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Chebyshevskii Sbornik, 2021, Volume 22, Issue 4, Pages 100–113
DOI: https://doi.org/10.22405/2226-8383-2021-22-4-100-113
(Mi cheb1095)
 

This article is cited in 1 scientific paper (total in 1 paper)

Nikol'skii constants for compact homogeneous spaces

D. V. Gorbachevab

a N. N. Krasovskii Institute of Mathematics and Mechanics (Yekaterinburg)
b Tula State University (Tula)
Full-text PDF (690 kB) Citations (1)
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Abstract: In this paper, we study the sharp $L^{p}$-Nikol'skii constants for the case of Riemannian symmetric manifolds $\mathbb{M}^{d}$ of rank $1$. These spaces are fully classified and include the unit Euclidean sphere $\mathbb{S}^{d}$, as well as the projective spaces $\mathbb{P}^{d}(\mathbb{R})$, $\mathbb{P}^{d}(\mathbb{C})$, $\mathbb{P}^{d}(\mathbb{H})$, $\mathbb{P}^{16}(\mathrm{Ca})$. There is a common harmonic analysis on these manifolds, in particular, the subspaces of polynomials $\Pi_{n}(\mathbb{M}^{d})$ of order at most $n$ are defined. In the general case, the sharp $ L^{p}$-Nikol'skii constant for the subspace $Y\subset L^{\infty}$ is defined by the equality
$$ \mathcal{C}(Y,L^{p})=\sup_{f\in (Y\cap L^{p})\setminus \{0\}}\frac{\|f\|_{\infty}}{\|f\|_{p}}. $$
V.A. Ivanov (1983) gave the asymptotics
$$ \mathcal{C}(\Pi_{n}(\mathbb{M}^{d}),L^{p}(\mathbb{M}^{d}))\asymp n^{d/p}, n\to \infty, p\in [1,\infty). $$
For the case of a sphere, this result was significantly improved by the author together with F. Dai and S. Tikhonov (2020):
$$ \mathcal{C}(\Pi_{n}(\mathbb{S}^{d}),L^{p}(\mathbb{S}^{d}))= \mathcal{C}(\mathcal{E}_{1}^{d},L^{p}(\mathbb{R}^{d}))n^{d/p}(1+o(1)), n\to \infty, p\in (0,\infty), $$
where $\mathcal{E}_{1}^{d}$ is the set of entire functions of exponential spherical type at most $1$ bounded on $\mathbb{R}^{d}$. M.I. Ganzburg (2020) transferred this equality to the case of the multidimensional torus $\mathbb{T}^{d}$ and trigonometric polynomials. For $d=1$, these results follow from the fundamental work of E. Levin and D. Lubinsky (2015). In a joint work of the author and I.A. Martyanov (2020), the following explicit boundaries of the spherical Nikol'skii constant were proved, which refine the above results for $p\ge 1$:
$$ n^{d/p}\le \frac{\mathcal{C}(\Pi_{n}(\mathbb{S}^{d}),L^{p}(\mathbb{S}^{d}))} {\mathcal{C}(\mathcal{E}_{1}^{d},L^{p}(\mathbb{R}^{d}))}\le \bigl(n+2\lceil \tfrac{d+1}{2p}\rceil\bigr)^{d/p}, n\in \mathbb{Z}_{+}, p\in [1,\infty). $$
This result was proved using a one-dimensional version of the problem for the case of a periodic Gegenbauer weight. The development of this method allows us to prove the following general result: for $p\ge 1$
$$ n^{d/p}\le \frac{\mathcal{C}(\Pi_{n}(\mathbb{M}^{d}),L^{p}(\mathbb{M}^{d}))} {\mathcal{C}(\mathcal{E}_{1}^{d},L^{p}(\mathbb{R}^{d}))}\le \bigl(n+\lceil \tfrac{\alpha_{d}+3/2}{p}\rceil+\lceil \tfrac{\beta_{d}+1/2}{p}\rceil\bigr)^{d/p}, $$
where $\alpha_{d}=d/2-1$, $\beta_{d}=d/2-1$, $-1/2$, $0$, $1$, $3$ respectively for $\mathbb{S}^{d}$, $\mathbb{P}^{d}(\mathbb{R})$, $\mathbb{P}^{d}(\mathbb{C})$, $\mathbb{P}^{d}(\mathbb{H})$, $\mathbb{P}^{16}(\mathrm {Ca})$. The proof of this result is based on the connection of harmonic analysis on $\mathbb{M}^{d}$ with Jacobi analysis on $[0,\pi] $ and $\mathbb{T}$ with periodic weight $\bigl|2\sin \tfrac{t}{2}\bigr|^{2\alpha+1}\bigl|\cos \tfrac{t}{2}\bigr|^{2\beta+1}$. Also we give related results for the trigonometric Nikol'skii constants in $L^{p}$ on $\mathbb{T}$ with Jacobi weight and Nikol'skii constants for entire functions of exponential type in $L^{p}$ on $\mathbb{R}$ with power weight.
Keywords: Nikolskii constant, homogeneous space, polynomial, entire function of exponential type, Jacobi weight.
Funding agency Grant number
Ministry of Science and Higher Education of the Russian Federation 075-02-2021-1383
The work was performed as part of research conducted in the Ural Mathematical Center with the financial support of the Ministry of Science and Higher Education of the Russian Federation (Agreement number 075-02-2021-1383).
Received: 27.08.2021
Accepted: 06.12.2021
Document Type: Article
UDC: 517.5
Language: Russian
Citation: D. V. Gorbachev, “Nikol'skii constants for compact homogeneous spaces”, Chebyshevskii Sb., 22:4 (2021), 100–113
Citation in format AMSBIB
\Bibitem{Gor21}
\by D.~V.~Gorbachev
\paper Nikol'skii constants for compact homogeneous spaces
\jour Chebyshevskii Sb.
\yr 2021
\vol 22
\issue 4
\pages 100--113
\mathnet{http://mi.mathnet.ru/cheb1095}
\crossref{https://doi.org/10.22405/2226-8383-2021-22-4-100-113}
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