共查询到20条相似文献,搜索用时 78 毫秒
1.
Michael I. Ganzburg 《Constructive Approximation》2008,27(3):289-321
Let B be a closed linear subspace of a Banach space F and let
be a group of continuous linear operators
, where G is a compact topological group. We prove that if
is invariant under
, then under some conditions on f, F, B, and G, there exists an element
of best approximation to f that has the same property. As applications, we compute the bivariate Bernstein constant for
polynomial approximation of
and solve a Braess problem on the exponential order of decay of the error of polynomial approximation of
. Other examples and
applications are discussed as well. 相似文献
2.
We show that every function in the Hardy space can be approximated by linear combinations of translates and dilates of a synthesizer
, provided only that
and
satisfies a mild regularity condition. Explicitly, we prove scale averaged approximation for each
,
where
is an arbitrary lacunary sequence (such as
) and the coefficients
are local averages of f. This formula holds in particular if the synthesizer
is in the Schwartz class, or if it has compact support and belongs to
for some
in terms of differences of
. 相似文献
3.
We continue the investigation of some problems in learning theory in the setting formulated by F. Cucker and S. Smale. The
goal is to find an estimator
on the base of given data
that approximates well the regression function
of an unknown Borel probability measure
defined on
We assume that
belongs to a function class
It is known from previous works that the behavior of the entropy numbers
of
in the uniform norm
plays an important role in the above problem. The standard way of measuring the error between a target function
and an estimator
is to use the
norm (
is the marginal probability measure on X generated by
). This method has been used in previous papers. We continue to use this method in this paper. The use of the
norm in measuring the error has motivated us to study the case when we make an assumption on the entropy numbers
of
in the
norm. This is the main new ingredient of thispaper. We construct good estimators in different settings: (1) we know both
and
; (2) we know
but we do not know
and (3) we only know that
is from a known collection of classes but we do not know
An estimator from the third setting is called a universal estimator. 相似文献
4.
Jacek Dziubanski 《Constructive Approximation》2008,27(3):269-287
Let
be the standard Laguerre functions of type a. We denote
. Let
and
be the semigroups associated with the orthonormal systems
and
. We say that a function f belongs to the Hardy space
associated with one of the semigroups if the corresponding maximal function belongs to
. We prove special atomic decompositions of the elements of the Hardy spaces. 相似文献
5.
Let
be a nontrivial probability measure on the unit circle
the density of its absolutely continuous part,
its Verblunsky coefficients, and
its monic orthogonal polynomials. In this paper we compute the coefficients of
in terms of the
. If the function
is in
, we do the same for its Fourier coefficients. As an application we prove that if
and if
is a polynomial, then with
and S the left-shift operator on sequences we have
We also study relative ratio asymptotics of the reversed polynomials
and provide a necessary and sufficient condition in terms of the Verblunsky coefficients of the measures
and
for this difference to converge to zero uniformly on compact subsets of
. 相似文献
6.
Almost exponentially localized polynomial kernels are constructed on the unit ball
in
with weights
, by smoothing out the coefficients of the corresponding orthogonal projectors. These kernels are utilized to the design of
cubature formulas on
with respect to
and to the construction of polynomial tight frames in
(called needlets) whose elements have nearly exponential localization. 相似文献
7.
We give conditions on radial nonnegative weights $W_1We give conditions on radial nonnegative weights
and
on
, for which the a priori inequality
holds with constant independent of
. Here
is the Laplace-Beltrami operator on the sphere
. Due to the relation between
and the tangential component of the gradient,
, we obtain some "Morawetz-type" estimates for
on
. As a consequence we establish some new estimates for the free Schr?dinger propagator
, which may be viewed as certain refinements of the
-(super)smoothness estimates of Kato and Yajima. These results, in turn, lead to the well-posedness of the initial value problem
for certain time dependent first order spherical perturbations of the
dimensional Schr?dinger equation. 相似文献
8.
Regular Semigroups with Inverse Transversals 总被引:2,自引:0,他引:2
Fenglin Zhu 《Semigroup Forum》2006,73(2):207-218
Let C be a semiband with an inverse transversal
. In [7], G.T. Song and F.L. Zhu construct a fundamental regular semigroup
with an inverse transversal
.
is isomorphic to a subsemigroup of the Hall semigroup of C but it is easier to handle. Its elements are partial transformations,
and the operation-although not the usual composition-is defined by means of composition. Any full regular subsemigroup T of
is a fundamental regular semigroup with inverse transversal
. Moreover, any regular semigroup S with an inverse transversal
is proved to be an idempotent-separating coextension of a full regular subsemigroup T of some
. By means of a full
regular subsemigroup T of some
and by means of an inverse semigroup K satisfying some conditions, in this paper, we construct a regular semigroup
with inverse transversal
such that
is isomorphic to K and
to T. Furthermore, it is proved that if S is a regular semigroup with an inverse transversal
then S can be constructed from the corresponding T and from
in this way. 相似文献
9.
Zachary Mesyan 《Semigroup Forum》2007,75(3):648-675
Let
be a countably infinite set,
the group of permutations of
, and
the monoid of self-maps of
. Given two subgroups
, let us write
if there exists a finite subset
such that the groups generated by
and
are equal. Bergman and Shelah showed that the subgroups which are closed in the function topology on S fall into exactly
four equivalence classes with respect to
. Letting
denote the obvious analog of
for submonoids of E, we prove an analogous result for a certain class of submonoids of E, from which the theorem for groups
can be recovered. Along the way, we show that given two subgroups
which are closed in the function topology on S, we have
if and only if
(as submonoids of E), and that
for every subgroup
(where
denotes the closure of G in the function topology in S and
its closure in the function topology in E). 相似文献
10.
Rostom Getsadze 《Journal of Fourier Analysis and Applications》2006,12(5):597-604
We prove the following theorem: For arbitrary
there exists a nonnegative
function
such that
and
almost everywhere on
where
is the double Walsh-Paley system.
This statement remains true also for the double trigonometric system. 相似文献
11.
In this paper we study the worst-case error (of numerical integration) on the unit sphere
for all functions in the unit ball of the Sobolev space
where
More precisely, we consider infinite sequences
of m(n)-point numerical integration rules
where: (i)
is exact for all spherical polynomials of degree
and (ii)
has positive weights or, alternatively to (ii), the sequence
satisfies a certain local regularity property. Then we show that the worst-case error (of numerical integration)
in
has the upper bound
where the constant c depends on s and d (and possibly the sequence
This extends the recent results for the sphere
by K. Hesse and I.H. Sloan to spheres
of arbitrary dimension
by using an alternative representation of the worst-case error. If the sequence
of numerical integration rules satisfies
an order-optimal rate of convergence is achieved. 相似文献
12.
Given a function ψ in
the affine (wavelet) system generated by ψ, associated to an invertible matrix a and a lattice Γ, is the collection of functions
In this paper we prove that the set of functions generating affine systems that are a Riesz basis of
${\cal L}^2({\Bbb R}^d)$ is dense in We also prove that a stronger result is true for affine systems that are a frame of
In this case we show that the generators associated to a fixed but arbitrary dilation are a dense set. Furthermore, we analyze
the orthogonal case in which we prove that the set of generators of orthogonal (not necessarily complete) affine systems,
that are compactly supported in frequency, are dense in the unit sphere of
with the induced metric. As a byproduct we introduce the p-Grammian of a function and prove a convergence result of this
Grammian as a function of the lattice. This result gives insight in the problem of oversampling of affine systems. 相似文献
13.
Jesus Jeronimo Castro 《Discrete and Computational Geometry》2007,37(3):409-417
Let
be a family of convex figures in the plane. We say that
has property T if there exists a line intersecting every member of
. Also, the family
has property T(k) if every k-membered subfamily of
has property T. Let B be the unit disc centered at the origin. In this paper we prove that if a finite family
of translates of B has property T(4) then the family
, where
, has property T. We also give some results concerning families of translates of the unit disc which has either property T(3)
or property T(5). 相似文献
14.
Arthur D. Grainger 《Semigroup Forum》2006,73(2):234-242
Let J be an infinite set and let
, i.e., I is the collection of all non empty finite subsets of
J. Let
denote the collection of all ultrafilters on the set I and let
be the compact (Hausdorff) right topological semigroup that is the Stone-Cech Compactification of the semigroup
equipped with the discrete topology. This paper continues the study of
that was started in [3] and [5]. In [5], Koppelberg established that
(where K( S) is the smallest ideal of a semigroup S) and for non empty
she established
. In this note, we show that for
such that
is infinite,
is a proper subset of
and
, where
. 相似文献
15.
A.J.E.M. Janssen 《Journal of Fourier Analysis and Applications》1994,1(4):403-436
Let
and let
In this paper we investigate the relation between the frame operator
and the matrix
whose entries
are given by
for
Here
, for any
We show that
is bounded as a mapping of
into
if and only if
is bounded as a mapping of
into
Also we show that
if and
only if
where
denotes the identity operator of
and
respectively, and
Next, when
generates a frame, we have that
has an upper frame bound, and the minimal dual function
can be computed as
The results of this paper extend, generalize, and rigourize results of Wexler and Raz and of Qian, D. Chen, K. Chen, and
Li on the computation of dual functions for finite, discrete-time Gabor expansions to the infinite, continuous-time case.
Furthermore, we present a framework in which one can show that certain smoothness and decay properties of a
generating a frame are inherited by
In particular, we show that
when
generates a frame
Schwartz space). The proofs of the main results of this paper rely heavily on a technique introduced by Tolimieri and Orr
for relating frame bound questions on complementary lattices by means of the Poisson summation formula. 相似文献
16.
Kernel and Trace Operators for Extensions of Brandt Semigroups 总被引:1,自引:0,他引:1
Mario Petrich 《Semigroup Forum》2007,75(1):18-44
Let S be an (ideal) extension of a Brandt semigroup S0 by a Brandt semigroup S1 and let
denote the congruence lattice of S. For
denote by
and
the least and the greatest congruences on S with the same kernel as
respectively, and let
and
have the analogous meaning relative to trace. We establish necessary and sufficient conditions on S in order that one or
more of the operators
be
- or
-homomorphisms on
The conditions are expressed directly in terms of a construction of an extension of S0 and S1 and the proofs make use of a construction of congruences on S expressed by means of congruences on S0 and S1. 相似文献
17.
18.
C. Carton-Lebrun 《Journal of Fourier Analysis and Applications》1995,2(1):49-64
For
define
where
Pointwise estimates and weighted inequalities describing the local Lipschitz continuity
of
are established. Sufficient conditions are found
for the boundedness of
from
into
and a spherical restriction property is proved. A study of the moment subspaces of
is next developed in the one-variable case, for
locally integrable,
a.e. It includes a decomposition theorem and a complete classification of all possible sequences of moment subspaces in
Characterizations are also given for each class. Applications related to the approximation and decomposition of
are discussed. 相似文献
19.
In this article we show that the distributional point values of a tempered distribution are characterized by their Fourier
transforms in the following way: If
and
, and
is locally integrable, then
distributionally if and only if there exists k such that
, for each a > 0, and similarly in the case when
is a general distribution. Here
means in the Cesaro sense. This result generalizes the characterization of Fourier series of distributions with a distributional
point value given in [5] by
. We also show that under some extra conditions, as if the sequence
belongs to the space
for some
and the tails satisfy the estimate
,\ as
, the asymmetric partial sums\ converge to
. We give convergence results in other cases and we also consider the convergence of the asymmetric partial integrals. We
apply these results to lacunary Fourier series of distributions. 相似文献
20.
D.S. Lubinsky 《Constructive Approximation》2007,25(3):303-366
Assume
is not an integer. In papers published in 1913 and 1938,
S.~N.~Bernstein established the limit
Here
denotes the error in best uniform approximation of
by polynomials
of degree
. Bernstein proved that
is itself the error in best uniform approximation of
by entire functions of exponential type at most 1,
on the whole real line. We prove that the best approximating entire function
is unique, and satisfies an alternation property. We show that the scaled
polynomials of best approximation converge to this unique entire function.
We derive a representation for
, as well
as its
analogue for
. 相似文献