Common periodic trajectories of two mappings

For a mapping f ∈ C ^r (I, I), r ≥ 0, we consider the problem of the existence of a mapping close to it that has periodic trajectories of given periods in common with f. Translated from Ukrains'kyi Matematychnyi Zhurnal, Vol. 60, No. 7, pp. 937–948, July, 2008.     

On the Hilbert scheme of curves in higher-dimensional projective space

In this paper we prove that, for anyn≥3, there exist infinitely manyr∈N and for each of them a smooth, connected curveC _r in ℙ ^r such thatC _r lies on exactlyn irreducible components of the Hilbert scheme Hilb(ℙ ^r ). This is proven by reducing the problem to an analogous statement for the moduli of surfaces of general type.     

Lower bounds for weak epsilon-nets and stair-convexity

A set N ⊂ ℝ ^d is called a weak ɛ-net (with respect to convex sets) for a finite X ⊂ ℝ ^d if N intersects every convex set C with |X ∩ C| ≥ ɛ|X|. For every fixed d ≥ 2 and every r ≥ 1 we construct sets X ⊂ ℝ ^d for which every weak 1/r -net has at least Ω(r log ^d−1 r) points; this is the first superlinear lower bound for weak ɛ-nets in a fixed dimension.     

On <Emphasis Type="Italic">C</Emphasis><Superscript><Emphasis Type="Italic">r</Emphasis></Superscript>-closing for flows on orientable and non-orientable 2-manifolds

We provide an affirmative answer to the C ^r -Closing Lemma, r ≥ 2, for a large class of flows defined on every closed surface.     

Conservation and bifurcation of an invariant torus of a vector field

We consider small perturbations with respect to a small parameter ε≥0 of a smooth vector field in ℝ^n+m possessing an invariant torusT ^m. The flow on the torusT ^m is assumed to be quasiperiodic withm basic frequencies satisfying certain conditions of Diophantine type; the matrix Ω of the variational equation with respect to the invariant torus is assumed to be constant. We investigate the existence problem for invariant tori of different dimensions for the case in which Ω is a nonsingular matrix that can have purely imaginary eigenvalues. Translated fromMatematicheskie Zametki, Vol. 61, No. 1, pp. 34–44, January, 1997. Translated by S. K. Lando     

Real perturbations of complex polynomials

In this article the dynamics of generic C^r (r ≥ 3) perturbations of complex polynomials are considered. The attention is focused on the determination of the existence of large or invariant components of the complement of the basin of ∞, where the interesting dynamics occur.     

On lower bounds for the approximation of individual functions by local splines with nonfixed nodes

For functions with the integrable βth power, where β = (r + 1 + 1/p)⁻¹, we obtain asymptotically exact lower bounds for the approximation by local splines of degreer and defectk ≥r/2 in the metric ofL _p. Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 51, No. 12, pp. 1628–1637, December, 1999.     

Negative result in pointwise 3-convex polynomial approximation

Let Δ³ be the set of functions three times continuously differentiable on [−1, 1] and such that f″′(x) ≥ 0, x ∈ [−1, 1]. We prove that, for any n ∈ ℕ and r ≥ 5, there exists a function f ∈ C ^r [−1, 1] ⋂ Δ³ [−1, 1] such that ∥f ^(r)∥ _{C[−1, 1]} ≤ 1 and, for an arbitrary algebraic polynomial P ∈ Δ³ [−1, 1], there exists x such that

MixingC rmaps of the interval without maximal measure

We construct aC ^rtransformation of the interval (or the torus) which is topologically mixing but has no invariant measure of maximal entropy. Whereas the assumption ofC ^∞ ensures existence of maximal measures for an interval map, it shows we cannot weaken the smoothness assumption. We also compute the local entropy of the example.     

Non-existence of KAM torus

Given an integrable Hamiltonian h ₀ with n-degrees of freedom and a Diophantine frequency ω, then, arbitrarily close to h ₀ in the C ^r topology with r < 2n, there exists an analytical Hamiltonian h _ε with no KAM torus of rotation vector ω. In contrast with it, KAM tori exist if perturbations are small in C ^r topology with r > 2n.     

On the smoothness of conjugation of circle diffeomorphisms with rigid rotations

We prove that any C^3+β-smooth diffeomorphism preserving the orientation of a circle with rotation number from the Diophantine class D_δ, 0 < β < δ < 1, is C^2+β−δ-smoothly conjugate to a rigid rotation of the circle by a certain angle. __________ Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, No. 2, pp. 268–282, February, 2008.     

The Hausdorff dimension of horseshoes of diffeomorphisms of surfaces

Letf be aC ^r diffeomorphism,r≥2, of a two dimensional manifoldM ², and let Λ be a horseshoe off (i.e. a transitive and isolated hyperbolic set with topological dimension zero). We prove that there exist aC ^r neighborhoodU off and a neighbourhoodU of Λ such that forg∈U, the Hausdorff dimension of ∩ _n g ⁿ (U) is aC ^r−1 function ofg.     

Singular hyperbolicity for transitive attractors with singular points of 3-dimensional <Emphasis Type="Italic">C</Emphasis><Superscript>2</Superscript>-flows

In the context of C^r-flows on 3-manifolds (r ≥ 1), the notion of singular hyperbolicity, inspired on the Lorenz Attractor, is the right generalization of hyperbolicity (in the sense of Smale) for C¹-robustly transitive sets with singularities. We estabish conditions (on the associated linear Poincaré flow and on the nature of the singular set) under which a transitive attractor with singularities of a C²-flow on a 3-manifold is singular hyperbolic.     

Bernstein and Gelfand widths of certain classes of analytic functions. II

The Gelfand widths of the unit ball of H²(ν) (the weighted Hardy space) with respect to the metric of the space L_∞(T_r) are considered (here T_r is the circle of radius r centered at the origin), as well as the Bernstein widths of the unit ball of H^∞ with respect to the metric of the space L₂(T_r, μ). Asymptotic formulas for the widths in question are established for arbitrary measures ν, μ. Bibliography: 5 titles. Translated fromZapiski Nauchnykh Seminarov POMI, Vol. 232, 1996, pp. 134–140. Translated by S. V. Kislyakov.     

Linear series computing the Clifford index of a projective curve

For a (smooth irreducible) curveC of genus g and Clifford indexc>2 with a linear seriesg _d ^r computing c (so ) it is well known thatc + 2 ≤d ≤2 (c + 2), and if then 2c + 1 ≤g ≤ 2c + 4 unlessd = 2c + 4 in which caseg = 2c + 5. Let c ≥ 0 andg be integers. If 2c + 1 ≤g ≤2c + 4 we prove that for any integerd <g such thatd ≡c mod 2 andc + 2 ≤d < 2(c + 2) there exists a curve of genus g and Clifford index c with a g_d ^r computing c. Ford ≥c + 6 (i.e.r ≥ 3) we construct this curve on a surface of degree 2r-2 in ℙ^r, and ford ≥c + 8 (i.e.r ≥ 4) we show that such a curve cannot be found on a surface in ℙ^r of smaller degree. In fact, if g_d ^r computes the Clifford index c of C such thatc + 8 ≤d ≤ 2c + 3 then the birational morphism defined by this series cannot map C onto a (maybe, singular) curve contained in a surface of degree at most 2r-3 in ℙ^r.     

3·2 r+1 -Cycle Systems of K n −C n

 In this paper we investigate the necessary and sufficient conditions for the existence of 3 · 2 ^{r +1}-cycle system of K _n – C _n , where C _n is any Hamilton cycle of K _n and r ≥ 0. Received: December 17, 1999 Final version received: July 25, 2000 Present address: 15 Guang Ming 10 St. Sec. 1, Chu-Bei, Hsin-Chu, 302, Taiwan, ROC     

The behavior of bounded solutions to the equation Δu−c(x)u=0 on riemannian manifolds of special type

In the paper we consider solutions of the equation Δu−c(x)u=0,c(x)≥0, on complete Riemannian manifolds constituted as follows: the exterior of some compact set is isometric to the direct product of the semiaxis by some compact manifold with the metricds ²=h ²(r)dr ²+g ²(r)dθ². Necessary and sufficient conditions under which bounded solutions of the equation have a limit independent of θ asr→∞ are obtained and also conditions under which the two-sided Liouville theorem is valid. Translated fromMatematicheskie Zametki, Vol. 65, No. 2, pp. 215–221, February, 1999.     

Pseudo-rotations of the open annulus

In this paper, we study pseudo-rotations of the open annulus, i.e. conservative homeomorphisms of the open annulus whose rotation set is reduced to a single irrational number (the angle of the pseudo-rotation). We prove in particular that, for every pseudo-rotation h of angle ρ, the rigid rotation of angle ρ is in the closure of the conjugacy class of h. We also prove that pseudo-rotations are not persistent in C^r topology for any r ≥ 0.     

Moduli of symplectic instanton vector bundles of higher rank on projective space ℙ<Superscript>3</Superscript>

Symplectic instanton vector bundles on the projective space ℙ³ constitute a natural generalization of mathematical instantons of rank-2. We study the moduli space I _n;r of rank-2r symplectic instanton vector bundles on ℙ³ with r ≥ 2 and second Chern class n ≥ r, n ≡ r (mod 2). We introduce the notion of tame symplectic instantons by excluding a kind of pathological monads and show that the locus I _n;r ^* of tame symplectic instantons is irreducible and has the expected dimension, equal to 4n(r + 1) −r(2r + 1).     

New Lower Bounds on the Multicolor Ramsey Numbers R r (C 2 m )

The multicolor Ramsey number R_r(H) is defined to be the smallest integer n=n(r) with the property that any r-coloring of the edges of the complete graph K_n must result in a monochromatic subgraph of K_n isomorphic to H. It is well known that 2^rm<R_r(C_2m+1)<2(r+2)!m and R_r(C_2m)≥(r−1)(m−1)+1. In this paper, we prove that R_r(C_2m)≥2(r−1)(m−1)+2. This research is supported by NSFC(60373096, 60573022) and SRFDP(20030141003)