Difference between revisions of "Hyperbolic cross"
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| + | A summation domain of multiple [[Fourier series|Fourier series]] (cf. also [[Partial Fourier sum|Partial Fourier sum]]). Let $f ( x )$ be an integrable periodic function of $n$ variables defined on $\mathbf{T} ^ { n }$, $\mathbf{T} = ( - \pi , \pi ]$. It has an expansion as a Fourier series, $\sum _ { \mathbf{k} } c_{ \mathbf{k} } e ^ { i \mathbf{kx} }$, $\mathbf{k} = ( k _ { 1 } , \dots , k _ { n } )$, ${\bf x} = ( x _ { 1 } , \ldots , x _ { n } )$, $\mathbf{k} \cdot \mathbf{x} = k _ { 1 } x _ { 1 } + \ldots + k _ { n } x _ { n }$. Unlike in the one-dimensional case, there is no natural ordering of the Fourier coefficients, so the choice of the order of summation is of great importance. | ||
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| + | Let $\mathbf{r} = ( r _ { 1 } , \dots , r _ { n } ) \in \mathbf{R} ^ { n }$ with all coordinates positive, $r_j > 0$. Consider the [[Differential operator|differential operator]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h130/h130130/h13013011.png"/> with periodic boundary conditions on $\mathbf{T} ^ { n }$. Then the eigenvalues (cf. [[Eigen value|Eigen value]]) of $D ^ { \mathbf{r} }$ are <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h130/h130130/h13013014.png"/>, while the corresponding eigenfunctions are $e ^ { i \mathbf k . \mathbf x }$. The partial sums of the Fourier series corresponding to the eigenfunctions with all eigenvalues $| \lambda _ { \mathbf{k} } | \leq N$ are called hyperbolic partial Fourier sums of order $N$ (or hyperbolic crosses). This approach, in which the method of summation of the Fourier series is defined by the differential operator, is due to K. Babenko [[#References|[a1]]], who applied it to various problems in [[Approximation theory|approximation theory]] (e.g., Kolmogorov widths, $\varepsilon$-entropy, etc.). Subsequently the hyperbolic cross itself became the object of study in connection with Lebesgue constants, the Bernshtein inequality, etc. Also, this approach initiated a detailed study and applications of spaces of functions with bounded mixed derivative (in $L _ { p }$). | ||
Many of these and related classes, as well as various problems in approximation theory, are considered in [[#References|[a2]]]. This method of summation has also been applied to other series expansions, e.g., multiple wavelets systems. | Many of these and related classes, as well as various problems in approximation theory, are considered in [[#References|[a2]]]. This method of summation has also been applied to other series expansions, e.g., multiple wavelets systems. | ||
====References==== | ====References==== | ||
| − | <table>< | + | <table><tr><td valign="top">[a1]</td> <td valign="top"> K. Babenko, "Approximation of periodic functions of many variables by trigonometric polynomials" ''Soviet Math.'' , '''1''' (1960) pp. 513–516 ''Dokl. Akad. Nauk. SSSR'' , '''132''' (1960) pp. 247–250</td></tr><tr><td valign="top">[a2]</td> <td valign="top"> V. Temlyakov, "Approximation of periodic functions" , Nova Sci. (1993)</td></tr></table> |
Revision as of 16:59, 1 July 2020
A summation domain of multiple Fourier series (cf. also Partial Fourier sum). Let $f ( x )$ be an integrable periodic function of $n$ variables defined on $\mathbf{T} ^ { n }$, $\mathbf{T} = ( - \pi , \pi ]$. It has an expansion as a Fourier series, $\sum _ { \mathbf{k} } c_{ \mathbf{k} } e ^ { i \mathbf{kx} }$, $\mathbf{k} = ( k _ { 1 } , \dots , k _ { n } )$, ${\bf x} = ( x _ { 1 } , \ldots , x _ { n } )$, $\mathbf{k} \cdot \mathbf{x} = k _ { 1 } x _ { 1 } + \ldots + k _ { n } x _ { n }$. Unlike in the one-dimensional case, there is no natural ordering of the Fourier coefficients, so the choice of the order of summation is of great importance.
Let $\mathbf{r} = ( r _ { 1 } , \dots , r _ { n } ) \in \mathbf{R} ^ { n }$ with all coordinates positive, $r_j > 0$. Consider the differential operator 
with periodic boundary conditions on $\mathbf{T} ^ { n }$. Then the eigenvalues (cf. Eigen value) of $D ^ { \mathbf{r} }$ are 
, while the corresponding eigenfunctions are $e ^ { i \mathbf k . \mathbf x }$. The partial sums of the Fourier series corresponding to the eigenfunctions with all eigenvalues $| \lambda _ { \mathbf{k} } | \leq N$ are called hyperbolic partial Fourier sums of order $N$ (or hyperbolic crosses). This approach, in which the method of summation of the Fourier series is defined by the differential operator, is due to K. Babenko [a1], who applied it to various problems in approximation theory (e.g., Kolmogorov widths, $\varepsilon$-entropy, etc.). Subsequently the hyperbolic cross itself became the object of study in connection with Lebesgue constants, the Bernshtein inequality, etc. Also, this approach initiated a detailed study and applications of spaces of functions with bounded mixed derivative (in $L _ { p }$).
Many of these and related classes, as well as various problems in approximation theory, are considered in [a2]. This method of summation has also been applied to other series expansions, e.g., multiple wavelets systems.
References
| [a1] | K. Babenko, "Approximation of periodic functions of many variables by trigonometric polynomials" Soviet Math. , 1 (1960) pp. 513–516 Dokl. Akad. Nauk. SSSR , 132 (1960) pp. 247–250 |
| [a2] | V. Temlyakov, "Approximation of periodic functions" , Nova Sci. (1993) |
How to Cite This Entry:
Hyperbolic cross. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Hyperbolic_cross&oldid=15654
Hyperbolic cross. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Hyperbolic_cross&oldid=15654
This article was adapted from an original article by E.S. Belinsky (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article