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## Colloquium Mathematicum

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## Two-dimensional Shannon type expansions via one-dimensional affine and wavelet lattice actions

### Tom 157 / 2019

Colloquium Mathematicum 157 (2019), 107-120 MSC: 42C40, 65T60. DOI: 10.4064/cm7529-5-2018 Opublikowany online: 1 March 2019

#### Streszczenie

It is rather unexpected, but true, that it is possible to construct reproducing formulae and orthonormal bases of $L^2 (\mathbb R^2)$ just by applying the standard one-dimensional wavelet action of translations and dilations to the first variable $x_1$ of the generating function $\psi(x_1,x_2)$, $\psi \in L^2 (\mathbb R^2)$, i.e., by making use of building blocks $$\psi_{u,s}(x_1,x_2)=s^{-1/2}\psi\bigg(\frac{x_1-u}{s},x_2\bigg), \quad\ \text{where } u\in \mathbb R,\, s \gt 0,$$ in the case of reproducing formulae, and $$\psi_{k,m}(x_1,x_2)=2^{-k/2} \psi\bigg(\frac{x_1-2^k m}{2^k},x_2 \bigg), \quad\ \text{where } k,m\in \mathbb Z,$$ in the case of orthonormal bases. It is possible to compensate the fact that the second variable $x_2$ is not acted upon by a careful selection of the generating function $\psi$. Shannon wavelet tiling of the time-frequency plane $\mathbb R^2$ is a standard illustration of orthogonality and completeness phenomena corresponding to the Shannon wavelet, $$\chi_{(2^km,2^k(m+1)]}(x) \chi_{2^{-k}I}(\xi),\quad\ k,m\in \mathbb Z, \,I=- (1/2,1]\cup (1/2,1],$$ with $x$ representing time and $\xi$ frequency. In our current context, of the wavelet action restricted to the first coordinate of $\mathbb R^2$, it is substituted by a phase space tiling of $\mathbb R^4$ with unbounded, hyperboloid type blocks of the form $$\chi_{(2^km,2^k(m+1)]}(x_1)\sum_{n,l}\chi_{2^{-k}I_{D(n,l)}}(\xi_1) \chi_{(n,n+1]}(x_2)\chi_{(l,l+1]}(\xi_2),\quad k,m\in \mathbb Z,$$ where $I_r=2^{-r}I$, $r\ge 1$, and $D:\mathbb Z \times \mathbb Z \rightarrow \N$ is a bijection, an additional parameter of the generating function, needed for the lift from $L^2(\mathbb R)$ to $L^2(\mathbb R^2)$. The variables $x_1,x_2$ are the coordinates of position, and the variables $\xi_1,\xi_2$ of momentum.

#### Autorzy

• K. NowakDepartment of Computer Science
Drexel University
3141 Chestnut Street
e-mail
• M. PapFaculty of Sciences
University of Pécs
Ifjúság út 6
7624 Pécs, Hungary
e-mail

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