Weighted approximation by entire functions interpolating at finitely or infinitely many points on the real line

We give error estimates for the weighted approximation of functions on the real line with Freud-type weights, by entire functions interpolating at finitely or infinitely many points on the real line. The error estimates involve weighted moduli of continuity corresponding to general Freud-type weights for which the density of polynomials is not always guaranteed.     

Interpolation of analytic functions with finitely many singularities

We consider an interpolation process for the class of functions with finitely many singular points by means of rational functions whose poles coincide with the singular points of the function under interpolation. The interpolation nodes form a triangular matrix. We find necessary and sufficient conditions for the uniform convergence of sequences of interpolation fractions to the function under interpolation on every compact set disjoint from the singular points of the function and other conditions for convergence. We generalize and improve the familiar results on the interpolation of functions with finitely many singular points by rational fractions and of entire functions by polynomials.     

Interpolation by L-spline functions of many variables

The choice of function space allows us to make conclusions in the multidimensional case that are analogous to results in the theory of spline functions of one variable. We establish the minimum norm property, the existence and uniqueness of a solution of the interpolation problem, the property of best approximation, and the convergence of interpolation processes.Translated from Matematicheskie Zametki, Vol. 14, No. 1, pp. 11–20, July, 1973.     

Toda flows with infinitely many variables

The Toda flow and related flows extend naturally to operators in Hilbert space and the purpose of this paper is to describe these flows and to analyse some of their special properties.     

Integrodifferential systems with infinitely many delays

Summary In this paper we consider the initial-value problems: (P ₁ )X(t)=(AX)(t) for t>0, X(0+)=I, X(t)=0 for t<0 and (P ₂ ) Y(t)=(QY)(t) for t>0, Y(0+)=I, Y(t)=0 for t<0, where A and Q are linear specified operators, I and0 — the identity and null matrices of order n, and X(t), Y(t) are unknown functions whose values are square matrices of order n. Sufficient conditions are established under which the problems (P ₁ ) and (P ₂ ) have the same unique solution, locally summable on the half-axis t ⩾0. Using this fact and some properties of the Laplace transform we find a new proof for the variation of constants formula given in[1, 2]. On the basis of this formula we derive certain results concerning a class of integrodifferential systems with infinite delay. Entrata in Redazione il 2 marzo 1977.     

Vector bundles with infinitely many souls

We construct the first examples of manifolds, the simplest one being , which admit infinitely many complete nonnegatively curved metrics with pairwise nonhomeomorphic souls.

     

Markets with countably infinitely many traders

In this paper we will show how to construct, in a canonical way, a continuous economy in the sense of R. J. Aumann [1]. This construction is based on W. Hildenbrand's [3] definition of “pure competition” for an exchange economy with countably infinitely many participants.     

Interpolation and frames in certain Banach spaces of entire functions

The important class of generalized bases known as frames was first introduced by Duffin and Schaeffer in their study of nonharmonic Fourier series in L ² (?π, π) [4]. Here we consider more generally the classical Banach spacesE ^p(1 ≤ p ≤ ∞) consisting of all entire functions of exponential type at most π that belong to L^p (?∞, ∞) on the real axis. By virtue of the Paley-Wiener theorem, the Fourier transform establishes an isometric isomorphism between L ² (?π, π) andE ² . When p is finite, a sequence {λ _n} of complex numbers will be called aframe forE ^p provided the inequalities $$A\left\| f \right\|^p \leqslant \sum {\left| {f\left( {\lambda _\pi } \right)} \right|^p } \leqslant B\left\| f \right\|^p $$ hold for some positive constants A and B and all functions f inE ^p. We say that {λ _n} is aninterpolating sequence forE ^p if the set of all scalar sequences {f (λ _n)}, with f εE ^p, coincides with ?^p. If in addition {λ _n} is a set of uniqueness forE ^p, that is, if the relations f(λ _n)=0(?∞<n<∞), with f εE ^p, imply that f ≡0, then we call {λ _n} acomplete interpolating sequence. Plancherel and Pólya [7] showed that the integers form a complete interpolating sequence forE ^p whenever1<p<∞. In Section 2 we show that every complete interpolating sequence forE ^p(1<p<∞) remains stable under a very general set of displacements of its elements. In Section 3 we use this result to prove a far-reaching generalization of another classical interpolation theorem due to Ingham [6].     

On weighted Hilbert spaces and integration of functions of infinitely many variables

We study aspects of the analytic foundations of integration and closely related problems for functions of infinitely many variables _x1,_x2,…∈D$x_1,x_2,\dots \in D$. The setting is based on a reproducing kernel k$k$ for functions on D$D$, a family of non-negative weights _γu${\gamma }_u$, where u$u$ varies over all finite subsets of N$\mathbb{N}$, and a probability measure ρ$\rho$ on D$D$. We consider the weighted superposition K=_∑uγuku$K={\sum }_u{\gamma }_u{k}_u$ of finite tensor products _ku$k_u$ of k$k$. Under mild assumptions we show that K$K$ is a reproducing kernel on a properly chosen domain in the sequence space D^N$D^{\mathbb{N}}$, and that the reproducing kernel Hilbert space H(K)$H\left(K\right)$ is the orthogonal sum of the spaces H(_γuku)$H\left({\gamma }_u{k}_u\right)$. Integration on H(K)$H\left(K\right)$ can be defined in two ways, via a canonical representer or with respect to the product measure ρ^N${\rho }^{\mathbb{N}}$ on D^N$D^{\mathbb{N}}$. We relate both approaches and provide sufficient conditions for the two approaches to coincide.     

Criteria for the nuclearity of spaces of functions of infinitely many variables

The present paper evolves from Berezanskii and Gali (Ukrainian Math. J.24 (4) (1972), 435–464) and Berezanskii, Gali, and Zuk (Soviet. Math. Dokl.13 (2) (1972)), in which, it was shown how one can construct a weighted infinite tensor product H_e,δ = ?_{n = 1:e,δ}^∞ = H_τn of Hilbert spaces H_τn with a given stabilizing sequence δ = (δ_n)_{n = 1}^∞(δ_n > 0). Here a weighted infinite tensor product ?_e = ?_{n = 1,e}^∞?_n of nuclear spaces ?_n is established first. Criteria for nuclearity of the constructed spaces are also given. Some examples of nuclear spaces of functions of infinite many variables K(T^∞) and A(T^∞) are obtained.     

On polling systems with infinitely many stations

The research was financially supported in part by the International Science Foundation (Grant NR8300) and the INTAS (Grant 93-820).