Local Instability in Curved Flow

Expressions are obtained for the rates of change of the vorticitycomponents about the tangent, principal normal, and binormalof inviscid flow of arbitrary configuration with curvature andtorsion of the particle paths, and non-uniform density. If a steady flow pattern exists the vorticity changes are exactlythose required to carry the fluid particles through it. A smallrotational displacement of a fluid element about each of thesedirections is then considered separately and if the additionalrate of change of vorticity has the same sign and order of magnitudeas the displacement the motion is unstable locally regardlessof the motion elsewhere. The equations depend on four quantities: the curvature, torsion,velocity and vorticity at the point where stability is beinginvestigated. These four quantities define a helical vortex,with the same stability properties, in which the flow is equivalent.There is therefore special merit in studying this case. It is found that a vortex with helical particle paths is unstablefor rotation of the fluid elements about directions which liebetween the vorticity vector and the direction of the axis ofthe motion, when the density is uniform. More generally, themotion is unstable for these disturbances if the stagnationpressure decreases radially outwards. A gradient of axial velocitycomponent always causes some local instability, and the mostlikely (fastest growing) disturbance is one in which rotationsoccur around a line everywhere bisecting the angle between thevorticity vector and the direction of the axis. The analysis shows that in two dimensional circular flow themost unstable disturbances are toroidal and in general curvedflow in two dimensions (without torsion) the most likely disturbancesare rotations around the tangent, i.e. longitudinal rolls. The criteria obtained for local instability do not agree witha Richardson-type criterion for local stability, but both showthe destabilizing effect of a radiating gradient of axial velocity.     

Toroidal Geometry Stabilizing a Latitudinal Ring of Point Vortices on a Torus

We carry out the linear stability analysis of a polygonal ring configuration of N point vortices, called an N-ring, along the line of latitude \(\theta _0\) on a torus with the aspect ratio \(\alpha \). Deriving a criterion for the stability depending on the parameters N, \(\theta _0\) and \(\alpha \), we reveal how the aspect ratio \(\alpha \) contributes to the stability of the N-ring. While the N-ring necessarily becomes unstable when N is sufficiently large for fixed \(\alpha \), the stability is closely associated with the geometric property of the torus for variable \(\alpha \); for low aspect ratio \(\alpha \sim 1\), \(N=7\) is a critical number determining the stability of the N-ring when it is located along a certain range of latitudes, which is an analogous result to those in a plane and on a sphere. On the other hand, the stability is determined by the sign of curvature for high aspect ratio \(\alpha \gg 1\). That is to say, the N-ring is neutrally stable if it is located on the inner side of the toroidal surface with a negative curvature, while the N-ring on its outer side with a positive curvature is unstable. Furthermore, based on the linear stability analysis, we describe nonlinear evolution of the N-ring when it becomes unstable. It is difficult to deal with this problem, since the evolution equation of the N point vortices is formulated as a Hamiltonian system with N degrees of freedom, which is in general non-integrable. Thus, we reduce the Hamiltonian system to a simple integrable system by introducing a cyclic symmetry. Owing to this reduction, we successfully find some periodic orbits in the reduced system, whose local bifurcations and global transitions for variable \(\alpha \) are characterized in terms of the fundamental group of the torus.     

Stability of unduloid bridges with free boundary in a Euclidean slab

We study the stability of unduloids with free boundary in the domain B between two parallel hyperplanes in R~(n+1). If the unduloid has one half of period in B and is sufficiently close to a cylinder, then for 2≤n≤10, it is unstable; while for n≥11, it is stable. If the unduloid has two or more halves of period in B and is sufficiently close to a cylinder, then for all n≥2, it is unstable.     

On the Heyde theorem for some locally compact Abelian groups

According to the well-known Heyde theorem Gaussian distribution on the real line is characterized by the symmetry of the conditional distribution of one linear form in n independent random variables given another. For n = 2 we prove analogs of this theorem in the case when random variables take values in a locally compact Abelian group X, and coefficients of the linear forms are topological automorphisms of the group X.     

On Stability of Thomson’s Vortex <Emphasis Type="Italic">N</Emphasis>-gon in the Geostrophic Model of the Point Bessel Vortices

A stability analysis of the stationary rotation of a system of N identical point Bessel vortices lying uniformly on a circle of radius R is presented. The vortices have identical intensity Γ and length scale γ^?1 > 0. The stability of the stationary motion is interpreted as equilibrium stability of a reduced system. The quadratic part of the Hamiltonian and eigenvalues of the linearization matrix are studied. The cases for N = 2,..., 6 are studied sequentially. The case of odd N = 2?+1 ≥ 7 vortices and the case of even N = 2_n ≥ 8 vortices are considered separately. It is shown that the (2? + 1)-gon is exponentially unstable for 0 < γR<R_*(N). However, this (2? + 1)-gon is stable for γR ≥ R_*(N) in the case of the linearized problem (the eigenvalues of the linearization matrix lie on the imaginary axis). The even N = 2n ≥ 8 vortex 2n-gon is exponentially unstable for R > 0.     

Orbital stability of solitary waves for derivative nonlinear Schrödinger equation

In this paper, we show the orbital stability of solitons arising in the cubic derivative nonlinear Schrödinger equations. We consider the zero mass case that is not covered by earlier works. As this case enjoys L² scaling invariance, we expect orbital stability (up to scaling symmetry) in addition to spatial and phase translations. We also show a self-similar type blow up criterion of solutions with the critical mass 4π.     

On the Stability of the Zero Solution of a Second-Order Differential Equation under a Periodic Perturbation of the Center

Small periodic perturbations of the oscillator \(\ddot x + {x^{2n}}\) sgn x = Y(t, x, \(\dot x\)) are considered, where n < 1 is a positive integer and the right-hand side is a small perturbation periodic in t, which is an analytic function in \(\dot x\) and x in a neighborhood of the origin. New Lyapunov-type periodic functions are introduced and used to investigate the stability of the equilibrium position of the given equation. Sufficient conditions for asymptotic stability and instability are given.     

On the structure of large sum-free sets of integers

A set of integers is called sum-free if it contains no triple (x, y, z) of not necessarily distinct elements with x + y = z. In this paper, we provide a structural characterisation of sum-free subsets of {1, 2,..., n} of density at least 2/5 ? c, where c is an absolute positive constant. As an application, we derive a stability version of Hu’s Theorem [Proc. Amer. Math. Soc. 80 (1980), 711–712] about the maximum size of a union of two sum-free sets in {1, 2,..., n}. We then use this result to show that the number of subsets of {1, 2,..., n} which can be partitioned into two sum-free sets is Θ(2^4n/5), confirming a conjecture of Hancock, Staden and Treglown [arXiv:1701.04754].     

Semigroup and Metric Characteristics of Locally Primitive Matrices and Digraphs

The notion of local primitivity for a quadratic 0, 1-matrix of size n × n is extended to any part of the matrix which need not be a rectangular submatrix. A similar generalization is carried out for any set B of pairs of initial and final vertices of the paths in an n-vertex digraph, B ? {(i, j) : 1 ≤ i, j ≤ n}. We establish the relationship between the local B-exponent of a matrix (digraph) and its characteristics such as the cyclic depth and period, the number of nonprimitive matrices, and the number of nonidempotentmatrices in the multiplicative semigroup of all quadratic 0, 1-matrices of order n, etc. We obtain a criterion of B-primitivity and an upper bound for the B-exponent. We also introduce some new metric characteristics for a locally primitive digraph Γ: the k, r-exporadius, the k, r-expocenter, where 1 ≤ k, r ≤ n, and the matex which is defined as the matrix of order n of all local exponents in the digraph Γ. An example of computation of the matex is given for the n-vertex Wielandt digraph. Using the introduced characteristics, we propose an idea for algorithmically constructing realizable s-boxes (elements of round functions of block ciphers) with a relatively wide range of sizes.     

Group classification of differential-difference equations: Low-dimensional Lie algebras

Differential-difference equations of the form u? _n = F _n (t, u_n?1, u _n , u_n+1, u?_n?1, u? _n , u?_n+1) are classified according to their intrinsic Lie point symmetries, equivalence group and some low-dimensional Lie algebras including the Abelian symmetry algebras, nilpotent nonAbelian symmetry algebras, solvable symmetry algebras with nonAbelian nilradicals, solvable symmetry algebras with Abelian nilradicals and nonsolvable symmetry algebras. Here F _n is a nonlinear function of its arguments and the dot over u denotes differentiation with respect to t.     

Note on the locally product and almost locally product structures

This paper is concerned with a study of some of the properties of locally product and almost locally product structures on a differentiable manifold X _n of class C ^k . Every locally product space has certain almost locally product structures which transform the local tangent space to X _n at an arbitrary point P in a set fashion: this is studied in Theorem (2.2). Theorem (2.3) considers the nature of transformations that exist between two co-ordinate systems at a point whenever an almost locally product structure has the same local representation in each of these co-ordinate systems. A necessary and sufficient condition for X _n to be a locally product manifold is obtained in terms of the pseudo-group of co-ordinate transformations on X _n and the subpseudo-groups [cf., Theoren (2.1)]. Section 3 is entirely devoted to the study of integrable almost locally product structures.     

Distinguishing graphs with intermediate growth

A graph G is said to be 2-distinguishable if there is a 2-labeling of its vertices which is not preserved by any nontrivial automorphism of G. We show that every locally finte graph with infinite nite motion and growth at most \(\mathcal{O}\left( {2^{(1 - \varepsilon )\tfrac{{\sqrt n }}{2}} } \right)\) is 2-distinguishable. Infinite motion means that every automorphism moves infinitely many vertices and growth refers to the cardinality of balls of radius n.     

A local mean value theorem for functions on non-archimedean field extensions of the real numbers

In this paper, we review the definition and properties of locally uniformly differentiable functions on N, a non-Archimedean field extension of the real numbers that is real closed and Cauchy complete in the topology induced by the order. Then we define and study n-times locally uniform differentiable functions at a point or on a subset of N. In particular, we study the properties of twice locally uniformly differentiable functions and we formulate and prove a local mean value theorem for such functions.     

A criterion of smoothness at infinity for an arithmetic quotient of the future tube

Let Γ be an arithmetic group of affine automorphisms of the n-dimensional future tube T. It is proved that the quotient space T/Γ is smooth at infinity if and only if the group Γ is generated by reflections and the fundamental polyhedral cone (“Weyl chamber”) of the group dΓ in the future cone is a simplicial cone (which is possible only for n ≤ 10). As a consequence of this result, a smoothness criterion for the Satake–Baily–Borel compactification of an arithmetic quotient of a symmetric domain of type IV is obtained.     

On locally balanced gray codes

We consider locally balanced Gray codes.We say that a Gray code is locally balanced if every “short” subword in its transition sequence contains all letters of the alphabet |1, 2,..., n~. The minimal length of these subwords is the window width of the code. We show that for each n ≥ 3 there exists a Gray code with window width at most n + 3?log n?.     

The inertial motion of a <Emphasis Type="Italic">roller racer</Emphasis>

This paper addresses the problem of the inertial motion of a roller racer, which reduces to investigating a dynamical system on a (two-dimensional) torus and to classifying singular points on it. It is shown that the motion of the roller racer in absolute space is asymptotic. A restriction on the system parameters in which this motion is bounded (compact) is presented.     

Explore Stochastic Instabilities of Periodic Points by Transition Path Theory

We consider the noise-induced transitions from a linearly stable periodic orbit consisting of T periodic points in randomly perturbed discrete logistic map. Traditional large deviation theory and asymptotic analysis at small noise limit cannot distinguish the quantitative difference in noise-induced stochastic instabilities among the T periodic points. To attack this problem, we generalize the transition path theory to the discrete-time continuous-space stochastic process. In our first criterion to quantify the relative instability among T periodic points, we use the distribution of the last passage location related to the transitions from the whole periodic orbit to a prescribed disjoint set. This distribution is related to individual contributions to the transition rate from each periodic points. The second criterion is based on the competency of the transition paths associated with each periodic point. Both criteria utilize the reactive probability current in the transition path theory. Our numerical results for the logistic map reveal the transition mechanism of escaping from the stable periodic orbit and identify which periodic point is more prone to lose stability so as to make successful transitions under random perturbations.     

Characterization of Day–James Spaces and Generalized Day–James Spaces

In this paper, we introduce the definition of generalized Day–James space on R~n(n ≥2) and give a characterization of it, which extend some known results. In addition, we provide a sufficient and necessary condition for Day–James space, which reappeared Day's construction for any two-dimensional normed space to make Birkhoff orthogonality symmetry.     

Complex vector measure and integral over manifolds with locally finite variations

It is well known that any compactly supported continuous complex differential n-form can be integrated over real n-dimensional C¹ manifolds in C^m (m ≥ n). For n = 1, the integral along any locally rectifiable curve is defined. Another generalization is the theory of currents (linear functionals on the space of compactly supported C^∞ differential forms). The topic of the article is the integration of measurable complex differential (n, 0)-forms (containing no \(d{\bar z_j}\)) over real n-dimensional C⁰ manifolds in C^m with locally finite n-dimensional variations (a generalization of locally rectifiable curves to dimensions n > 1). The last result is that a real n-dimensional manifold C¹ embedded in C^m has locally finite variations, and the integral of a measurable complex differential (n, 0)-form defined in the article can be calculated by a well-known formula.