
arXiv: 1907.03832
Let $Π_n^d$ denote the space of spherical polynomials of degree at most $n$ on the unit sphere $\mathbb{S}^d\subset \mathbb{R}^{d+1}$ that is equipped with the surface Lebesgue measure $dσ$ normalized by $\int_{\mathbb{S}^d} \, dσ(x)=1$. This paper establishes a close connection between the asymptotic Nikolskii constant, $$ \mathcal{L}^\ast(d):=\lim_{n\to \infty} \frac 1 {\dim Π_n^d} \sup_{f\in Π_n^d} \frac { \|f\|_{L^\infty(\mathbb{S}^d)}}{\|f\|_{L^1(\mathbb{S}^d)}},$$ and the following extremal problem: $$ \mathcal{I}_α:=\inf_{a_k} \Bigl\| j_{α+1} (t)- \sum_{k=1}^\infty a_k j_α \bigl( q_{α+1,k}t/q_{α+1,1}\bigr)\Bigr\|_{L^\infty(\mathbb{R}_+)} $$ with the infimum being taken over all sequences $\{a_k\}_{k=1}^\infty\subset \mathbb{R}$ such that the infinite series converges absolutely a.e. on $\mathbb{R}_+$. Here $j_α$ denotes the Bessel function of the first kind normalized so that $j_α(0)=1$, and $\{q_{α+1,k}\}_{k=1}^\infty$ denotes the strict increasing sequence of all positive zeros of $j_{α+1}$. We prove that for $α\ge -0.272$, $$\mathcal{I}_α= \frac{\int_{0}^{q_{α+1,1}}j_{α+1}(t)t^{2α+1}\,dt}{\int_{0}^{q_{α+1,1}}t^{2α+1}\,dt}= {}_{1}F_{2}\Bigl(α+1;α+2,α+2;-\frac{q_{α+1,1}^{2}}{4}\Bigr). $$ As a result, we deduce that the constant $\mathcal{L}^\ast(d)$ goes to zero exponentially fast as $d\to\infty$: \[ 0.5^d\le \mathcal{L}^{*}(d)\le (0.857\cdots)^{d\,(1+\varepsilon_d)} \ \ \ \ \ \text{with $\varepsilon_d =O(d^{-2/3})$.} \]
27 pages
Orthogonal polynomials and functions in several variables expressible in terms of special functions in one variable, spherical harmonics, 33C55, 33C50, 42B15, 42C10, Multipliers for harmonic analysis in several variables, best Nikolskii constants, Fourier series in special orthogonal functions (Legendre polynomials, Walsh functions, etc.), Bessel functions, Mathematics - Classical Analysis and ODEs, Classical Analysis and ODEs (math.CA), FOS: Mathematics, Spherical harmonics, entire functions of exponential type, Nikolskii inequalities
Orthogonal polynomials and functions in several variables expressible in terms of special functions in one variable, spherical harmonics, 33C55, 33C50, 42B15, 42C10, Multipliers for harmonic analysis in several variables, best Nikolskii constants, Fourier series in special orthogonal functions (Legendre polynomials, Walsh functions, etc.), Bessel functions, Mathematics - Classical Analysis and ODEs, Classical Analysis and ODEs (math.CA), FOS: Mathematics, Spherical harmonics, entire functions of exponential type, Nikolskii inequalities
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