共查询到20条相似文献,搜索用时 31 毫秒
1.
A packing (resp. covering) ? of a normed space X consisting of unit balls is called completely saturated (resp. completely reduced) if no finite set of its members can be replaced by a more numerous (resp. less numerous) set of unit balls of X without losing the packing property (resp. covering property) of ?. We show that a normed space X admits completely saturated packings with disjoint closed unit balls as well as completely reduced coverings with open unit balls, provided that there exists a tiling of X with unit balls. Completely reduced coverings by open balls are of interest in the context of an approximation theory for continuous real‐valued functions that rests on so‐called controllable coverings of compact metric spaces. The close relation between controllable coverings and completely reduced coverings allows an extension of the approximation theory to non‐compact spaces. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
2.
This paper contains some theorems related to the best approximation ρn(f;E) to a function f in the uniform metric on a compact set by rational functions of degree at most n. We obtain results characterizing the relationship between ρn(f;K) and ρn(f;E) in the case when complements of compact sets K and E are connected, K is a subset of the interior Ω of E, and f is analytic in Ω and continuous on E. 相似文献
3.
A. A. Ligun 《Mathematical Notes》1973,14(1):563-569
For all odd r we construct a linear operator Br,r(f) which maps the set of 2-periodic functionsf(t) X(r) (X(r)=C(r) or L1
(r)) into a set of trigonometric polynomials of order not higher than n-1 such that where X is the C or L1 metric, En(f)X and (f, )X are the best approximation by means of trigonometric polynomials of order not higher than n-1 and the modulus of continuity of the functionf in the X metric, respectively; Kr are the known Favard constants.Translated from Matematicheskie Zametki, Vol. 14, No. 1, pp. 21–30, July, 1973.In conclusion, the author wishes to express his deep gratitude to N. P. Korneichuk under whose guidance this paper was written. 相似文献
4.
Wang Yangeng 《数学学报(英文版)》1997,13(3):333-336
The space of continuous maps from a topological spaceX to topological spaceY is denoted byC(X,Y) with the compact-open topology. In this paper we prove thatC(X,Y) is an absolute retract ifX is a locally compact separable metric space andY a convex set in a Banach space. From the above fact we know thatC(X,Y) is homomorphic to Hilbert spacel
2 ifX is a locally compact separable metric space andY a separable Banach space; in particular,C(R
n,Rm) is homomorphic to Hilbert spacel
2.
This research is supported by the Science Foundation of Shanxi Province's Scientific Committee 相似文献
5.
Suzanne Larson 《代数通讯》2013,41(8):2611-2627
Let X be a compact topological space and let C(X) denote the f-ring of all continuous real-valued functions defined on X. A point x in X is said to have rank n if, in C(X), there are n minimal prime ?-ideals contained in the maximal ?-ideal M x = {f ? C(X):f(x) = 0}. The space X has finite rank if there is an n ? N such that every point x ? X has rank at most n. We call X an SV space (for survaluation space) if C(X)/P is a valuation domain for each minimal prime ideal P of C(X). Every compact SV space has finite rank. For a bounded continuous function h defined on a cozeroset U of X, we say there is an h-rift at the point z if h cannot be extended continuously to U ∪ {z}. We use sets of points with h-rift to investigate spaces of finite rank and SV spaces. We show that the set of points with h-rift is a subset of the set of points of rank greater than 1 and that whether or not a compact space of finite rank is SV depends on a characteristic of the closure of the set of points with h-rift for each such h. If X has finite rank and the set of points with h-rift is an F-space for each h, then X is an SV space. Moreover, if every x ? X has rank at most 2, then X is an SV space if and only if for each h, the set of points with h-rift is an F-space. 相似文献
6.
A subspace If(X) of the space of idempotent probability measures on a given compact space X is constructed. It is proved that if the initial compact space X is contractible, then If(X) is a contractible compact space as well. It is shown that the shapes of the compact spaces X and If(X) are equal. It is also proved that, given a compact space X, the compact space If(X) is an absolute neighborhood retract if and only if so is X.
相似文献7.
Risong Li 《Communications in Nonlinear Science & Numerical Simulation》2012,17(7):2815-2823
Let f : X → X be a continuous map of a compact metric space X. The map f induces in a natural way a map fM on the space M(X) of probability measures on X, and a transformation fK on the space K(X) of closed subsets of X. In this paper, we show that if (X, f) is a chain transitive system with shadowing property, then exactly one of the following two statements holds:
- (a)
- fn and (fK)n are syndetically sensitive for all n ? 1.
- (b)
- fn and (fK)n are equicontinuous for all n ? 1.
8.
Ra. Siegmund-Schultze 《Mathematische Nachrichten》1991,151(1):101-120
We characterize those Banach spaces X for which K(X ⊕p X) is an M-ideal in L(X ⊕p X) by means of a variant of the compact metric approximation property. As a consequence we obtain that such a space X must be hereditarily lp-rich. 相似文献
9.
U. Feiste 《Mathematische Nachrichten》1978,83(1):161-165
Compact metric spaces χ of such a kind, that ??f =??(X), are characterized, ??(X) is the σ-field of BOREL sets and ??f(X) is the field generated by all open subset of X. Our main result is Theorem 5: If χ is a compact metric space, then the following conditions are equivalent:
- 1 ??f(X) =??(X).
- 2 card (X) ≦x0 and there are k, m?N such that card (X(k)) = m.
- 3 There are k, m?N such that χ is homeomorphic to ωk · m + 1.
10.
F. Vogl 《Journal of Approximation Theory》2000,107(2):281
For a class of analytic functions f(z) defined by Laplace–Stieltjes integrals the uniform convergence on compact subsets of the complex plane of the Bruwier series (B-series) ∑∞n=0 λn(f)
, λn(f)=f(n)(nc)+cf(n+1)(nc), generated by f(z) and the uniform approximation of the generating function f(z) by its B-series in cones |arg z|<
is shown. 相似文献
11.
Let T be a nonempty set of real numbers, X a metric space with metric d and XT the set of all functions from T into X. If fXT and n is a positive integer, we set , where the supremum is taken over all numbers a1,…,an,b1,…,bn from T such that a1b1a2b2anbn. The sequence is called the modulus of variation of f in the sense of Chanturiya. We prove the following pointwise selection principle: If a sequence of functions is such that the closure in X of the set is compact for each tT and (∗) then there exists a subsequence of , which converges in X pointwise on T to a function fXT satisfying limn→∞ν(n,f)/n=0. We show that condition (*) is optimal (the best possible) and that all known pointwise selection theorems follow from this result (including Helly's theorem). Also, we establish several variants of the above theorem for the almost everywhere convergence and weak pointwise convergence when X is a reflexive separable Banach space. 相似文献
12.
U. Luther 《Annali di Matematica Pura ed Applicata》2003,182(2):161-200
We show that the representation theorem for classical approximation spaces can be generalized to spaces A(X,l
q
(ℬ))={f∈X:{E
n
(f)}∈l
q
(ℬ)} in which the weighted l
q
-space l
q
(ℬ) can be (more or less) arbitrary. We use this theorem to show that generalized approximation spaces can be viewed as real
interpolation spaces (defined with K-functionals or main-part K-functionals) between couples of quasi-normed spaces which satisfy certain Jackson and Bernstein-type inequalities. Especially,
interpolation between an approximation space and the underlying quasi-normed space leads again to an approximation space.
Together with a general reiteration theorem, which we also prove in the present paper, we obtain formulas for interpolation
of two generalized approximation spaces.
Received: December 6, 2001; in final form: April 2, 2002?Published online: March 14, 2003 相似文献
13.
Valeria Marraffa 《Journal of Theoretical Probability》1988,1(3):255-261
It is shown here that for any Banach spaceE-valued amart (X
n) of classB, almost sure convergence off(Xn) tof(X) for eachf in a total subset ofE
* implies scalar convergence toX. 相似文献
14.
R. Lucchetti P. Shunmugaraj Y. Sonntag 《Numerical Functional Analysis & Optimization》2013,34(1-2):103-115
Given a continuous function f: X→R, sufficient conditions are offered for the continuity of the value function v(A):=inf{f{x): x ε A} and of the level set multifunction Lev(A, α) := {x ε A: f(x)?α}, with respect to recently defined topologies on the closed sets of a metric space. 相似文献
15.
Lipschitz continuity and Gateaux differentiability of the best approximation operator in vector-valued Chebyshev approximation 总被引:1,自引:1,他引:0
When G is a finite-dimensional Haar subspace of C(X,Rk), the vector-valued functions (including complex-valued functions when k is 2) from a finite set X to Euclidean k-dimensional space, it is well-known that at any function f in C(X,Rk) the best approximation operator satisfies the strong unicity condition of order 2 and a Lipschitz (Hőlder) condition of order . This note shows that in fact the best approximation operator satisfies the usual Lipschitz condition of order 1 and has a Gateaux derivative on a dense set of functions in C(X,Rk). 相似文献
16.
LetC(X,Y) be the space of continuous functions from a metric space (X,d) to a metric space (Y, e).C(X, Y) can be thought as subset of the hyperspaceCL(X×Y) of closed and nonempty subsets ofX×Y by identifying each element ofC(X,Y) with its graph. We considerC(X,Y) with the topology inherited from the Wijsman topology induced onCL(X×Y) by the box metric ofd ande. We study the relationships between the Wijsman topology and the compact-open topology onC(X,Y) and also conditions under which the Wijsman topology coincide with the Fell topology. Sufficient conditions under which the
compactopen topology onC(X,Y) is weaker than the Wijsman topology are given (IfY is totally bounded, then for every metric spaceX the compactopen topology onC(X,Y) is weaker than the Wijsman topology and the same is true forX locally connected andY rim-totally bounded). We prove that a metric spaceX is boundedly compact iff the Wijsman topology onC(X, ℝ) is weaker than the compact-open topology. We show that ifX is a σ-compact complete metric space andY a compact metric space, then the Wijsman topology onC(X,Y) is Polish. 相似文献
17.
Let (X, ρ) be a metric space and ↓USCC(X) and ↓CC(X) be the families of the regions below all upper semi-continuous compact-supported maps and below all continuous compact-supported maps from X to I = [0, 1], respectively. With the Hausdorff-metric, they are topological spaces. In this paper, we prove that, if X is an infinite compact metric space with a dense set of isolated points, then (↓USCC(X), ↓CC(X)) ≈ (Q, c0 ∪ (Q \ Σ)), i.e., there is a homeomorphism h :↓USCC(X) → Q such that h(↓CC(X)) = c0 ∪ (Q \ Σ... 相似文献
18.
We consider the problem of embedding a certain finite metric space to the Euclidean space, trying to keep the bi-Lipschitz
constant as small as possible. We introduce the notationc
2(X, d) for the least distortion with which the metric space (X, d) may be embedded in a Euclidean space. It is known that if (X, d) is a metric space withn points, thenc
2(X, d)≤0(logn) and the bound is tight. LetT be a tree withn vertices, andd be the metric induced by it. We show thatc
2(T, d)≤0(log logn), that is we provide an embeddingf of its vertices to the Euclidean space, such thatd(x, y)≤‖f(x)−f(y) ‖≤c log lognd(x, y) for some constantc.
Supported in part by grants from the Israeli Academy of Sciences and the US-Israel Binational Science Foundation.
Supported in part by NSF under grants CCR-9215293 and by DIMACS, which is supported by NSF grant STC-91-19999 and by the New
Jersey Commission on Science and Technology. 相似文献
19.
Let ℂ[−1,1] be the space of continuous functions on [−,1], and denote by Δ2 the set of convex functions f ∈ ℂ[−,1]. Also, let E
n
(f) and E
n
(2) (f) denote the degrees of best unconstrained and convex approximation of f ∈ Δ2 by algebraic polynomials of degree < n, respectively. Clearly, En (f) ≦ E
n
(2) (f), and Lorentz and Zeller proved that the inverse inequality E
n
(2) (f) ≦ cE
n
(f) is invalid even with the constant c = c(f) which depends on the function f ∈ Δ2.
In this paper we prove, for every α > 0 and function f ∈ Δ2, that
where c(α) is a constant depending only on α. Validity of similar results for the class of piecewise convex functions having s convexity changes inside (−1,1) is also investigated. It turns out that there are substantial differences between the cases
s≦ 1 and s ≧ 2.
Dedicated to Jóska Szabados on his 70th birthday 相似文献
20.
João B. Prolla 《Numerical Functional Analysis & Optimization》2013,34(3-4):349-353
Let X be a compact HausdorfF space and let D(X) be the set of all continuous real-valued functions f defined on X and such that 0 ≤ f(x) ≤ 1, for all x ? X. The set D(X) is equipped with the uniform topology. We characterize the uniform closure of subsets A ? D(X) containing 0 and 1 and ?ψ + (1 ? ?)η, whenever they contain ?, ψ and η 相似文献