Multivariate Wavelet Frames

We proved that for any matrix dilation and for any positive integer $n$, there exists a compactly supported tight wavelet frame with approximation order $n$. Explicit methods for construction of dual and tight wavelet frames with a given number of vanishing moments are suggested.Comment: LaTex file, 28 page

Multivariate exact and falsified sampling approximation

Approximation properties of the expansions $\sum_{k\in{\mathbb z}^d}c_k\phi(M^jx+k)$, where $M$ is a matrix dilation, $c_k$ is either the sampled value of a signal $f$ at $M^{-j}k$ or the integral average of $f$ near $M^{-j}k$ (falsified sampled value), are studied. Error estimations in $L_p$-norm, $2\le p\le\infty$, are given in terms of the Fourier transform of $f$. The approximation order depends on how smooth is $f$, on the order of Strang-Fix condition for $\phi$ and on $M$. Some special properties of $\phi$ are required. To estimate the approximation order of falsified sampling expansions we compare them with a differential expansions $\sum_{k\in\,{\mathbb z}^d} Lf(M^{-j}\cdot)(-k)\phi(M^jx+k)$, where $L$ is an appropriate differential operator. Some concrete functions $\phi$ applicable for implementations are constructed. In particular, compactly supported splines and band-limited functions can be taken as $\phi$. Some of these functions provide expansions interpolating a signal at the points $M^{-j}k$.Comment: 23 page

Approximation by multivariate Kantorovich-Kotelnikov operators

Approximation properties of multivariate Kantorovich-Kotelnikov type operators generated by different band-limited functions are studied. In particular, a wide class of functions with discontinuous Fourier transform is considered. The $L_p$-rate of convergence for these operators is given in terms of the classical moduli of smoothness. Several examples of the Kantorovich-Kotelnikov operators generated by the ${\rm sinc}$-function and its linear combinations are provided

Quasi-projection operators in the weighted LpL_p spaces

Approximation properties of multivariate quasi-projection operators are studied in the paper. Wide classes of such operators are considered, including the sampling and the Kantorovich-Kotelnikov type operators generated by different band-limited functions.The rate of convergence in the weighted $L_p$-spaces for these operators is investigated. The results allow to estimate the error for reconstruction of signals (approximated functions) whose decay is not enough to be in $L_p$

Walsh and wavelet methods for differential equations on the Cantor group

Ordinary and partial differential equation for unknown functions defined on the Cantor dyadic group are studied. We consider two types of equations: related to the Gibbs derivatives and to the fractional modified Gibbs derivatives (or pseudo differential-operators). We find solutions in classes of distributions and study under what assumptions these solutions are regular functions with some "good" properties.Comment: 24 page

On orthogonal pp-adic wavelet bases

A variety of different orthogonal wavelet bases has been found in L_2(R) for the last three decades. It appeared that similar constructions also exist for functions defined on some other algebraic structures, such as the Cantor and Vilenkin groups and local fields of positive characteristic. In the present paper we show that the situation is quite different for the field of $p$-adic numbers. Namely, it is proved that any orthogonal wavelet basis consisting of band-limited (periodic) functions is a modification of Haar basis. This is a little bit unexpected because from the wavelet theory point of view, the additive group of $p$-adic numbers looks very similar to the Vilenkin group where analogs of the Daubechies wavelets (and even band-limited ones) do exist. We note that all $p$-adic wavelet bases and frames appeared in the literature consist of Schwartz-Bruhat functions (i.e., band-limited and compactly supported ones)

Differential and falsified sampling expansions

Differential and falsified sampling expansions $\sum_{k\in \mathbb{Z}^d}c_k\phi(M^jx+k)$, where $M$ is a matrix dilation, are studied. In the case of differential expansions, $c_k=Lf(M^{-j}\cdot)(-k)$, where $L$ is an appropriate differential operator. For a large class of functions $\phi$, the approximation order of differential expansions was recently studied. Some smoothness of the Fourier transform of $\phi$ from this class is required. In the present paper, we obtain similar results for a class of band-limited functions $\phi$ with the discontinuous Fourier transform. In the case of falsified expansions, $c_k$ is the mathematical expectation of random integral average of a signal $f$ near the point $M^{-j}k$. To estimate the approximation order of the falsified sampling expansions we compare them with the differential expansions. Error estimations in $L_p$-norm are given in terms of the Fourier transform of $f$.Comment: arXiv admin note: substantial text overlap with arXiv:1407.032

pp-Adic Haar multiresolution analysis and pseudo-differential operators

The notion of {\em $p$-adic multiresolution analysis (MRA)} is introduced. We discuss a ``natural'' refinement equation whose solution (a refinable function) is the characteristic function of the unit disc. This equation reflects the fact that the characteristic function of the unit disc is a sum of $p$ characteristic functions of mutually disjoint discs of radius $p^{-1}$. This refinement equation generates a MRA. The case $p=2$ is studied in detail. Our MRA is a 2-adic analog of the real Haar MRA. But in contrast to the real setting, the refinable function generating our Haar MRA is 1-periodic, which never holds for real refinable functions. This fact implies that there exist infinity many different 2-adic orthonormal wavelet bases in {\cL}^2(\bQ_2) generated by the same Haar MRA. All of these bases are described. We also constructed multidimensional 2-adic Haar orthonormal bases for {\cL}^2(\bQ_2^n) by means of the tensor product of one-dimensional MRAs. A criterion for a multidimensional $p$-adic wavelet to be an eigenfunction for a pseudo-differential operator is derived. We proved also that these wavelets are eigenfunctions of the Taibleson multidimensional fractional operator. These facts create the necessary prerequisites for intensive using our bases in applications

pp-Adic multiresolution analysis and wavelet frames

We study $p$-adic multiresolution analyses (MRAs). A complete characterisation of test functions generating MRAs (scaling functions) is given. We prove that only 1-periodic test functions may be taken as orthogonal scaling functions. We also suggest a method for the construction of wavelet functions and prove that any wavelet function generates a $p$-adic wavelet frame.Comment: 16 page

Uniform approximation by multivariate quasi-projection operators

Approximation properties of quasi-projection operators $Q_j(f,\varphi, \widetilde{\varphi})$ are studied. Such an operator is associated with a function $\varphi$ satisfying the Strang-Fix conditions and a tempered distribution $\widetilde{\varphi}$ such that compatibility conditions with $\varphi$ hold. Error estimates in the uniform norm are obtained for a wide class of quasi-projection operators defined on the space of uniformly continuous functions and on the anisotropic Besov spaces. Under additional assumptions on $\varphi$ and $\widetilde{\varphi}$, two-sided estimates in terms of realizations of the $K$-functional are also obtained.Comment: arXiv admin note: text overlap with arXiv:2006.1327