Asymptotic stability at infinity for differentiable vector fields of the plane

Let be a differentiable (but not necessarily C¹) vector field, where σ>0 and . Denote by R(z) the real part of z∈C. If for some ?>0 and for all , no eigenvalue of DpX belongs to , then: (a) for all , there is a unique positive semi-trajectory of X starting at p; (b) it is associated to X, a well-defined number I(X) of the extended real line [−∞,∞) (called the index of X at infinity) such that for some constant vector v∈R² the following is satisfied: if I(X) is less than zero (respectively greater or equal to zero), then the point at infinity ∞ of the Riemann sphere R²∪{∞} is a repellor (respectively an attractor) of the vector field X+v.     

Towards a Ryll‐Nardzewski‐type theorem for weakly oligomorphic structures

A structure is called weakly oligomorphic if its endomorphism monoid has only finitely many invariant relations of every arity. The goal of this paper is to show that the notions of homomorphism‐homogeneity, and weak oligomorphy are not only completely analogous to the classical notions of homogeneity and oligomorphy, but are actually closely related. We first prove a Fraïssé‐type theorem for homomorphism‐homogeneous relational structures. We then show that the countable models of the theories of countable weakly oligomorphic structures are mutually homomorphism‐equivalent (we call first order theories with this property weakly ω‐categorical). Furthermore we show that every weakly oligomorphic homomorphism‐homogeneous structure contains (up to isomorphism) a unique homogeneous, homomorphism‐homogeneous core, to which it is homomorphism‐equivalent. As a consequence we obtain that every countable weakly oligomorphic structure is homomorphism‐equivalent to a finite or ω‐categorical structure. As a corollary we obtain a characterization of positive existential theories of weakly oligomorphic structures as the positive existential parts of ω‐categorical theories.     

Connected components of sets of finite perimeter and applications to image processing

This paper contains a systematic analysis of a natural measure theoretic notion of connectedness for sets of finite perimeter in ℝ ^N , introduced by H. Federer in the more general framework of the theory of currents. We provide a new and simpler proof of the existence and uniqueness of the decomposition into the so-called M-connected components. Moreover, we study carefully the structure of the essential boundary of these components and give in particular a reconstruction formula of a set of finite perimeter from the family of the boundaries of its components. In the two dimensional case we show that this notion of connectedness is comparable with the topological one, modulo the choice of a suitable representative in the equivalence class. Our strong motivation for this study is a mathematical justification of all those operations in image processing that involve connectedness and boundaries. As an application, we use this weak notion of connectedness to provide a rigorous mathematical basis to a large class of denoising filters acting on connected components of level sets. We introduce a natural domain for these filters, the space WBV(Ω) of functions of weakly bounded variation in Ω, and show that these filters are also well behaved in the classical Sobolev and BV spaces. Received July 27, 1999 / final version received June 8, 2000?Published online November 8, 2000     

Existence of invariant tori for differentiable Hamiltonian vector fields without action-angle variables

We proved the theorem for existence of invariant tori in differentiable Hamiltonian vector fields without action-angle variables. It is a generalization of the result of de la Llave et al. [4] that deals with analytic vector fields.     

Fréchet-Urysohn for finite sets, II

We continue our study [G. Gruenhage, P.J. Szeptycki, Fréchet Urysohn for finite sets, Topology Appl. 151 (2005) 238-259] of several variants of the property of the title. We answer a question from that paper by showing that a space defined in a natural way from a certain Hausdorff gap is a Fréchet α₂ space which is not Fréchet-Urysohn for 2-point sets (FU₂), and answer a question of Hrušák by showing that under MAω₁, no such “gap space” is FU₂. We also introduce versions of the properties which are defined in terms of “selection principles”, give examples when possible showing that the properties are distinct, and discuss relationships of these properties to convergence in product spaces, to the αi-spaces of A.V. Arhangel'skii, and to topological games.     

Integral invariants and limit sets of planar vector fields

We show that if a planar system of differential equations admits an inverse integrating factor V defined in a neighborhood of a singular point with exactly one zero eigenvalue then V vanishes along any separatrix of the singular point. Additionally we prove that if K is a compact α- or ω-limit set that contains a regular point (or has an elliptic or parabolic sector if not), and if V is defined on a neighborhood of K, then V vanishes at at least one point of K (and on all of K if V is real analytic or Morse).     

A poincaré-bendixson theorem for analytic families of vector fields

We provide a characterization of the limit periodic sets for analytic families of vector fields under the hypothesis that the first jet is non-vanishing at any singular point. Also, applying the family desingularization method, we reduce the complexity of some of these sets.     

A resolution theorem for absolutely isolated singularities of holomorphic vector fields

In this paper, the desingularization problem for an absolutely isolated singularity of an-dimensional holomorphic vector field is solved. Also, we exhibit final forms under blowing-up for this type of singularities.     

Central limit theorem for nonlinear transforms of gaussian random vector fields

We prove a central limit theorem for integral functionals of nonlinear transforms of homogeneous, isotropic Gaussian random vector fields.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 42, No. 8, pp. 1057–1063, August, 1990.     

An F. and M. Riesz theorem for planar vector fields

We prove that solutions of the homogeneous equation Lu=0, where L is a locally integrable vector field with smooth coefficients in two variables possess the F. and M. Riesz property. That is, if is an open subset of the plane with smooth boundary, satisfiesLu=0 on , has tempered growth at the boundary, and its weak boundary value is a measure , then is absolutely continuous with respect to Lebesgue measure on the noncharacteristic portion of . Received March 10, 2000 / Published online April 12, 2001     

Large sets of t-designs over finite fields exist for all t

Designs, Codes and Cryptography - A t- $$(n,k,\lambda )_q$$ design is a set of k-dimensional subspaces, called blocks, of an n-dimensional vector space V over the finite field $$\mathbb {F}_q$$...     

Algebraic groups over finite fields, a quick proof of Lang's theorem

We give an easy proof of Lang's theorem about the surjectivity of the Lang map on a linear algebraic group defined over a finite field, where is a Frobenius endomorphism.

     

An analogue of a theorem of Szüsz for formal Laurent series over finite fields

About 40 years ago, Szüsz proved an extension of the well-known Gauss-Kuzmin theorem. This result played a crucial role in several subsequent papers (for instance, papers due to Szüsz, Philipp, and the author). In this note, we provide an analogue in the field of formal Laurent series and outline applications to the metric theory of continued fractions and to the metric theory of diophantine approximation.     

Upwind finite volume schemes for weakly coupled hyperbolic systems of conservation laws in 2D

Summary. Systems of nonlinear hyperbolic conservation laws in two space dimensions are considered which are characterized by the fact that the coupling of the equations is only due to source terms. To solve these weakly coupled systems numerically a class of explicit and implicit upwind finite volume methods on unstructured grids is presented. Provided an unique entropy solution of the system of conservation laws exists we prove that the approximations obtained by these schemes converge for vanishing discretization parameter to this entropy solution. These results are applied to examples from combustion theory and hydrology where the existence of entropy solutions can be shown. The proofs rely on an extension of a result due to DiPerna concerning measure valued solutions to the case of weakly coupled hyperbolic systems. Received April 29, 1997     

The index of singularities of vector fields and finite jets

We describe when the index of a singularity of a smooth vector field is determined by a finite jet at the singularity. We also give some criteria to determine some terms from the formal series expansion of the vector field at the singularity which determine the index.     

Liouville's theorem and the maximum modulus principle for a system of complex vector fields

This work is concerned with Liouville's theorem and the maximum principle for the homogeneous solutions of systems of complex vector fields . Necessary and sufficient conditions are provedfor tube structures to have the Liouville property. Maximum principles are proved for a general system of complex vector fields which are integrable. As an application, in the case of vector fields, we get new characterizations of the local solvability property (P) of Nirenberg andTreves. Another application concerns a solvability condition (P_n-1) introduced by P.Cordaro and J. Hounie in differential complexes associated to locally integrable structures.