Foliations by stationary disks of almost complex domains

We study the problem of existence of stationary disks for domains in almost complex manifolds. As a consequence of our results, we prove that any almost complex domain which is a small deformation of a strictly linearly convex domain D⊂Cn with standard complex structure admits a singular foliation by stationary disks passing through any given internal point. Similar results are given for foliations by stationary disks through a given boundary point.     

Divisibility properties of hyperharmonic numbers

We extend Wolstenholme’s theorem to hyperharmonic numbers. Then, we obtain infinitely many congruence classes for hyperharmonic numbers using combinatorial methods. In particular, we show that the numerator of any hyperharmonic number in its reduced fractional form is odd. Then we give quantitative estimates for the number of pairs (n, r) lying in a rectangle where the corresponding hyperharmonic number \({ h_n^{(r)} }\) is divisible by a given prime number p. We also provide p-adic value lower bounds for certain hyperharmonic numbers. It is an open problem that given a prime number p, there are only finitely many harmonic numbers h _n which are divisible by p. We show that if we go to the higher levels r ≥  2, there are infinitely many hyperharmonic numbers \({ h_n^{(r)} }\) which are divisible by p. We also prove a finiteness result which is effective.     

Area-preserving isotopies of self-transverse immersions of S 1 in ℝ2

Let C and C′ be two smooth self-transverse immersions of S ¹ into ?². Both C and C′ subdivide the plane into a number of disks and one unbounded component. An isotopy of the plane which takes C to C′ induces a one-to-one correspondence between the disks of C and C′. An obvious necessary condition for there to exist an area-preserving isotopy of the plane taking C to C′ is that there exists an isotopy for which the area of every disk of C equals that of the corresponding disk of C′. In this paper we show that this is also a sufficient condition.     

Existence of solutions for linear hyperbolic initial-boundary value problems in a rectangle

In a recent article, we achieved the well-posedness of linear hyperbolic initial and boundary value problems (IBVP) in a rectangle via semigroup method, and we found that there are only two elementary modes called hyperbolic and elliptic modes in the system. It seems that, there is only one set of boundary conditions for the hyperbolic mode, while there are infinitely many sets of boundary conditions for the elliptic mode, which can lead to well-posedness. In this article, we continue to consider linear hyperbolic IBVP in a rectangle in the constant coefficients case and we show that there are also infinitely many sets of boundary conditions for hyperbolic mode which will lead to the existence of a solution. We also have uniqueness in some special cases. The boundary conditions satisfy the reflection conditions introduced in Section 3, which turn out to be equivalent to the strictly dissipative conditions.     

Holomorphic curvature of Finsler metrics and complex geodesics

In his famous 1981 paper, Lempert proved that given a point in a strongly convex domain the complex geodesics (i.e., the extremal disks) for the Kobayashi metric passing through that point provide a very useful fibration of the domain. In this paper we address the question whether, given a smooth complex Finsler metric on a complex manifoldM, it is possible to find purely differential geometric properties of the metric ensuring the existence of such a fibration in complex geodesies ofM. We first discuss at some length the notion of holomorphic sectional curvature for a complex Finsler metric; then, using the differential equation of complex geodesies we obtained in [AP], we show that for every pair (p;v) ∈T M, withv ≠ 0, there is a (only a segment if the metric is not complete) complex geodesic passing throughp tangent tov iff the Finsler metric is Kähler, has constant holomorphic sectional curvature ?4, and its curvature tensor satisfies a specific simmetry condition—which are the differential geometric conditions we were after. Finally, we show that a complex Finsler metric of constant holomorphic sectional curvature ?4 satisfying the given simmetry condition on the curvature is necessarily the Kobayashi metric.     

Many L-Shaped Polyominoes Have Odd Rectangular Packings

A polyomino is called odd if it can tile a rectangle using an odd number of copies. We give a very general family of odd polyominoes. Specifically, consider an L-shaped polyomino, i.e., a rectangle that has a rectangular piece removed from one corner. For some of these polyominoes, two copies tile a rectangle, called a basic rectangle. We prove that such a polyomino is odd if its basic rectangle has relatively prime side lengths. This general family encompasses several previously known families of odd polyominoes, as well as many individual examples. We prove a stronger result for a narrower family of polyominoes. Let L _n denote the polyomino formed by removing a 1 ×  (n?2) corner from a 2 ×  (n?1) rectangle. We show that when n is odd, L _n tiles all rectangles both of whose sides are at least 8n ³, and whose area is a multiple of n. If we only allow L _n to be rotated, but not reflected, then the same is true, provided that both sides of the rectangle are at least 16n ⁴. We also give several isolated examples of odd polyominoes.     

Approximate grid solution of a nonlocal boundary value problem for Laplace’s equation on a rectangle

A nonlocal boundary value problem for Laplace’s equation on a rectangle is considered. Dirichlet boundary conditions are set on three sides of the rectangle, while the boundary values on the fourth side are sought using the condition that they are equal to the trace of the solution on the parallel midline of the rectangle. A simple proof of the existence and uniqueness of a solution to this problem is given. Assuming that the boundary values given on three sides have a second derivative satisfying a Hölder condition, a finite difference method is proposed that produces a uniform approximation (on a square mesh) of the solution to the problem with second order accuracy in space. The method can be used to find an approximate solution of a similar nonlocal boundary value problem for Poisson’s equation.     

Regular combinatorial maps

The classical approach to maps is by cell decomposition of a surface. A combinatorial map is a graph-theoretic generalization of a map on a surface. Besides maps on orientable and non-orientable surfaces, combinatorial maps include tessellations, hypermaps, higher dimensional analogues of maps, and certain toroidal complexes of Coxeter, Shephard, and Grünbaum. In a previous paper the incidence structure, diagram, and underlying topological space of a combinatorial map were investigated. This paper treats highly symmetric combinatorial maps. With regularity defined in terms of the automorphism group, necessary and sufficient conditions for a combinatorial map to be regular are given both graph- and group-theoretically. A classification of regular combinatorial maps on closed simply connected manifolds generalizes the well-known classification of metrically regular polytopes. On any closed manifold with nonzero Euler characteristic there are at most finitely many regular combinatorial maps. However, it is shown that, for nearly any diagram D, there are infinitely many regular combinatorial maps with diagram D. A necessary and sufficient condition for the regularity of rank 3 combinatorial maps is given in terms of Coxeter groups. This condition reveals the difficulty in classifying the regular maps on surfaces. In light of this difficulty an algorithm for generating a large class of regular combinatorial maps that are obtained as cyclic coverings of a given regular combinatorial map is given.     

On periodic Teichmüller disks of modular transformations

We study the periodic Teichmüller disks of modular transformations. Especially, we prove that a parabolic modular transformation has either no periodic Teichmullüller disk or infinitely many periodic Teichmüller disks which can be chosen to cover infinitely many arithmetic Teichmüller curves in the Riemann moduli space M _g . Some related topics are also discussed.     

Rational Profiling of Reinforced Rotating Disks

The problem of rational profiling of thermoelastic three-layered reinforced rotating disks of gas turbines is formulated. The conditions of equal-stressed reinforcement of the midlayer and the condition of equal-strength binder of the reinforced layer or external isotropic layers are used as criteria for rational designing. An iterative method is proposed for solving such problems. The problem of rational profiling of disks with radial and radially circumferential reinforcements, on condition that the fibers of the radial family are equally stressed, is solved analytically. The calculation results for B-Al and B-Ti disks, obtained by using both the criteria of rational design, are given for plane-parallel and 3D reinforcements. It is shown that, at an equal load-carrying capacity, the weight of such disks can be 30-60% smaller than that of reinforced disks of constant thickness, or, at an equal weight, they can have a considerably higher load-carrying capacity. The criterion of equal-stressed reinforcement, as a rule, is more efficient than the condition of equal-strength binder.     

Convergence of solutions for a network approximation of the two-dimensional Laplace equation in a domain with a system of absolutely conducting disks

A boundary value problem for the Laplace equation describing the (electric, thermal, etc.) field of a system of ideally conducting disks of radius R is considered. The solution to the problem is analyzed under the condition that the characteristic distance δ between the disks is small. It was previously proved that the original continuous problem can be approximated as δ → 0 by a finite-dimensional network problem in the sense that the effective conductivities (energies) of the continuous problem are close to those of its network model. It is shown that the potentials of the ideally conducting disks determined from the continuous problem and the network model are also close to each other as δ → 0, and the difference between the potentials is O(ε^1/4), where ε = δ/R is the characteristic relative distance between the disks.     

Analysis of flow and heat transfer of viscous fluid between contracting rotating disks

The present work investigates the effects of disks contracting, rotation and heat transfer on the viscous fluid between heated contracting rotating disks. By introducing the Von Kármán type similarity transformations through which we reduced the highly nonlinear partial differential equation to a system of ordinary differential equations. This system of differential equations with appropriate boundary conditions is responsible for the flow behavior between large but finite coaxial rotating and heated disks. It is important to note that the lower disk is rotating with angular velocity Ω while the upper one with SΩ, the disks are also contracting and the temperatures of the upper and lower disks are T₁ and T₀, respectively. The agents which driven the flow are the contraction and also the rotation of the disks. On the other hand the velocity components and especially radial component of velocity strongly influence the temperature distribution inside the flow regime. The basic equations which govern the flow are the Navier Stokes equations with well known continuity equation for incompressible flow. The final system of ordinary differential equations is then solved numerically with given boundary conditions. In addition, the effect of physical parameters, the Reynolds number (Re), the wall contraction ratio (γ) and the rotation ratio (S) on the velocity and pressure gradient, as well as, the effect of Prandtl number (Pr) on temperature distribution are also observed.     

Latin squares and low discrepancy allocation of two-dimensional data

Fast browsing and retrieval of geographically referenced information can require the allocation of data on different storage devices for concurrent retrieval. By dividing the two-dimensional space into tiles (essentially an array), a system can allow users to specify regions of interest using a query rectangle and then retrieving information related to tiles included in the rectangle. Suppose that there are m I/O devices. A tile is labeled by i if the data corresponding to this area is stored in the ith I/O device. A labeling is efficient if the discrepancy of the numbers of occurrences of different labels in any given rectangle is small. In the present paper, constructions are given to make this discrepancy O(logm). The constructions use Latin squares. A lower bound of Ω(logm) on the discrepancy is given for constructions of this Latin square type.     

Viscous limits for piecewise smooth solutions of the p-system

We study the zero-dissipation problem for a one-dimensional model system for the isentropic flow of a compressible viscous gas, the so-called p-system with viscosity. When the solution of the inviscid problem is piecewise smooth and having finitely many noninteracting shocks satisfying the entropy condition, there exists unique solution to the viscous problem which converges to the given inviscid solution away from shock discontinuities at a rate of order ε as the viscosity coefficient ε goes to zero. The proof is given by a matched asymptotic analysis and an elementary energy method. And we do not need the smallness condition on the shock strength.     

Optimization of the Hausdorff distance between sets in Euclidean space

Set approximation problems play an important role in many areas of mathematics and mechanics. For example, approximation problems for solvability sets and reachable sets of control systems are intensively studied in differential game theory and optimal control theory. In N.N. Krasovskii and A.I. Subbotin’s investigations devoted to positional differential games, one of the key problems was the problem of identification of solvability sets, which are maximal stable bridges. Since this problem can be solved exactly in rare cases only, the question of the approximate calculation of solvability sets arises. In papers by A.B. Kurzhanskii and F.L. Chernous’ko and their colleagues, reachable sets were approximated by ellipsoids and parallelepipeds.In the present paper, we consider problems in which it is required to approximate a given set by arbitrary polytopes. Two sets, polytopes A and B, are given in Euclidean space. It is required to find a position of the polytopes that provides a minimum Hausdorff distance between them. Though the statement of the problem is geometric, methods of convex and nonsmooth analysis are used for its investigation.One of the approaches to dealing with planar sets in control theory is their approximation by families of disks of equal radii. A basic component of constructing such families is best n-nets and their generalizations, which were described, in particular, by A.L. Garkavi. The authors designed an algorithm for constructing best nets based on decomposing a given set into subsets and calculating their Chebyshev centers. Qualitative estimates for the deviation of sets from their best n-nets as n grows to infinity were given in the general case by A.N. Kolmogorov. We derive a numerical estimate for the Hausdorff deviation of one class of sets. Examples of constructing best n-nets are given.     

Reconstructing Matrices from Minors

Which collections of mn minors of an m-by-n matrix uniquely determine the matrix, given some regularity conditions? For m=n=3, the 585 such collections, that are distinct up to symmetry, are determined. For general m, n, a necessary and a sufficient condition for reconstruction are given in terms of matchings in a bipartite graph. Among other particular results, those collections of entries for which there are minors that permit reconstruction one entry at a time are characterized.     

On a class of rational p-adic dynamical systems

In this paper we investigate the behavior of trajectories of one class of rational p-adic dynamical systems in complex p-adic field Cp. We studied Siegel disks and attractors of such dynamical systems. We found the basin of the attractor of the system. It is proved that such dynamical systems are not ergodic on a unit sphere with respect to the Haar measure.     

Two regularization methods for a Cauchy problem for the Laplace equation

A Cauchy problem for the Laplace equation in a rectangle is considered. Cauchy data are given for y=0, and boundary data are for x=0 and x=π. The solution for 0<y?1 is sought. We propose two different regularization methods on the ill-posed problem based on separation of variables. Both methods are applied to formulate regularized solutions which are stably convergent to the exact one with explicit error estimates.     

Indefinite Boundary Eigenvalue Problems in a Pontrjagin Space Setting

We study eigenvalue problems Fy = λ Gy consisting of Hamiltonian systems of ordinary differential equations on a compact interval with symmetric λ-linear boundary conditions. The problems we are interested in are non-definite: neither left-nor right-definite. Instead of this, we give some weak condition on one coefficient of the Hamiltonian system which ensures that a hermitian form associated with the operator F has at most finitely many negative squares. This enables us to study the problem by the help of a compact self-adjoint operator in a Pontrjagin space and we obtain as a main result uniformly convergent eigenfunction expansions. In the final section, applications to formally self-adjoint differential equations of higher order are given.