori; a complete formula is presented for $ A_1$ in the spherical case.
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9. Semiconjugate Rational Functions: A Dynamical Approach
   F. Pakovich
   Received: 7 January 2018 / Accepted: 29 January 2018
   Abstract
   
   Using dynamical methods we give a new proof of the theorem saying that if $A,B,X$ are rational functions of complex variable $z$ of degree at least two such that $A\circ X=X\circ B$ and $\mathbb C(B,X)=\mathbb C(z)$, then the Galois closure of the field extension $\mathbb C(z)/\mathbb C(X)$ has genus zero or one.
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10. Secant Degeneracy Index of the Standard Strata in The Space of Binary Forms
    Gleb Nenashev, Boris Shapiro, Michael Shapiro
    Received: 30 January 2017 / Revised: 3 November 2017 / Accepted: 7 November 2017
    Abstract
    
    The space $ Pol_d\simeq \mathbb{C} P^d$ of all complex-valued binary forms of degree $ d$ (considered up to a constant factor) has a standard stratification, each stratum of which contains all forms whose set of multiplicities of their distinct roots is given by a fixed partition $ \mu \vdash d$. For each such stratum $ S_\mu,$ we introduce its secant degeneracy index $ \ell_\mu$ which is the minimal number of projectively dependent pairwise distinct points on $ S_\mu$, i.e., points whose projective span has dimension smaller than $ \ell_\mu-1$. In what follows, we discuss the secant degeneracy index $ \ell_\mu$ and the secant degeneracy index $ \ell_{{{\bar{\mu}}}}$ of the closure $ {{\bar{S}}}_\mu$.
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11. Orbifold Jacobian Algebras for Exceptional Unimodal Singularities
    Alexey Basalaev, Atsushi Takahashi, Elisabeth Werner
    Received: 4 March 2017 / Revised: 27 September 2017 / Accepted: 8 October 2017
    Abstract
    
    This note shows that the orbifold Jacobian algebra associated to each invertible polynomial defining an exceptional unimodal singularity is isomorphic to the (usual) Jacobian algebra of the Berglund–Hübsch transform of an invertible polynomial defining the strange dual singularity in the sense of Arnold.
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12. On Tangent Cones of Schubert Varieties
    Dmitry Fuchs, Alexandre Kirillov, Sophie Morier-Genoud, Valentin Ovsienko
    Received: 23 March 2017 / Revised: 10 June 2017 / Accepted: 2 August 2017 / First Online: 22 August 2017
    Abstract
    
    We consider tangent cones of Schubert varieties in the complete flag variety, and investigate the problem when the tangent cones of two different Schubert varieties coincide. We give a sufficient condition for such coincidence, and formulate a conjecture that provides a necessary condition. In particular, we show that all Schubert varieties corresponding to the Coxeter elements of the Weyl group have the same tangent cone. Our main tool is the notion of pillar entries in the rank matrix counting the dimensions of the intersections of a given flag with the standard one. This notion is a version of Fulton's essential set. We calculate the dimension of a Schubert variety in terms of the pillar entries of the rank matrix.
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13. On Irreducible Components of Real Exponential Hypersurfaces
    Cordian Riener, Nicolai Vorobjov
    Arnold Math J. (2017) 3:3, 423–443
    Received: 31 December 2016 / Revised: 28 July 2017 / Accepted: 2 August 2017 / First Online: 09 August 2017
    Abstract
    
    Fix any real algebraic extension $ \mathbb K$ of the field $ \mathbb Q$ of rationals. Polynomials with coefficients from $ \mathbb K$ in $ n$ variables and in $ n$ exponential functions are called exponential polynomials over $ {\mathbb K}$. We study zero sets in $ \mathbb R^n$ of exponential polynomials over $ \mathbb K$, which we call exponential-algebraic sets . Complements of all exponential-algebraic sets in $ \mathbb R^n$ form a Zariski-type topology on $ \mathbb R^n$. Let $ P \in {\mathbb K}[X_1, \ldots ,X_n,U_1, \ldots ,U_n]$ be a polynomial and denote \begin{eqnarray*} V:=\{ (x_1, \ldots , x_n) \in \mathbb R^n|\> P(x_1, \ldots ,x_n,, e^{x_1}, \ldots ,e^{x_n})=0 \}. \end{eqnarray*} The main result of this paper states that, if the real zero set of a polynomial $ P$ is irreducible over $ \mathbb K$ and the exponential-algebraic set $ V$ has codimension 1, then, under Schanuel's conjecture over the reals, either $ V$ is irreducible (with respect to the Zariski topology) or each of its irreducible components of codimension 1 is a rational hyperplane through the origin. The family of all possible hyperplanes is determined by monomials of $ P$. In the case of a single exponential (i.e., when $ P$ is independent of $ U_2, \ldots , U_n$) stronger statements are shown which are independent of Schanuel's conjecture.
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14. Integral Geometry of Euler Equations
    Nikolai Nadirashvili, Serge Vlăduţ
    Arnold Math J. (2017) 3:3, 397–421
    Received: 23 May 2017 / Revised: 24 July 2017 / Accepted: 29 July / First Online: 09 August 2017
    Abstract
    
    We develop an integral geometry of stationary Euler equations defining some function $ w$ on the Grassmannian of affine lines in $ \mathbb R^3$ depending on a putative compactly supported solution $ (v;p)$ of the system and deduce some linear differential equations for $ w$. We conjecture that $ w=0$ everywhere and prove that this conjecture implies that $ v=0.$
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15. Origami, Affine Maps, and Complex Dynamics
    William Floyd, Gregory Kelsey, Sarah Koch, Russell Lodge, Walter Parry, Kevin M. Pilgrim, Edgar Saenz
    Arnold Math J. (2017) 3:3, 365–395
    Received: Received: 19 December 2016 / Accepted: 27 July 2017/ First Online: 22 August 2017
    Abstract
    
    We investigate the combinatorial and dynamical properties of so-called nearly Euclidean Thurston maps , or NET maps . These maps are perturbations of many-to-one folding maps of an affine two- sphere to itself. The close relationship between NET maps and affine maps makes computation of many invariants tractable. In addition to this, NET maps are quite diverse, exhibiting many different behaviors. We discuss data, findings, and new phenomena.
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16. Moduli Space of a Planar Polygonal Linkage: A Combinatorial Description
    Gaiane Panina
    Arnold Math J. (2017) 3:3, 351–364
    Received: 11 December 2016 / Revised: 8 April 2017 / Accepted: 18 May 2017
    Abstract
    
    We describe and study an explicit structure of a regular cell complex $\mathcal{K}(L)$ on the moduli space $M(L)$ of a planar polygonal linkage $L$. The combinatorics is very much related (but not equal) to the combinatorics of the permutohedron. In particular, the cells of maximal dimension are labeled by elements of the symmetric group. For example, if the moduli space $M$ is a sphere, the complex $\mathcal{K}$ is dual to the boundary complex of the permutohedron.The dual complex $\mathcal{K}^*$ is patched of Cartesian products of permutohedra. It can be explicitly realized in the Euclidean space via a surgery on the permutohedron.
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17. When is the Intersection of Two Finitely Generated Subalgebras of a Polynomial Ring Also Finitely Generated?
    Pinaki Mondal
    Arnold Math J. (2017) 3:3, 333–350
    Received: 6 November 2016 / Revised: 23 March 2017 / Accepted: 27 March 2017
    Abstract
    
    We study two variants of the following question: "Given two finitely generated $ \mathbb C$-subalgebras $ R_1, R_2$ of $ \mathbb C[x_1, \ldots, x_n]$, is their intersection also finitely generated?" We show that the smallest value of $ n$ for which there is a counterexample is $ 2$ in the general case, and $ 3$ in the case that $ R_1$ and $ R_2$ are integrally closed. We also explain the relation of this question to the problem of constructing algebraic compactifications of $ \mathbb C^n$ and to the moment problem on semialgebraic subsets of $ \mathbb R^n$. The counterexample for the general case is a simple modification of a construction of Neena Gupta, whereas the counterexample for the case of integrally closed subalgebras uses the theory of normal analytic compactifications of $ \mathbb C^2$ via key forms of valuations centered at infinity.
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18. On a Triply Periodic Polyhedral Surface Whose Vertices are Weierstrass Points
    Dami Lee
    Arnold Math J. (2017) 3:3, 319–331
    Received: 3 May 2016 / Revised: 12 March 2017 / Accepted: 23 March 2017
    Abstract
    
    In this paper, we will construct an example of a closed Riemann surface $ X$ that can be realized as a quotient of a triply periodic polyhedral surface $ \Pi \subset \mathbb R^3$ where the Weierstrass points of $ X$ coincide with the vertices of $ \Pi.$ First we construct $ \Pi$ by attaching Platonic solids in a periodic manner and consider the surface of this solid. Due to periodicity we can find a compact quotient of this surface. The symmetries of $ X$ allow us to construct hyperbolic structures and various translation structures on $ X$ that are compatible with its conformal type. The translation structures are the geometric representations of the holomorphic 1-forms of $ X.$ Via the basis of 1-forms we find an explicit algebraic description of the surface that suggests the Fermat's quartic. Moreover the 1-forms allow us to identify the Weierstrass points.
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19. Dynamics of Polynomial Diffeomorphisms of $ \mathbb{C}^2$: Combinatorial and Topological Aspects
    Yutaka Ishii
    Arnold Math J. (2017) 3:1, 119–173
    Received: 7 October 2016 / Revised: 10 February 2017 / Accepted: 23 March 2017
    Abstract
    
    The Fig. 1 was drawn by Shigehiro Ushiki using his software called HenonExplorer . This complicated object is the Julia set of a complex Hénon map $ f_{c, b}(x, y)=(x^2+c-by, x)$ defined on $ \mathbb{C}^2$ together with its stable and unstable manifolds, hence it is a fractal set in the real $ 4$-dimensional space! The purpose of this paper is to survey some results, questions and problems on the dynamics of polynomial diffeomorphisms of $ \mathbb{C}^2$ including complex Hénon maps with an emphasis on the combinatorial and topological aspects of their Julia sets.
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20. Vanishing Cycles and Cartan Eigenvectors
    Laura Brillon, Revaz Ramazashvili, Vadim Schechtman, Alexander Varchenko
    Arnold Math J. (2017) 3:2, 251–280
    Received: 19 December 2015 / Revised: 11 July 2016 / Accepted: 20 July 2016
    Abstract
    
    Using the ideas coming from the singularity theory, we study the eigenvectors of the Cartan matrices of finite root systems, and of q-deformations of these matrices.
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21. Polynomial Splitting Measures and Cohomology of the Pure Braid Group
    Trevor Hyde, Jeffrey C. Lagarias
    Arnold Math J. (2017) 3:2, 219–249
    Received: 10 August 2016 / Revised: 27 December 2016 / Accepted: 1 February 2017
    Abstract
    
    We study for each $n$ a one-parameter family of complex-valued measures on the symmetric group $S_n$, which interpolate the probability of a monic, degree $n$, square-free polynomial in $\mathbb F_q[x]$ having a given factorization type. For a fixed factorization type, indexed by a partition $\lambda$ of $n$, the measure is known to be a Laurent polynomial. We express the coefficients of this polynomial in terms of characters associated to $S_n$-subrepresentations of the cohomology of the pure braid group $H^{\bullet}(P_n, \mathbb Q)$. We deduce that the splitting measures for all parameter values $z= -\frac{1}{m}$ (resp. $z= \frac{1}{m}$), after rescaling, are characters of $S_n$-representations (resp. virtual $S_n$-representations).
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22. Combinatorics of the Lipschitz Polytope
    J. Gordon, F. Petrov
    Arnold Math J. (2017) 3:2, 205–218
    Received: 18 July 2016 / Revised: 13 November 2016 / Accepted: 17 January 2017
    Abstract
    
    Let $ \rho$ be a metric on the set $ X=\{1,2,\dots,n+1\}$. Consider the $ n$- dimensional polytope of functions $ f:X\rightarrow \mathbb{R}$, which satisfy the conditions $ f(n+1)=0$, $ |f(x)-f(y)|\leq \rho(x,y)$. The question on classifying metrics depending on the combinatorics of this polytope have been recently posed by (Vershik, Arnold Math J 1(1):75-81, 2015). We prove that for any "generic" metric the number of $ (n-m)$-dimensional faces, $ 0\leq m\leq n$, equals $ \binom{n+m}{m,m,n-m}=(n+m)!/m!m!(n-m)!$. This fact is intimately related to regular triangulations of the root polytope (convex hull of the roots of $ A_n$ root system). Also we get two-sided estimates for the logarithm of the number of Vershik classes of metrics: $ n^3\log n$ from above and $ n^2$ from below.
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23. Convex Shapes and Harmonic Caps
    Laura DeMarco, Kathryn Lindsey
    Arnold Math J. (2017) 3:1, 97–117
    Received: 4 February 2016 / Revised: 30 November 2016 / Accepted: 23 December 2016
    Abstract
    
    Any planar shape $P\subset{\mathbb{C}}$ can be embedded isometrically as part of the boundary surface $S$ of a convex subset of $\mathbb{R}^{3}$ such that $\partial P$ supports the positive curvature of $S$. The complement $Q=S{\setminus}P$ is the associated cap. We study the cap construction when the curvature is harmonic measure on the boundary of $({\hat{{\mathbb{C}}}}{\setminus}P,\infty)$. Of particular interest is the case when $P$ is a filled polynomial Julia set and the curvature is proportional to the measure of maximal entropy.
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24. Random Chain Complexes
    Viktor L. Ginzburg, Dmitrii V. Pasechnik
    Arnold Math J. (2017) 3:2, 197–204
    Received: 16 March 2016 / Revised: 9 December 2016 / Accepted: 23 December 2016
    Abstract
    
    We study random, finite-dimensional, ungraded chain complexes over a finite field and show that for a uniformly distributed differential a complex has the smallest possible homology with the highest probability: either zero or one-dimensional homology depending on the parity of the dimension of the complex. We prove that as the order of the field goes to infinity the probability distribution concentrates in the smallest possible dimension of the homology. On the other hand, the limit probability distribution, as the dimension of the complex goes to infinity, is a super-exponentially decreasing, but strictly positive, function of the dimension of the homology.
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25. The $4n^{2}$-Inequality for Complete Intersection Singularities
    Aleksandr V. Pukhlikov
    Arnold Math J. (2017) 3:2, 187–196
    Received: 11 July 2016 / Revised: 25 October 2016 / Accepted: 17 November 2016
    Abstract
    
    The famous $4n^{2}$-inequality is extended to generic complete intersection singularities: it is shown that the multiplicity of the self-intersection of a mobile linear system with a maximal singularity is greater than $4n^{2}\mu$, where $\mu$ is the multiplicity of the singular point.
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26. The Geometry of Axisymmetric Ideal Fluid Flows with Swirl
    Pearce Washabaugh, Stephen C. Preston
    Arnold Math J. (2017) 3:2, 175–185
    Received: 3 February 2016 / Revised: 21 August 2016 / Accepted: 15 October 2016
    Abstract
    
    The sectional curvature of the volume preserving diffeomorphism group of a Riemannian manifold $M$ can give information about the stability of inviscid, incompressible fluid flows on $M$. We demonstrate that the submanifold of the volumorphism group of the solid flat torus generated by axisymmetric fluid flows with swirl, denoted by $\mathcal{D}_{\mu,E}(M)$, has positive sectional curvature in every section containing the field $X=u(r)\partial_{\theta}$ iff $\partial_{r}(ru^{2})>0$. This is in sharp contrast to the situation on $\mathcal{D}_{\mu}(M)$, where only Killing fields $X$ have nonnegative sectional curvature in all sections containing it. We also show that this criterion guarantees the existence of conjugate points on $\mathcal{D}_{\mu,E}(M)$ along the geodesic defined by $X$.
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27. On Postsingularly Finite Exponential Maps
    Walter Bergweile
    Arnold Math J. (2017) 3:1, 83–95
    Received: 5 December 2015 / Revised: 15 August 2016 / Accepted: 6 September 2016
    Abstract
    
    We consider parameters $\lambda$ for which 0 is preperiodic under the map $z\mapsto\lambda e^{z}$. Given $k$ and $l$, let $n(r)$ be the number of $\lambda$ satisfying $0<|\lambda|\leq r$ such that 0 is mapped after $k$ iterations to a periodic point of period $l$. We determine the asymptotic behavior of $n(r)$ as $r$ tends to $\infty$.
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28. Spherical Rectangles
    Alexandre Eremenko, Andrei Gabrielov
    Arnold Math J. (2016) 2:4, 463–486
    Received: 24 January 2016 / Revised: 9 August 2016 / Accepted: 30 August 2016
    Abstract
    
    We study spherical quadrilaterals whose angles are odd multiples of $\pi/2$, and the equivalent accessory parameter problem for the Heun equation. We obtain a classification of these quadrilaterals up to isometry. For given angles, there are finitely many one-dimensional continuous families which we enumerate. In each family the conformal modulus is either bounded from above or bounded from below, but not both, and the numbers of families of these two types are equal. The results can be translated to classification of Heunâ~@~Ys equations with real parameters, whose exponent differences are odd multiples of $1/2$, with unitary monodromy.
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29. q-Polynomial Invariant of Rooted Trees
    Jözef H. Przytycki
    Arnold Math J. (2016) 2:4, 449–461
    Received: 7 December 2015 / Revised: 28 July 2016 / Accepted: 2 August 2016
    Abstract
    
    We describe in this note a new invariant of rooted trees. We argue that the invariant is interesting on it own, and that it has connections to knot theory and homological algebra. However, the real reason that we propose this invariant to readers of Arnold Journal of Mathematics is that we deal here with an elementary, interesting, new mathematics, and after reading this essay readers can take part in developing the topic, inventing new results and connections to other disciplines of mathematics, and likely, statistical mechanics, and combinatorial biology.
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30. On the Roots of a Hyperbolic Polynomial Pencil
    Victor Katsnelson
    Arnold Math J. (2016) 2:4, 439–448
    Received: 03 May 2016 / Accepted: 20 July 2016 / Published Online: 02 August 2016
    Abstract
    

    Let $ \nu_0(t),\nu_1(t),\ldots,\nu_n(t)$ be the roots of the equation $ R(z)=t$, where $ R(z)$ is a rational function of the form

    $\displaystyle \begin{eqnarray*} R(z)=z-\sum\limits_{k=1}^n\frac{\alpha_k}{z-\mu_k}, \end{eqnarray*}$

    $ \mu_k$ are pairwise distinct real numbers, $ \alpha_k> 0,\,1\leq{}k\leq{}n$. Then for each real $ \xi$, the function $ e^{\xi\nu_0(t)}+e^{\xi\nu_1(t)}+\,\cdots\,+e^{\xi\nu_n(t)}$ is exponentially convex on the interval $ -\infty< t< \infty$.

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31. A Generalisation of the Cauchy-Kovalevskaïa Theorem
    Mauricio Garay
    Arnold Math J. (2016) 2:3, 407–438
    Received: 1 July 2015 / Revised: 15 May 2016 / Accepted: 23 June 2016 / Published Online: 09 August 2016
    Abstract
    
    We prove that time evolution of a linear analytic initial value problem leadsto sectorial holomorphic solutions in time.
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32. A Classification of Spherical Curves Based on Gauss Diagrams
    Guy Valette
    Arnold Math J. (2016) 2:3, 383–405
    Received: 28 August 2015 / Revised: 4 May 2016 / Accepted: 23 June 2016 / Published Online: 11 July 2016
    Abstract
    
    We consider generic smooth closed curves on the sphere $S^{2}$. These curves (oriented or not) are classified relatively to the group $\mbox{Diff}(S^{2})$ or its subgroup $\mbox{Diff}^{+}(S^{2})$, with the Gauss diagrams as main tool. V. I. Arnold determined the numbers of orbits of curves with $n$ double points when $n<6$. This paper explains how a preliminary classification of the Gauss diagrams of order 5, 6 and 7 allows to draw up the list of the realizable chord diagrams of these orders. For each such diagram $\Gamma$ and for each Arnold symmetry type $T$, we determine the number of orbits of spherical curves of type $T$ realizing $\Gamma$. As a consequence, we obtain the total numbers of curves (oriented or not) with 6 or 7 double points on the sphere (oriented or not) and also the number of curves with special properties (e.g. having no simple loop).
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33. On Malfatti's Marble Problem
    Uuganbaatar Ninjbat
    Arnold Math J. (2016) 2:3, 309–327
    Received: 3 April 2015 / Revised: 9 April 2016 / Accepted: 20 June 2016 / Published Online: 11 July 2016
    Abstract
    
    Consider the problem of finding three non-overlapping circles in a given triangle with the maximum total area. This is Malfatti's marble problem, and it is known that the greedy arrangement is the solution. In this paper, we provide a simpler proof of this result by synthesizing earlier insights with more recent developments. We also discuss some related geometric extremum problems, and show that the greedy arrangement solves the problem of finding two non-overlapping circles in a tangential polygon with the maximum total radii and/or area. In the light of this discussion, we formulate a natural extension of Melissen's conjecture.
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34. Volume Polynomials and Duality Algebras of Multi-Fans
    Anton Ayzenberg, Mikiya Masuda
    Arnold Math J. (2016) 2:3, 329–381
    Received: 17 October 2015 / Revised: 12 November 2015 / Accepted: 23 June 2016 / Published Online: 11 July 2016
    Abstract
    
    We introduce a theory of volume polynomials and corresponding duality algebras of multi-fans. Any complete simplicial multi-fan $\Delta$ determines a volume polynomial $V_\Delta$ whose values are the volumes of multi-polytopes based on $\Delta$. This homogeneous polynomial is further used to construct a Poincare duality algebra $\mathcal{A}^*(\Delta)$. We study the structure and properties of $V_\Delta$ and $\mathcal{A}^*(\Delta)$ and give applications and connections to other subjects, such as Macaulay duality, Novik-Swartz theory of face rings of simplicial manifolds, generalizations of Minkowski's theorem on convex polytopes, cohomology of torus manifolds, computations of volumes, and linear relations on the powers of linear forms. In particular, we prove that the analogue of the $g$-theorem does not hold for multi-polytopes.
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35. Generalizations of Tucker-Fan-Shashkin Lemmas
    Oleg R. Musin
    Arnold Math J. (2016) 2:3, 299–308
    Received: 26 November 2014 / Revised: 25 April 2016 / Accepted: 27 May 2016 / Published online: 16 June 2016
    Abstract
    
    Tucker and Ky Fan's lemma are combinatorial analogs of the Borsuk-Ulam theorem (BUT). In 1996, Yu. A. Shashkin proved a version of Fan's lemma, which is a combinatorial analog of the odd mapping theorem (OMT). We consider generalizations of these lemmas for BUT-manifolds, i.e. for manifolds that satisfy BUT. Proofs rely on a generalization of the OMT and on a lemma about the doubling of manifolds with boundaries that are BUT-manifolds.
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36. Strange Duality Between Hypersurface and Complete Intersection Singularities
    Wolfgang Ebeling, Atsushi Takahashi
    Arnold Math J. (2016) 2:3, 277–298
    Received: 22 September 2015 / Revised: 9 May 2016 / Accepted: 12 May 2016 / Published online: 24 May 2016
    Abstract
    
    W. Ebeling and C. T. C. Wall discovered an extension of Arnold's strange duality embracing on one hand series of bimodal hypersurface singularities and on the other, isolated complete intersection singularities. In this paper, we derive this duality from the mirror symmetry and the Berglund-Hübsch transposition of invertible polynomials.
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37. The Coadjoint Operator, Conjugate Points, and the Stability of Ideal Fluids
    James Benn
    Arnold Math J. (2016) 2:2, 249–266
    Received: 11 August 2015 / Revised: 25 February 2016 / Accepted: 11 April 2016 / Published online: 18 May 2016
    Abstract
    
    We give a new description of the coadjoint operator $Ad^*_{\eta^{-1}(t)}$ along a geodesic $\eta(t)$ of the $L^2$ metric in the group of volume-preserving diffeomorphisms, important in hydrodynamics. When the underlying manifold is two dimensional the coadjoint operator is given by the solution operator to the linearized Euler equations modulo a compact operator; when the manifold is three dimensional the coadjoint operator is given by the solution operator to the linearized Euler equations plus a bounded operator. We give two applications of this result when the underlying manifold is two dimensional: conjugate points along geodesics of the $L^2$ metric are characterized in terms of the coadjoint operator and thus determining the conjugate locus is a purely algebraic question. We also prove that Eulerian and Lagrangian stability of the $2D$ Euler equations are equivalent and that instabilities in the $2D$ Euler equations are contained and small.
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38. Internal Addresses of the Mandelbrot Set and Galois Groups of Polynomials
    Dierk Schleicher
    Received: 15 October 2015 / Revised: 9 February 2016 / Accepted: 7 April 2016 / Published online: 02 August 2016
    Abstract
    
    We describe an interesting interplay between symbolic dynamics, the structure of the Mandelbrot set, permutations of periodic points achieved by analytic continuation, and Galois groups of certain polynomials. Internal addresses are a convenient and efficient way of describing the combinatorial structure of the Mandelbrot set, and of giving geometric meaning to the ubiquitous kneading sequences in human-readable form (Sects. 3 and 4). A simple extension, angled internal addresses, distinguishes combinatorial classes of the Mandelbrot set and in particular distinguishes hyperbolic components in a concise and dynamically meaningful way. This combinatorial description of the Mandelbrot set makes it possible to derive existence theorems for certain kneading sequences and internal addresses in the Mandelbrot set (Sect. 6) and to give an explicit description of the associated parameters. These in turn help to establish some algebraic results about permutations of periodic points and to determine Galois groups of certain polynomials (Sect. 7). Through internal addresses, various areas of mathematics are thus related in this manuscript, including symbolic dynamics and permutations, combinatorics of the Mandelbrot set, and Galois groups.
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39. Non-avoided Crossings for $n$-Body Balanced Configurations in $\mathbb R^3$ Near a Central Configuration
    Alain Chenciner
    Arnold Math J. (2016) 2:2, 213–248
    Received: 4 September 2015 / Revised: 14 January 2016 / Accepted: 10 March 2016 / Published online: 8 April 2016
    Abstract
    
    The balanced configurations are those $n$-body configurations which admit a relative equilibrium motion in a Euclidean space $E$ of high enough dimension $2 p$. They are characterized by the commutation of two symmetric endomorphisms of the $(n-1)$-dimensional Euclidean space of codispositions, the intrinsic inertia endomorphism $B$ which encodes the shape and the Wintner-Conley endomorphism $A$ which encodes the forces. In general, $p$ is the dimension $d$ of the configuration, which is also the rank of B. Lowering to $2(d-1)$ the dimension of $E$ occurs when the restriction of $A$ to the (invariant) image of $B$ possesses a double eigenvalue. It is shown that, while in the space of all $d\times d$ symmetric endomorphisms, having a double eigenvalue is a condition of codimension 2 (the avoided crossings of physicists), here it becomes of codimension 1 provided some condition $(H)$ is satisfied. As the condition is always satisfied for configurations of the maximal dimension (i.e. if $d = n-1$), this implies in particular the existence, in the neighborhood of the regular tetrahedron configuration of four bodies with no three of the masses equal, of exactly three families of balanced configurations which admit relative equilibrium motion in a four dimensional space.
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40. Geodesics on Regular Polyhedra with Endpoints at the Vertices
    Dmitry Fuchs
    Arnold Math J. (2016) 2:2, 201–211
    Received: 3 October 2015 / Revised: 23 October 2015 / Accepted: 3 March 2016 / Published online: 23 March 2016
    Abstract
    
    In a recent work of Davis et al. (2016), the authors consider geodesics on regular polyhedra which begin and end at vertices (and do not touch other vertices). The cases of regular tetrahedra and cubes are considered. The authors prove that (in these cases) a geodesic as above never begins at ends at the same vertex and compute the probabilities with which a geodesic emanating from a given vertex ends at every other vertex. The main observation of the present article is that there exists a close relation between the problem considered in Davis et al. (2016) and the problem of classification of closed geodesics on regular polyhedra considered in articles (Fuchs and Fuchs, Mosc Math J 7:265-279, 2007; Fuchs, Geom Dedic 170:319-333, 2014). This approach yields different proofs of result of Davis et al. (2016) and permits to obtain similar results for regular octahedra and icosahedra (in particular, such a geodesic never ends where it begins).
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41. On Foliations in Neighborhoods of Elliptic Curves
    M. Mishustin
    Arnold Math J. (2016) 2:2, 195–199
    Received: 13 April 2015 / Revised: 24 August 2015 / Accepted: 13 January 2016 / Published online: 26 January 2016
    Abstract
    
    A counterexample is given to a conjecture from the comments to Arnold's problem 1989-11 about the existence of a tangent foliation in a zero type neighborhood of an elliptic curve.
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42. Skewers
    Serge Tabachnikov
    Arnold Math J. (2016) 2:2, 171–193
    Received: 19 September 2015 / Revised: 29 December 2015 / Accepted: 11 January 2016 / Published online: 27 January 2016
    Abstract
    
    The skewer of a pair of skew lines in space is their common perpendicular. To configuration theorems of plane projective geometry involving points and lines (such as Pappus or Desargues) there correspond configuration theorems in space: points and lines in the plane are replaced by lines is space, the incidence between a line and a point translates as the intersection of two lines at right angle, and the operations of connecting two points by a line or by intersecting two lines at a point translate as taking the skewer of two lines. These configuration theorems hold in elliptic, Euclidean, and hyperbolic geometries. This correspondence principle extends to plane configuration theorems involving polarity. For example, the theorem that the three altitudes of a triangle are concurrent corresponds to the Petersen-Morley theorem that the common normals of the opposite sides of a space right-angled hexagon have a common normal. We define analogs of plane circles (they are 2-parameter families of lines in space) and extend the correspondence principle to plane theorems involving circles. We also discuss the skewer versions of the Sylvester problem: given a finite collection of pairwise skew lines such that the skewer of any pair intersects at least one other line at right angle, do all lines have to share a skewer? The answer is positive in the elliptic and Euclidean geometries, but negative in the hyperbolic one.
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43. An Invariant of Colored Links via Skein Relation
    Francesca Aicardi
    Arnold Math J. (2016) 2:2, 159–169
    Received: 30 May 2015 / Accepted: 14 December 2015 / Published online: 1 March 2016
    Abstract
    
    In this note, we define a polynomial invariant for colored links by a skein relation. It specializes to the Jones polynomial for classical links.
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44. N-Division Points of Hypocycloids
    N. Mani, S. Rubinstein-Salzedo
    Arnold Math J. (2016) 2:2, 149–158
    Received: 4 May 2015 / Revised: 19 October 2015 / Accepted: 6 December 2015 / Published online: 04 January 2016
    Abstract
    
    We show that the n-division points of all rational hypocycloids are constructible with an unmarked straightedge and compass for all integers n, given a pre-drawn hypocycloid. We also consider the question of constructibility of n-division points of hypocycloids without a pre-drawn hypocycloid in the case of a tricuspoid, concluding that only the 1, 2, 3, and 6-division points of a tricuspoid are constructible in this manner.
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45. Polynomials Invertible in k-Radicals
    Y. Burda, A. Khovanskii
    Arnold Math J. (2016) 2:1, 121–138
    Received: 18 May 2015 / Revised: 22 December 2015 / Accepted: 25 December 2015 / Published online: 09 February 2016 2015
    Abstract
    
    A classic result of Ritt describes polynomials invertible in radicals: they are compositions of power polynomials, Chebyshev polynomials and polynomials of degree at most 4. In this paper we prove that a polynomial invertible in radicals and solutions of equations of degree at most k is a composition of power polynomials, Chebyshev polynomials, polynomials of degree at most k and, if \(k\le 14\), certain polynomials with exceptional monodromy groups. A description of these exceptional polynomials is given. The proofs rely on classification of monodromy groups of primitive polynomials obtained by Müller based on group-theoretical results of Feit and on previous work on primitive polynomials with exceptional monodromy groups by many authors.
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46. Generalized Plumbings and Murasugi Sums
    B. Ozbagci, P. Popescu-Pampu
    Arnold Math J. (2016) 2:1, 69–119
    Received: 6 July 2015 / Revised: 28 October 2015 / Accepted: 23 November 2015 / Published online: 23 December 2015
    Abstract
    
    We propose a generalization of the classical notions of plumbing and Murasugi summing operations to smooth manifolds of arbitrary dimensions, so that in this general context Gabai's credo "the Murasugi sum is a natural geometric operation" holds. In particular, we prove that the sum of the pages of two open books is again a page of an open book and that there is an associated summing operation of Morse maps. We conclude with several open questions relating this work with singularity theory and contact topology.
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47. The Gabrielov-Khovanskii Problem for Polynomials
    A. V. Pukhlikov
    Arnold Math J. (2016) 2:1, 29–68
    Received: 19 June 2015 / Revised: 24 October 2015 / Accepted: 6 November 2015 / Published online: 27 November 2015
    Abstract
    
    We state and consider the Gabrielov-Khovanskii problem of estimating the multiplicity of a common zero for a tuple of polynomials in a subvariety of a given codimension in the space of tuples of polynomials. For a bounded codimension we obtain estimates of the multiplicity of the common zero, which are close to optimal ones. We consider certain generalizations and open questions.
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48. Galois Correspondence Theorem for Picard-Vessiot Extensions
    T. Crespo, Z. Hajto, E. Sowa-Adamus
    Arnold Math J. (2016) 2:1, 21–27
    Received: 16 April 2015 / Revised: 23 September 2015 / Accepted: 23 October 2015 / Published online: 03 November 2015
    Abstract
    
    For a homogeneous linear differential equation defined over a differential field K, a Picard-Vessiot extension is a differential field extension of K differentially generated by a fundamental system of solutions of the equation and not adding constants. When K has characteristic 0 and the field of constants of K is algebraically closed, it is well known that a Picard-Vessiot extension exists and is unique up to K-differential isomorphism. In this case the differential Galois group is defined as the group of K-differential automorphisms of the Picard-Vessiot extension and a Galois correspondence theorem is settled. Recently, Crespo, Hajto and van der Put have proved the existence and unicity of the Picard-Vessiot extension for formally real (resp. formally p-adic) differential fields with a real closed (resp. p-adically closed) field of constants. This result widens the scope of application of Picard-Vessiot theory beyond the complex field. It is then necessary to give an accessible presentation of Picard-Vessiot theory for arbitrary differential fields of characteristic zero which eases its use in physical or arithmetic problems. In this paper, we give such a presentation avoiding both the notions of differential universal extension and specializations used by Kolchin and the theories of schemes and Hopf algebras used by other authors. More precisely, we give an adequate definition of the differential Galois group as a linear algebraic group and a new proof of the Galois correspondence theorem for a Picard-Vessiot extension of a differential field with non algebraically closed field of constants, which is more elementary than the existing ones.
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49. On Maps Taking Lines to Plane Curves
    V. Petrushchenko, V. Timorin
    Arnold Math J. (2016) 2:1, 1–20
    Received: 24 March 2015 / Accepted: 16 October 2015 / Published online: 03 November 2015
    Abstract
    
    We study cubic rational maps that take lines to plane curves. A complete description of such cubic rational maps concludes the classification of all planarizations, i.e., maps taking lines to plane curves.
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50. Solvability of Linear Differential Systems with Small Exponents in the Liouvillian Sense
    R. R. Gontsov, I. V. Vyugin
    Arnold Math J. (2015) 1:4, 445–471
    Received: 25 November 2014 / Revised: 20 August 2015 / Accepted: 11 November 2015 / Published online: 26 November 2015
    Abstract
    
    The paper is devoted to solvability of linear differential systems by quadratures, one of the classical problems of differential Galois theory. As known, solvability of a system depends entirely on properties of its differential Galois group. However, detecting solvability or non-solvability of a given system is a difficult problem, because the dependence of its differential Galois group on the coefficients of the system remains unknown. We consider systems with regular singular points as well as those with non-resonant irregular ones, whose exponents (respectively, so-called formal exponents in the irregular case) are sufficiently small. It turns out that for systems satisfying such restrictions criteria of solvability can be given in terms of the coefficient matrix.
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51. Finite and Infinitesimal Flexibility of Semidiscrete Surfaces
    O. Karpenkov
    Arnold Math J. (2015) 1:4, 403–444
    Received: 18 April 2015 / Revised: 28 July 2015 / Accepted: 24 August 2015 / Published online: 3 September 2015
    Abstract
    
    In this paper we study infinitesimal and finite flexibility for regular semidiscrete surfaces. We prove that regular 2-ribbon semidiscrete surfaces have one degree of infinitesimal and finite flexibility. In particular we write down a system of differential equations describing isometric deformations in the case of existence. Further we find a necessary condition of 3-ribbon infinitesimal flexibility. For an arbitrary \(n\ge 3\) we prove that every regular n-ribbon surface has at most one degree of finite/infinitesimal flexibility. Finally, we discuss the relation between general semidiscrete surface flexibility and 3-ribbon subsurface flexibility. We conclude this paper with one surprising property of isometric deformations of developable semidiscrete surfaces.
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52. Constructive Geometrization of Thurston Maps and Decidability of Thurston Equivalence
    N. Selinger, M. Yampolsky
    Arnold Math J. (2015) 1:4, 361–402
    Received: 14 November 2014 / Revised: 3 June 2015 / Accepted: 4 August 2015 / Published online: 7 September 2015
    Abstract
    
    The key result in the present paper is a direct analogue of the celebrated Thurston's Theorem Douady and Hubbard (Acta Math 171:263-297, 1993) for marked Thurston maps with parabolic orbifolds. Combining this result with previously developed techniques, we prove that every Thurston map can be constructively geometrized in a canonical fashion. As a consequence, we give a partial resolution of the general problem of decidability of Thurston equivalence of two postcritically finite branched covers of \(S^2\) (cf. Bonnot et al. Moscow Math J 12:747-763, 2012).
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53. Bollobás – Riordan and Relative Tutte Polynomials
    C. Butler, S. Chmutov
    Arnold Math J. (2015) 1:3, 283–298
    Received: 8 December 2014 / Revised: 29 June 2015 / Accepted: 5 July 2015 / Published online: 28 July 2015
    Abstract
    
    We establish a relation between the Bollobás – Riordan polynomial of a ribbon graph with the relative Tutte polynomial of a plane graph obtained from the ribbon graph using its projection to the plane in a nontrivial way. Also we give a duality formula for the relative Tutte polynomial of dual plane graphs and an expression of the Kauffman bracket of a virtual link as a specialization of the relative Tutte polynomial.
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54. Critical Set of the Master Function and Characteristic Variety of the Associated Gauss-Manin Differential Equations
    A. Varchenko
    Arnold Math J. (2015) 1:3, 253–282
    Received: 7 November 2014 / Accepted: 15 June 2015 / Published online: 7 July 2015
    Abstract
    
    We consider a weighted family of n parallelly transported hyperplanes in a k-dimensional affine space and describe the characteristic variety of the Gauss–Manin differential equations for associated hypergeometric integrals. The characteristic variety is given as the zero set of Laurent polynomials, whose coefficients are determined by weights and the associated point in the Grassmannian Gr(k, n). The Laurent polynomials are in involution. These statements generalize (Varchenko, Mathematics 2:218-231, 2014), where such a description was obtained for a weighted generic family of parallelly transported hyperplanes. An intermediate object between the differential equations and the characteristic variety is the algebra of functions on the critical set of the associated master function. We construct a linear isomorphism between the vector space of the Gauss–Manin differential equations and the algebra of functions. The isomorphism allows us to describe the characteristic variety. It also allowed us to define an integral structure on the vector space of the algebra and the associated (combinatorial) connection on the family of such algebras.
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55. The Exponential Map Near Conjugate Points In 2D Hydrodynamics
    G. Misiołek
    Arnold Math J. (2015) 1:3, 243–251
    Received: 14 January 2015 / Accepted: 3 May 2015 / Published online: 5 August 2015
    Abstract
    
    We prove that the weak-Riemannian exponential map of the \(L^2\) metric on the group of volume-preserving diffeomorphisms of a compact two-dimensional manifold is not injective in any neighbourhood of its conjugate vectors. This can be viewed as a hydrodynamical analogue of the classical result of Morse and Littauer.
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56. Homology of Spaces of Non-Resultant Homogeneous Polynomial Systems in ${\mathbb R}^2$ and ${\mathbb C}^2$
    V. A. Vassiliev
    Arnold Math J. (2015) 1:3, 233–242
    Received: 7 November 2014 / Accepted: 3 June 2015 / Published online: 11 August 2015
    Abstract
    
    The resultant variety in the space of systems of homogeneous polynomials of some given degrees consists of such systems having non-trivial solutions. We calculate the integer cohomology groups of all spaces of non-resultant systems of polynomials \({\mathbb R}^2 \rightarrow {\mathbb R}\), and also the rational cohomology rings of spaces of non-resultant systems and non-m-discriminant polynomials in \({\mathbb C}^2\).
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57. Local Invariants of Framed Fronts in 3-Manifolds
    V. Goryunov, S. Alsaeed
    Arnold Math J. (2015) 1:3, 211–232
    Received: 14 January 2015 / Accepted: 3 May 2015 / Published online: 5 August 2015
    Abstract
    
    The front invariants under consideration are those whose increments in generic homotopies are determined entirely by diffeomorphism types of local bifurcations of the fronts. Such invariants are dual to trivial codimension 1 cycles supported on the discriminant in the space of corresponding Legendrian maps. We describe the spaces of the discriminantal cycles (possibly non-trivial) for framed fronts in an arbitrary oriented 3-manifold, both for the integer and mod2 coefficients. For the majority of these cycles we find homotopy-independent interpretations which guarantee the triviality required. In particular, we show that all integer local invariants of Legendrian maps without corank 2 points are essentially exhausted by the numbers of points of isolated singularity types of the fronts.
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58. A Formula for the HOMFLY Polynomial of rational links
    Sergei Duzhin, Mikhail Shkolnikov
    Arnold Math J. (2015) 1:4, 345–359
    Received: 10 November 2014 / Accepted: 7 April 2015 / Published online: 24 April 2015
    Abstract
    
    We give an explicit formula for the HOMFLY polynomial of a rational link (in particular, knot) in terms of a special continued fraction for the rational number that defines the given link [after this work was accomplished, the authors learned about a paper by Nakabo (J. Knot Theory Ramif 11(4):565-574, 2002) where a similar result was proved. However, Nakabo's formula is different from ours, and his proof is longer and less clear].
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59. Abundance of 3-Planes on Real Projective Hypersurfaces
    S. Finashin, V. Kharlamov
    Arnold Math J. (2015) 1:3, 171–199
    Received: 7 November 2014 / Accepted: 2 May 2015 / Published online: 2 June 2015
    Abstract
    
    We show that a generic real projective $n$-dimensional hypersurface of odd degree $d$, such that $4(n-2) ={{d+3}\choose3}$, contains ``many'' real 3-planes, namely, in the logarithmic scale their number has the same rate of growth, $d^3\log d$, as the number of complex 3-planes. This estimate is based on the interpretation of a suitable signed count of the 3-planes as the Euler number of an appropriate bundle.
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    Erratum to: Abundance of 3-Planes on Real Projective Hypersurfaces
    Arnold Math J. (2015) 1:3, 343
    Published online: 31 July 2015
    Abstract
    
    When we published this article, there was a typo in the first line of Theorem 5.3.1. Please find the corrected text in the pdf. The publisher apologises for this mistake.
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60. On Local Weyl Equivalence of Higher Order Fuchsian Equations
    Shira Tanny, Sergei Yakovenko
    Arnold Math J. (2015) 1:2, 141–170
    Received: 26 December 2014 / Accepted: 15 April 2015/ Published online: 08 May 2015
    Abstract
    
    We study the local classification of higher order Fuchsian linear differential equations under various refinements of the classical notion of the "type of differential equation" introduced by Frobenius. The main source of difficulties is the fact that there is no natural group action generating this classification. We establish a number of results on higher order equations which are similar but not completely parallel to the known results on local (holomorphic and meromorphic) gauge equivalence of systems of first order equations.
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61. On an Equivariant Version of the Zeta Function of a Transformation
    S. M. Gusein-Zade, I. Luengo, A. Melle-Hernández
    Arnold Math J. (2015) 1:2, 127–140
    Received: 17 December 2014 / Accepted: 4 April 2015 / Published online: 28 April 2015
    Abstract
    
    Earlier the authors offered an equivariant version of the classical monodromy zeta function of a G-invariant function germ with a finite group G as a power series with the coefficients from the Burnside ring of the group G tensored by the field of rational numbers. One of the main ingredients of the definition was the definition of the equivariant Lefschetz number of a G-equivariant transformation given by W. Lück and J. Rosenberg. Here we use another approach to a definition of the equivariant Lefschetz number of a transformation and describe the corresponding notions of the equivariant zeta function. This zeta-function is a power series with the coefficients from the Burnside ring itself. We give an A'Campo type formula for the equivariant monodromy zeta function of a function germ in terms of a resolution. Finally we discuss orbifold versions of the Lefschetz number and of the monodromy zeta function corresponding to the two equivariant ones.
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62. Vortex Dynamics of Oscillating Flows
    V. A. Vladimirov, M. R. E. Proctor, D. W. Hughes
    Arnold Math J. (2015) 1:2, 113–126
    Received: 22 December 2014 / Accepted: 23 March 2015 / Published online: 10 April 2015
    Abstract
    
    We employ the method of multiple scales (two-timing) to analyse the vortex dynamics of inviscid, incompressible flows that oscillate in time. Consideration of distinguished limits for Euler's equation of hydrodynamics shows the existence of two main asymptotic models for the averaged flows: strong vortex dynamics (SVD) and weak vortex dynamics (WVD). In SVD the averaged vorticity is 'frozen' into the averaged velocity field. By contrast, in WVD the averaged vorticity is 'frozen' into the 'averaged velocity + drift'. The derivation of the WVD recovers the Craik-Leibovich equation in a systematic and quite general manner. We show that the averaged equations and boundary conditions lead to an energy-type integral, with implications for stability.
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63. Remarks on the Circumcenter of Mass
    Serge Tabachnikov, Emmanuel Tsukerman
    Arnold Math J. (2015) 1:2, 101–112
    Received: 15 December 2014 / Accepted: 23 March 2015 / Published online: 31 March 2015
    Abstract
    
    Suppose that to every non-degenerate simplex $\Delta\subset\mathbb R^n$ a 'center' $C(\Delta)$ is assigned so that the following assumptions hold:

    1. The map $\Delta\to C(\Delta)$ commutes with similarities and is invariant under the permutations of the vertices of the simplex;
    2. The map $\Delta\to \operatorname{Vol}(\Delta)C(\Delta)$ is polynomial in the coordinates of the vertices of the simplex.

    Then $C(\Delta)$ is an affine combination of the center of mass $CM(\Delta)$ and the circumcenter $CC(\Delta)$ of the simplex: $$ C(\Delta)=tCM(\Delta)+(1-t)CC(\Delta), $$ where the constant $t\in\mathbb R$ depends on the map $\Delta\mapsto C(\Delta)$ (and does not depend on the simplex $\Delta$).
    The motivation for this theorem comes from the recent study of the circumcenter of mass of simplicial polytopes by the authors and by A. Akopyan.
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64. Quadratic Cohomology
    A. A. Agrachev
    Arnold Math J. (2015) 1:1, 37–58
    Received: 10 November 2014 / Accepted: 16 December 2014
    Abstract
    
    We study homological invariants of smooth families of real quadratic forms as a step towards a "Lagrange multipliers rule in the large" that intends to describe topology of smooth maps in terms of scalar Lagrange functions.
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65. Riemannian Geometry of the Contactomorphism Group
    David G. Ebin, Stephen C. Preston
    Arnold Math J. (2015) 1:1, 5–36
    Received: 11 November 2014 / Accepted: 8 December 2014
    Abstract
    
    Given an odd-dimensional compact manifold and a contact form, we consider the group of contact transformations of the manifold (contactomorphisms) and the subgroup of those transformations that precisely preserve the contact form (quantomorphisms). If the manifold also has a Riemannian metric, we can consider the L² inner product of vector fields on it, which by restriction gives an inner product on the tangent space at the identity of each of the groups that we consider. We then obtain right-invariant metrics on both the contactomorphism and quantomorphism groups. We show that the contactomorphism group has geodesics at least for short time and that the quantomorphism group is a totally geodesic subgroup of it. Furthermore we show that the geodesics in this smaller group exist globally. Our methodology is to use the right invariance to derive an "Euler-Arnold" equation from the geodesic equation and to show using ODE methods that it has solutions which depend smoothly on the initial conditions. For global existence we then derive a "quasi-Lipschitz" estimate on the stream function, which leads to a Beale-Kato-Majda criterion which is automatically satisfied for quantomorphisms. Special cases of these Euler-Arnold equations are the Camassa-Holm equation (when the manifold is one-dimensional) and the quasi-geostrophic equation in geophysics.
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1. Proof of van der Waerden's Theorem in Nine Figures
   Ari Blondal, Veselin Jungić
   Arnold Math J. (2018) 4:161-168
      Received: 15 July 2018 / Revised: 19 August 2018 / Accepted: 24 August 2018 / Published online: 3 September 2018
   Abstract
   
   This note contains a proof of van der Waerden's theorem, "one of the most elegant pieces of mathematics ever produced," in nine figures. The proof follows van der Waerden's original idea to establish the existence of what are now called van der Waerden numbers by using double induction. It also contains ideas and terminology introduced by I. Leader and T. Tao.
   Download PDF of the paper (1200KB) .
2. Two-Valued Groups, Kummer Varieties, and Integrable Billiards
   V. M. Buchstaber, V. Dragović
   Arnold Math J. (2018) 4:27-57
   Received: 10 July 2017 / Revised: 30 November 2017 / Accepted: 10 March 2018 / Published online: 9 April 2018
   Abstract
   
   A natural and important question of study two-valued groups associated with hyperelliptic Jacobians and their relationship with integrable systems is motivated by seminal examples of relationship between algebraic two-valued groups related to elliptic curves and integrable systems such as elliptic billiards and celebrated Kowalevski top. The present paper is devoted to the case of genus 2, to the investigation of algebraic two-valued group structures on Kummer varieties. One of our approaches is based on the theory of $\sigma$-functions. It enables us to study the dependence of parameters of the curves, including rational limits. Following this line, we are introducing a notion of $n$-groupoid as natural multivalued analogue of the notion of topological groupoid. Our second approach is geometric. It is based on a geometric approach to addition laws on hyperelliptic Jacobians and on a recent notion of billiard algebra. Especially important is connection with integrable billiard systems within confocal quadrics. The third approach is based on the realization of the Kummer variety in the framework of moduli of semi-stable bundles, after Narasimhan and Ramanan. This construction of the two-valued structure is remarkably similar to the historically first example of topological formal two-valued group from 1971, with a significant difference: the resulting bundles in the 1971 case were "virtual", while in the present case the resulting bundles are effectively realizable.
   Download PDF of the paper (600KB) .
3. Proof of the Broué - Malle - Rouquier Conjecture in Characteristic Zero (After I. Losev and I. Marin - G. Pfeiffer)
   Pavel Etingof
   Arnold Math J. (2017) 3:3, 445–449
   Received: 3 March 2017 / Revised: 11 March 2017 / Accepted: 4 April 2017
   Abstract
   
   We explain a proof of the Broué – Malle – Rouquier conjecture on Hecke algebras of complex reflection groups, stating that the Hecke algebra of a finite complex reflection group $ W$ is free of rank $ |W|$ over the algebra of parameters, over a field of characteristic zero. This is based on previous work of Losev, Marin – Pfeiffer, and Rains and the author.
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4. Flows in Flatland: A Romance of Few Dimensions
   Gabriel Katz
   Arnold Math J. (2017) 3:2, 281–317
   Received: 2 March 2016 / Revised: 15 October 2016 / Accepted: 23 October 2016
   Abstract
   
   This paper is about gradient-like vector fields and flows they generate on smooth compact surfaces with boundary. We use this particular 2-dimensional setting to present and explain our general results about non-vanishing gradient-like vector fields on $n$-dimensional manifolds with boundary. We take advantage of the relative simplicity of 2-dimensional worlds to popularize our approach to the Morse theory on smooth manifolds with boundary. In this approach, the boundary effects take the central stage.
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5. Some Recent Generalizations of the Classical Rigid Body Systems
   Vladimir Dragović, Borislav Gajić
   Arnold Math J. (2016) 2:4, 511–578
   Received: 20 November 2014 / Revised: 13 July 2016 / Accepted: 25 August 2016 / Published online: 19 September 2016
   Abstract
   
   Some recent generalizations of the classical rigid body systems are reviewed. The cases presented include dynamics of a heavy rigid body fixed at a point in three-dimensional space, the Kirchhoff equations of motion of a rigid body in an ideal incompressible fluid as well as their higher-dimensional generalizations.
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6. Building Thermodynamics for Non-uniformly Hyperbolic Maps
   Vaughn Climenhaga, Yakov Pesin
   Research Exposition,   Received: 4 February 2016 / Accepted: 20 July 2016 / Published online: 09 August 2016
   Abstract
   
   We briefly survey the theory of thermodynamic formalism for uniformly hyperbolic systems, and then describe several recent approaches to the problem of extending this theory to non-uniform hyperbolicity. The first of these approaches involves Markov models such as Young towers, countable-state Markov shifts, and inducing schemes. The other two are less fully developed but have seen significant progress in the last few years: these involve coarse-graining techniques (expansivity and specification) and geometric arguments involving push-forward of densities on admissible manifolds.
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7. Kepler's Laws and Conic Sections
   A. Givental
   Arnold Math J. (2016) 2:1, 139–148
   Received: 5 July 2015 / Revised: 7 September 2015 / Accepted: 24 October 2015 / Published online: 23 December 2015
   Abstract
   
   The geometry of Kepler's problem is elucidated by lifting the motion from the (x, y)-plane to the cone \(r^2=x^2+y^2\).
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8. The Conley Conjecture and Beyond
   V. L. Ginzburg, B. Z. Gürel
   Arnold Math J. (2015) 1:3, 299–337
   Received: 25 November 2014 / Accepted: 19 May 2015 / Published online: 4 June 2015
   Abstract
   
   This is (mainly) a survey of recent results on the problem of the existence of infinitely many periodic orbits for Hamiltonian diffeomorphisms and Reeb flows. We focus on the Conley conjecture, proved for a broad class of closed symplectic manifolds, asserting that under some natural conditions on the manifold every Hamiltonian diffeomorphism has infinitely many (simple) periodic orbits. We discuss in detail the established cases of the conjecture and related results including an analog of the conjecture for Reeb flows, the cases where the conjecture is known to fail, the question of the generic existence of infinitely many periodic orbits, and local geometrical conditions that force the existence of infinitely many periodic orbits. We also show how a recently established variant of the Conley conjecture for Reeb flows can be applied to prove the existence of infinitely many periodic orbits of a low-energy charge in a non-vanishing magnetic field on a surface other than a sphere.
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The journal intends to publish interesting and understandable results in all areas of Mathematics. The following are the most desirable features of publications that will serve as selection criteria:

• Accessibility

  The journal articles should be accessible to a very wide community of mathematicians. Not only formal definitions that are necessary for understanding must be provided but also informal motivations even if they are well-known to the experts in the field. If a general statement is given, then the simplest examples of it are also welcome.

• Interdisciplinary and multidisciplinary mathematics

  We would like to have many research expositions that connect different mathematical subjects. Connections that are useful in both ways are of particular importance. Multidisciplinary research (even if the disciplines all belong to pure mathematics) is generally hard to evaluate, except for the most popular combinations such as algebraic geometry and mathematical physics, analysis and dynamical systems, algebra and combinatorics, and the like. For this reason, this kind of research is often under-represented in specialized mathematical journals. The ArMJ will try to compensate for this.

• Problems, objectives, work in progress

  Most scholarly publications present results of a research project in their "final" form, in which all posed questions are answered. Some open questions and conjectures may be even mentioned but the very process of mathematical discovery remains hidden. Following Arnold, we will try to unhide this process and made it public by encouraging the authors to include informal discussion of their motivation, possibly unsuccessful lines of attack, experimental data and close by research directions. The journal intends to publish well-motivated research problems on a rather regular basis. Problems do not need to be original; an old problem with a new and exciting motivation is worth re-stating. Following Arnold's principle, a general formulation is less desirable than the simplest partial case that is still unknown.

• Being interesting

  The most important requirement is that the article be interesting. It does not have to be limited by original research contributions of the author; however, the author's responsibility is to carefully acknowledge the authorship of all results. Neither does the article need to consist entirely of formal and rigorous arguments. It can contain parts, in which an informal author's understanding of the overall picture is presented; however, these parts must be clearly indicated. Including motivations, informal parts, descriptions of other lines of research, possibly conducted by other mathematicians, should serve this principal objective: being interesting.

There are many great mathematicians of the twentieth century. The choice of the name may look random (why not, say, "Gelfand Mathematical Journal"? - we are often asked) but we have very specific reasons for using the name of Vladimir Arnold.

1. The principles, according to which the journal operates, are most accurately associated with Vladimir Arnold. He had been actively promoting these or similar principles.

2. For many years, V. Arnold had been the Editor-in-Chief of the journal Functional Analysis and its Applications (FAA). In 2006, V. Arnold launched a new journal, Functional Analysis and Other Mathematics (FAOM). The initial composition of the ArMJ Editorial Board consists mostly of former editors of the FAOM.

3. Despite the close connections with the FAA and the FAOM, we decided to avoid mentioning "Functional Analysis" in the name of the journal. These names have appeared historically, and have nothing to do with scientific principles of the journals. More than that, the names are even confusing: not all mathematicians could guess that, say, Functional Analysis and its Applications welcomes papers in all areas of mathematics, including algebra and number theory. On the other hand, we wanted to have an indication of these connections in the name of the journal. The name of Vladimir Arnold serves as this indication.

The journal is published quarterly, every issue consists of 100-150 pages. Manuscripts should be submitted online at http://www.editorialmanager.com/armj. Accepted file formats are LaTeX source (preferred) and MS Word.

Submission of a manuscript implies: that the work described has not been published before; that it is not under consideration for publication anywhere else; that its publication has been approved by all co-authors, if any, as well as by the responsible authorities - tacitly or explicitly - at the institute where the work has been carried out.

Authors wishing to include figures, tables, or text passages that have already been published elsewhere are required to obtain permission from the copyright owner(s) for both the print and online format and to include evidence that such permission has been granted when submitting their papers. Any material received without such evidence will be assumed to originate from the authors.