Contributions to differential geometry of isotropic curves in the complex space

This work deals with classical differential geometry of isotropic curves in the complex space C⁴. First, we study spherical isotropic curves and pseudo helices. Besides, in this section we introduce some special isotropic helices (type-1, type-2 and type-3 isotropic slant helices) and express some characterizations of them in terms of É. Cartan equations. Thereafter, we prove that position vector of an isotropic curve satisfies a vector differential equation of fourth order. Finally, we investigate position vector of an arbitrary curve with respect to É. Cartan frame by a system of complex differential equations whose solution gives components of the position vector. Solutions of the mentioned system and vector differential equation have not yet been found. Therefore, in terms of special cases, we present some special characterizations.     

Position vector of a time-like slant helix in Minkowski 3-space

In this paper, position vector of a time-like slant helix with respect to standard frame of Minkowski space E³₁ is studied in terms of Frenet equations. First, a vector differential equation of third order is constructed to determine position vector of an arbitrary time-like slant helix. In terms of solution, we determine the parametric representation of the slant helices from the intrinsic equations. Thereafter, we apply this method to find the representation of a time-like Salkowski and time-like anti-Salkowski curves as examples of a slant helices, by means of intrinsic equations. Moreover, we present some new characterizations of slant helices and illustrate some examples of our main results.     

Construction of Banach spaces using a generalization of the scalar product

A vector is assigned to every pair of elements of a linear space. If this vector is in l₂ and the relation between the element pair and the vector satisfies a certain system of axioms, then we call the space a Banach space. For such a space we introduce, as in the case of the usual scalar product, a series of concepts, and prove, as an example a theorem concerning orthogonal expansion.Translated from Matematicheskie Zametki, Vol. 8, No. 1, pp. 67–76, July, 1970.     

Vector Topical Function,Abstract Convexity and Image Space Analysis

In this paper, we introduce a new type of vector topical function. It contains some other categories of topical functions as special cases and can be interpreted as weak separation functions in image space analysis. We establish its envelope result and investigate its properties in the frame of abstract convexity. Then, we present the corresponding conjugation and subdifferential, and observe the relationships among these concepts. Finally, as applications, we obtain some dual results for some vector optimization, where the object is expressed as the difference of vector topical functions.     

On the differential geometry of frame bundles

Summary The problem to ascertain an admissible structure of frame bundles is solved in this paper, presenting a tensor field H of type-(1.1) which satisfies H³ = H. It acts on the horizontal tensor field as an annihilator and on the vertical tensor field as an almost product structure. When a metric is endowed on the base manifold, it is always possible to assign the metric in the frame bundle such that its element of length obeys the Pythagorean rule when the measurement is done along horizontal and vertical distributions, and by such a general metric it can be proved that the tangent bundle of a frame bundle F(X_n) is reduced to0(n²) ×0(n); especially for n=2m, it is reduced to U(mn) ×0(n). The Lie derivative of H and the parallelism of the three lifts, horizontal, vertical and cnmplete, are examined in terms of their corresponding projection vector fields in the base space.     

On spherical representations of quivers and generalized complexes

For any quiverQ we consider spherical representationsV ofQ such that the isomorphism class ofV is a spherical variety. We suggest an approach for classifying such representations for anyQ and obtain a classification forQ being an equioriented Dynkin diagramA _n. In particular, all complexes are spherical representations. We introduce a category of representations that we call generalized complexes and classify spherical generalized complexes. For the quivers that we call crumbly we prove that any spherical generalized complex has a polynomial algebra of covariants on the closure of its isomorphism class.Partially supported by INTAS-OPEN grant 97-1570 and RFFI grant 98-01-00598.     

Landau-Lifshitz方程派生的球面锥对称族及其演化

直至2008年,尚未见到讨论同时具有外磁场和各向异性场的多维Landau-Lifshitz方程的显式动态解的文献.当缺少外加磁场或各向异性场时,许多作者曾经尝试过Hirota的著名方法,Lie代数结构与Backlund变换,双线性变换等,但精确解仍然没有构造出来.本文首先给出了一种求解具有外磁场和各向异性场的Landa...     

Quasi-convolutions and applications to coded images

In this paper, we introduce a new integral operator, which we call a quasi-convolution. The kernel of this operator is analogous to the kernel of the operator of usual convolution but has a variable scale. Classical inversion by the usual Fourier transform is not possible in this case, but we give examples of inversion for special cases in dimensions one and two. We also give applications to the problem of recognition of images in medicine, where quasi-convolutions are connected with the transformation of coded images. Bibliography: 7 titles. Translated fromZapiski Nauchnykh Seminarov POMI, Vol. 244, 1997, pp. 167–180. Translated by S. Yu. Pilyugin.     

Tannaka duality for proper Lie groupoids

By replacing the category of smooth vector bundles of finite rank over a manifold with the category of what we call smooth Euclidean fields, which is a proper enlargement of the former, and by considering smooth actions of Lie groupoids on smooth Euclidean fields, we are able to prove a Tannaka duality theorem for proper Lie groupoids. The notion of smooth Euclidean field we introduce here is the smooth, finite dimensional analogue of the usual notion of continuous Hilbert field.     

Quilted Gabor frames – A new concept for adaptive time-frequency representation

Certain signal classes such as audio signals call for signal representations with the ability to adapt to the signalʼs properties. In this article we introduce the new concept of quilted frames, which aim at adaptivity in time-frequency representations. As opposed to Gabor or wavelet frames, this new class of frames allows for the adaptation of the signal analysis to the local requirements of signals under consideration. Quilted frames are constructed directly in the time-frequency domain in a signal-adaptive manner. Validity of the frame property guarantees the possibility to reconstruct the original signal. The frame property is shown for specific situations and the Bessel property is proved for the general setting. Strategies for reconstruction from coefficients obtained with quilted Gabor frames and numerical simulations are provided as well.     

Pseudo-spherical Darboux images and lightcone images of principal-directional curves of nonlightlike curves in Minkowski 3-space

Choosing an alternative frame, which is the Frenet frame of the principal-directional curve along a nonlightlike Frenet curve γ , we define de Sitter Darboux images, hyperbolic Darboux images, and lightcone images generated by the principal directional curves of nonlightlike Frenet curves and investigate geometric properties of these associated curves under considerations of singularity theory, contact, and Legendrian duality. It is shown that pseudo-spherical Darboux images and lightcone images can occur singularities (ordinary cusp) characterized by some important invariants. More interestingly, the cusp is closely related to the contact between nonlightlike Frenet curve γ and a slant helix, the principal-directional curve ψ of γ and a helix or the principal-directional curve ψ and a slant helix. In addition, some relations of Legendrian dualities between C-curves and pseudo-spherical Darboux images or lightcone images are shown. Some concrete examples are provided to illustrate our results.     

Spinor Bishop Equations of Curves in Euclidean 3-Space

In this paper, we study spinor Bishop equations of curves in ${\mathbb{E}^3}$ . We research the spinor formulations of curves according to Bishop frames in ${\mathbb{E}^3}$ . Also, the relations between spinor formulations of Bishop frames and Frenet frame are expressed.     

Surfaces de la classe VII0 et automorphismes

In this Note we construct surfaces S of class VII containing a global spherical shell starting with a Hénon automorphism H(x,y)=(x²+c−ay,x) of C². For a special choice of the parameter a we show the existence of a non-trivial holomorphic vector field X on S. This contributes to the (open) problem of classifying all compact complex surfaces with a positive dimensional automorphism group. For general results concerning vector fields and foliations on surfaces containing a global spherical shell we refer to our note [2].     

A generalization of the spherical inversion

In the present article, we introduce a generalization of the spherical inversion. In particular, we define an inversion with respect to an ellipsoid, and prove several properties of this new transformation. The inversion in an ellipsoid is the generalization of the elliptic inversion to the three-dimensional space. We also study the inverse images of planes, spheres, ellipsoids and other curves. We use the software Mathematica to generate these graphics. Finally, we generalize the Pappus chain theorem to ellipsoids.     

Periodic Solutions for Second Order Differential Inclusions with Nonconvex and Unbounded Multifunction

In this paper we consider a second order multivalued periodic boundary value problem with a nonconvex and unbounded orientor field (set-valued vector field). Using a directionally continuous selector, through its Filippov regularization we produce a convex-valued, bounded multifunction and with this as orientor field we introduce a new multivalued periodic problem. Using the Leray-Schauder principle, we solve the convex problem and then we show that its solutions also solve the original nonconvex problem.     

Signed symmetric covariation coefficient for alpha-stable dependence modeling

We introduce a new measure of dependence between the components of a symmetric α-stable random vector that we call the signed symmetric covariation coefficient. We show that this coefficient satisfies the properties of the classical Pearson coefficient. Moreover, we show that in the case of sub-Gaussian random vectors, this coefficient coincide with the association parameter and the generalized association parameter. To cite this article: B. Garel, B. Kodia, C. R. Acad. Sci. Paris, Ser. I 347 (2009).     

Wavelet Decomposition of Spherical Vector Fields with Respect to Sources

This article is concerned with an approach of modelling the Earth’s magnetic field as measured by satellites in terms of a special system of vector spherical harmonics and in terms of vector kernel functions, called vector scaling functions and wavelets. The main ingredient is the presentation of a system of vector spherical harmonics which separates a given spherical vector field with respect to its sources, i.e., the spherical vector field is separated into a part which is induced by sources inside the sphere, a part which is induced by sources outside the sphere and a part which is induced by sources on the sphere, which are, for example, currents crossing the sphere. Using this special system of vector spherical harmonics vector scaling functions and wavelets are constructed which keep the advantageous property of separating with respect to sources but which also allow a locally reflected modelling of the respective vector field. At the end of the article, the method is tested on real magnetic field data measured by the German geoscientific research satellite CHAMP.     

The generalized triangle inequalities in thick Euclidean buildings of rank 2

We describe the set of possible vector valued side lengths of n-gons in thick Euclidean buildings of rank 2. This set is determined by a finite set of homogeneous linear inequalities, which we call the generalized triangle inequalities. These inequalities are given in terms of the combinatorics of the spherical Coxeter complex associated to the Euclidean building.     

CR singular images of generic submanifolds under holomorphic maps

The purpose of this paper is to organize some results on the local geometry of CR singular real-analytic manifolds that are images of CR manifolds via a CR map that is a diffeomorphism onto its image. We find a necessary (sufficient in dimension 2) condition for the diffeomorphism to extend to a finite holomorphic map. The multiplicity of this map is a biholomorphic invariant that is precisely the Moser invariant of the image, when it is a Bishop surface with vanishing Bishop invariant. In higher dimensions, we study Levi-flat CR singular images and we prove that the set of CR singular points must be large, and in the case of codimension 2, necessarily Levi-flat or complex. We also show that there exist real-analytic CR functions on such images that satisfy the tangential CR conditions at the singular points, yet fail to extend to holomorphic functions in a neighborhood. We provide many examples to illustrate the phenomena that arise.     

An edge driven wavelet frame model for image restoration

Wavelet frame systems are known to be effective in capturing singularities from noisy and degraded images. In this paper, we introduce a new edge driven wavelet frame model for image restoration by approximating images as piecewise smooth functions. With an implicit representation of image singularities sets, the proposed model inflicts different strength of regularization on smooth and singular image regions and edges. The proposed edge driven model is robust to both image approximation and singularity estimation. The implicit formulation also enables an asymptotic analysis of the proposed models and a rigorous connection between the discrete model and a general continuous variational model. Finally, numerical results on image inpainting and deblurring show that the proposed model is compared favorably against several popular image restoration models.