Topology and geometry of the Berkovich ramification locus for rational functions, I

We initiate a detailed study of the ramification locus for projective endomorphisms of the Berkovich projective line—the non-Archimedean analog of the Riemann sphere.     

Monopoles, particles and rational functions

We prove a recent conjecture of Manton and Murray: if a polynomialp(z) of degreek — 1 is given, then anSU (2) monopole corresponding to a rational functionp(z)/q(z) with well-separated poles \₁,...,\_k is approximately made up from charge 1 monopoles located at points (1/2 In p(\_i), \_i). We show how the rate of approximation changes with the numeratorp(z) with the result that, as long as the values of the numerator remain close together relative to the distances between poles, the above statement remains true and ceases to be so otherwise.We also show that the spectral curve of the monopole approaches the union of curves of charge 1 monopoles exponentially fast. This remains true forSU (N) monopoles.     

Orthogonal on the circle rational functions with fixed poles II

The paper is devoted to the algebraic properties of rational functions which are orthogonal on the unit circle and have fixed poles.     

Uniqueness theorems for rational,algebraic, and algebroid functions

A number of pointsA, for which one must define the sets of simpleA-points to determine a rational function, a polynomial, and an algebraic or algebroid function uniquely, is obtained.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 46, No. 3, pp. 212–226, March, 1994.     

Quadrature rules for rational functions

Summary. It is shown how recent ideas on rational Gauss-type quadrature rules can be extended to Gauss-Kronrod, Gauss-Turán, and Cauchy principal value quadrature rules. Numerical examples illustrate the advantages in accuracy thus achievable. Received June 21, 1999 / Revised version received September 14, 1999 / Published online June 21, 2000     

Finite orbits for rational functions

Let K be a number field and φ ∈ K(z) a rational function. Let S be the set of all archimedean places of K and all non-archimedean places associated to the prime ideals of bad reduction for φ. We prove an upper bound for the length of finite orbits for φ in ?₁ (K) depending only on the cardinality of S.     

Linear extrapolation by rational functions,exponentials and logarithmic functions

In this paper linear extrapolation by rational functions with given poles is considered from an arithmetical point of view. It is shown that the classical interpolation algorithms of Lagrange, Neville-Aitken and Newton which are well known for polynomial interpolation can be extended in a natural way to this problem yielding recursive methods of nearly the same complexity. The proofs are based upon explicit representations of generalized Vandermonde-determinants which are calculated by the elimination method combined with analytical considerations. As an application a regularity criterion for certain linear sequence-transformations is given. Also, by the same method simplified recurrence relations for linear extrapolation by exponentials and logarithmic functions at special knots are derived.     

-resultant for rational functions

In this paper we introduce the -resultant of two rational functions and show how it can be used to decide if or if and to find the singularities of the parametric algebraic curve define by . In the course of our work we extend a result about implicitization of polynomial parametric curves to the rational case, which has its own interest.

     

On the density principle for rational functions

Let E be a subspace of C(X) and define R(E):={g/h: g,hE;h>0}. We prove that R(E) is dense in C(X) if for every X ₀X there exists xX ₀ such that E contains an approximation to a -function at the point x on the set X ₀. We use this principle to study the density of Müntz rationals in two variables.     

Determination of the limits for multivariate rational functions

The purpose of this paper is to solve the problem of determining the limits of multivariate rational functions.It is essential to decide whether or not limxˉ→0f g=0 for two non-zero polynomials f,g∈R[x1,...,xn]with f(0,...,0)=g(0,...,0)=0.For two such polynomials f and g,we establish two necessary and sufcient conditions for the rational functionf g to have its limit 0 at the origin.Based on these theoretic results,we present an algorithm for deciding whether or not lim(x1,...,xn)→(0,...,0)f g=0,where f,g∈R[x1,...,xn]are two non-zero polynomials.The design of our algorithm involves two existing algorithms:one for computing the rational univariate representations of a complete chain of polynomials,another for catching strictly critical points in a real algebraic variety.The two algorithms are based on the well-known Wu’s method.With the aid of the computer algebraic system Maple,our algorithm has been made into a general program.In the final section of this paper,several examples are given to illustrate the efectiveness of our algorithm.     

Residues and telescopers for bivariate rational functions

We give necessary and sufficient conditions for the existence of telescopers for rational functions of two variables in the continuous, discrete and q-discrete settings and characterize which operators can occur as telescopers. Using this latter characterization, we reprove results of Furstenberg and Zeilberger concerning diagonals of power series representing rational functions. The key concept behind these considerations is a generalization of the notion of residue in the continuous case to an analogous concept in the discrete and q-discrete cases.     

Elliptic grids, rational functions, and the Padé interpolation

We consider rational functions with n prescribed poles for which there exists a divided difference operator transforming them to rational functions with n−1 poles. The poles of such functions are shown to lie on the elliptic grids. There is a one-to-one correspondence between this problem of admissible grids and the Poncelet problem on two quadrics. Additionally, we outline an explicit scheme of the Padé interpolation with prescribed poles and zeros on the elliptic grids. Dedicated to Richard Askey on the occasion of his seventieth birthday. 2000 Mathematics Subject Classification Primary—42C05; Secondary—39A13, 41A05, 41A21.     

Derivations and linear functions along rational functions

The main purpose of this paper is to give characterization theorems on derivations as well as on linear functions. Among others the following problem will be investigated: Let ${n \in \mathbb{Z}, f, g\colon\mathbb{R} \to\mathbb{R}}$ be additive functions, ${\left(\begin{array}{cc} a&b\\ c&d \end{array} \right) \in \mathbf{GL}_{2}(\mathbb{Q})}$ be arbitrarily fixed, and let us assume that the mapping $$ \phi(x)=g\left(\frac{ax^{n}+b}{cx^{n}+d}\right)-\frac{x^{n-1}f(x)}{(cx^{n}+d)^{2}} \quad \left(x\in\mathbb{R}, cx^{n}+d\neq 0\right)$$ satisfies some regularity on its domain (e.g. (locally) boundedness, continuity, measurability). Is it true that in this case the above functions can be represented as a sum of a derivation and a linear function? Analogous statements ensuring linearity will also be presented.     

Conditions for the representability of analytic functions by series of rational functions

With a functionf(z), analytic in the unit circle, we associate by a specific rule the series \(\sum\nolimits_{n = 1}^\infty {\frac{{A_n }}{{1 - \lambda _n z}},\left| {\lambda _n } \right|< 1} \) . we derive a (necessary and sufficient) condition for the convergence of the series in the unit circle. We derive further conditions under which the series converges to the functionf(z) itself.