This article is cited 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)
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.
UDC:
517.5
Received: 27.08.2021
Accepted: 06.12.2021
DOI:
10.22405/2226-8383-2021-22-4-100-113