Maximizing the Product of Two Linear Functions In 0-1 Variables

We study both the continuous and discrete problems of maximizing the product of two linear functions subject to all variables being between 0 and 1. We first give linear and low-order polynomial algorithms for the solution of the continuous problem. In addition, we describe penalties that help to fix variables in the discrete problem. Extensive computational tests demonstrate the effectiveness of these results.     

On Lattice Structure of the Space of Pointwise Limits of Harmonic Functions

Let H¹(U) denote the space of all pointwise limits of bounded sequences from H(U), where H(U) consists of all continuous functions on the closure [`(U)]\overline{U} of a bounded open set U⊂ℝ^m that are harmonic on U. It is shown that the space H¹(U) is a lattice in the natural ordering if and only if the set ∂_regU of all regular points of U is an F_σ-set.     

On Continuity and Decomposition of Positive Definite Functions

Let G be a locally compact commutative group and let g and h be positive definite functions on G, which are not identically zero. We show that continuity of gh? implies the existence of a character γ of G_d (the discrete version of G) such that γg and γh are continuous. As corollary we get a special case of a result of K. de Leeuw and I. Glicksberg concerning almost continuous group representations. In the second part of the paper we prove decomposition theorems for positive definite functions defined on a neighbourhood of the zero.     

Discrete Systems and their Characteristic Spectral Functions

We define first-order discrete systems in the matrix-valued case. They are characterized by sequences of pair of matrices, called admissible sequences. We present two important examples of such sequences, called Szeg? and Nehari sequences. We introduce the characteristic spectral functions associated to a first-order system. We define in particular the scattering function, the Weyl function and the reflection coefficient function and we study the relationships between these functions. Daniel Alpay wishes to thank the Earl Katz family for endowing the chair which supported his research.     

Relationship between canonical forms of linear differential observation systems and canonical forms of their discrete approximations

We establish a relationship between the canonical form of a linear differential system and the canonical form of its discrete approximation based on the replacement of the derivative by Euler’s finite difference. We prove that if there exist limits of certain sequences of discrete functions constructed with the use of coefficients of the canonical form of the discrete system, then these limits define the canonical form of the differential system.     

Continuous and discrete N-convex approximations

We show that the best L_p-approximant to continuous functions by n-convex functions is the limit of discrete n-convex approximations. The techniques of the proof are then used to show the existence of near interpolants to discrete n-convex data by continuous n-convex functions if the data points are close.     

On the Limits at the Martin Boundary for a Class of Functions

In this paper we prove a criterion for existence of pathwise limits at the Martin boundary for functions with gradient in L _loc ². (This implies that such functions have fine limits at almost all Martin boundary points.)     

有限离散函数的导数和性质

通过引入有限离散函数的导数概念,分别从几何直观和性质两个角度,比较了有限离散函数的导数概念和常规连续函数导数的相似性.结果表明,在局部情况下,有限离散函数导数近似等于连续情形下的导数.在运算性质上,有限离散函数导数的性质非常相似于连续情形时的导数性质.最后的例子给出了有限离散函数导数的一个应用.     

Regularity of Harmonic Functions for Some Markov Chains with Unbounded Range

We consider a class of continuous time Markov chains on ? ^d . These chains are the discrete space analogue of Markov processes with jumps. Under some conditions, as we show, harmonic functions associated with these Markov chains are Hölder continuous.     

Symbolic Computation for Moments and Filter Coefficients of Scaling Functions

Algebraic relations between discrete and continuous moments of scaling functions are investigated based on the construction of Bell polynomials. We introduce families of scaling functions which are parametrized by moments. Filter coefficients of scaling functions and wavelets are computed with computer algebra methods (in particular Gröbner bases) using relations between moments. Moreover, we propose a novel concept for data compression based on parametrized wavelets.Received December 15, 2003     

Structure of rings of functions with Riemann Stieltjes convolution products

In this note three sets of complex valued functions with pointwise addition and a Riemann Stieltjes convolution product are considered. The functions considered are discrete analytic functions, sequences, and continuous functions of bounded variation defined on the nonnegative real numbers. Each forms a commutative algebra with identity. The discrete analytic functions form a principal ideal ring with five maximal ideals, nine prime ideals, and is essentially a direct sum of four discrete valuation rings. The ring of sequences is isomorphic to an ideal of the ring of discrete analytic functions; it has two maximal and three prime ideals. Both contain divisors of zero. The units, associates, irreducible elements and primes in these two rings are described. The results are used to study the continuous functions; partial results are obtained concerning units and divisors of zero. The product satisfies a convolution theorem.     

Singular Strictly Monotone Functions

We describe a universal approach to constructing continuous strictly monotone increasing singular functions on the closed interval [-1,1]. The “generator” of the method is the series ∑_k=1∞±2^-k with random permutation of signs, and the corresponding functions are generated as distribution functions of such series. As examples, we consider two stochastic methods of arranging signs: independent and Markov.     

On the Representation of Functions with Gaussian Wave Packets

We introduce Gaussian wave packets in pursuit of representations of functions, in which the representation is invariant under translation, modulation, scale, rotation and anisotropic dilation. Properties of both continuous and discrete representations are discussed. For the discrete (two-dimensional) case, we develop fast algorithms for the application of the analysis and synthesis operators. A main objective for using Gaussian wave packets is to obtain sparse approximations of functions. However, due to the many invariance properties, the representations will have a high degree of redundancy. Therefore, we also introduce sparse methods for highly redundant representations, that employ some of the analytic properties of Gaussian wave packet for gaining computational efficiency.     

Discrete approximation of relaxed optimal control problems

We consider a general nonlinear optimal control problem for systems governed by ordinary differential equations with terminal state constraints. No convexity assumptions are made. The problem, in its so-called relaxed form, is discretized and necessary conditions for discrete relaxed optimality are derived. We then prove that discrete optimality [resp., extremality] in the limit carries over to continuous optimality [resp., extremality]. Finally, we prove that limits of sequences of Gamkrelidze discrete relaxed controls can be approximated by classical controls.     

Painlevé Functions and Conformal Blocks

We outline recent developments relating Painlevé equations and 2D conformal field theory. Generic tau functions of Painlevé VI and Painlevé III₃ are written as linear combinations of c=1 conformal blocks and their irregular limits. This provides explicit combinatorial series representations of the tau functions, and helps to establish a connection formula for the tau function in the Painlevé VI case.     

无穷小与复合函数的极限

通过把无穷小(大)进行分类,并利用无穷小(大)的种类得到复合函数极限的各种形式,更全面准确地理解复合函数的极限.     

Existence and smoothness of continuous and discrete solutions of a two-dimensional shallow water problem over movable beds

We consider a two-dimensional shallow water system over movable beds. We begin with a continuous system and prove the existence of the solutions, and then we investigate their smoothness. Then, we employ a Galerkin method to obtain a finite-dimensional problem which is solved using a Brouwer fixed point theorem. Therefore, we show that the limits of the resulting solution sequences satisfy the model equations.After solving the continuous problem, we focus on the corresponding discrete problem. We employ a local discontinuous Galerkin scheme for numerical solution of the discrete system and conduct an error analysis of the numerical scheme. We prove that the method is convergent and that the error is bounded according to a specific norm defined herein.     

连续函数压缩映射问题的讨论

就连续函数应用问题的教学内容进行探讨.介绍数列和连续函数的压缩映射原理及其在方程求解,数学建模等方面的应用,并给出压缩映射的一般形式.续而通过压缩映射原理在图像处理和导航系统中的应用,阐明创新思想的重要性,以求开拓学生的思路和想象空间.     

Discrete semi-stable distributions

The purpose of this paper is to introduce and study the concepts of discrete semi-stability and geometric semi-stability for distributions with support inZ ₊. We offer several properties, including characterizations, of discrete semi-stable distributions. We establish that these distributions posses the property of infinite divisibility and that their probability generating functions admit canonical representations that are analogous to those of their continuous counterparts. Properties of discrete geometric semi-stable distributions are deduced from the results obtained for discrete semi-stability. Several limit theorems are established and some examples are constructed.     

Spaces of Vector-Valued Integrable Functions and Localization of Bounded Subsets

We study the structure of bounded sets in the space L¹{E} of absolutely integrable Lusin-measurable functions with values in a locally convex space E. The main idea is to extend the notion of property (B) of Pietsch, defined within the context of vector-valued sequences, to spaces of vector-valued functions. We prove that this extension, that at first sight looks more restrictive, coincides with the original property (B) for quasicomplete spaces. Then we show that when dealing with a locally convex space, property (B) provides the link to prove the equivalence between Radon–Nikodym property (the existence of a density function for certain vector measures) and the integral representation of continuous linear operators T: L¹ → E, a fact well-known for Banach spaces. We also study the relationship between Radon–Nikodym property and the characterization of the dual of L¹{E} as the space L^∞{E′_b}.