A Geometric characterization of Banach spaces with $p$-Bohr radius

Vasudevarao Allu, Himadri Halder, Subhadip Pal

Published 2022-01-11, updated 2022-01-19Version 2

For any complex Banach space $X$ and each $p \in [1,\infty)$, we introduce the $p$-Bohr radius of order $N(\in \mathbb{N})$ is $\widetilde{R}_{p,N}(X)$ defined by $$ \widetilde{R}_{p,N}(X)=\sup \left\{r\geq 0: \sum_{k=0}^{N}\norm{x_k}^p r^{pk} \leq \norm{f}^p_{H^{\infty}(\mathbb{D}, X)}\right\}, $$ where $f(z)=\sum_{k=0}^{\infty} x_{k}z^k \in H^{\infty}(\mathbb{D}, X)$. We also introduce the following geometric notion of $p$-uniformly $\mathbb{C}$-convexity of order $N$ for a complex Banach space $X$ for some $N \in \mathbb{N}$. For $p\in [2,\infty)$, a complex Banach space $X$ is called $p$-uniformly $\mathbb{C}$-convex of order $N$ if there exists a constant $\lambda > 0$ such that \begin{align}\label{e-0.1} \left(\norm{x_0}^p + \lambda \norm{x_1}^p + {\lambda}^2 \norm{x_2}^p + \cdots + {\lambda}^N \norm{x_N}^p \right)^{1/p} \leq \max_{\theta \in [0,2\pi)} \norm{x_0 + \sum_{k=1}^{N}e^{i \theta}x_k} \end{align} for all $x_0$, $x_1$,$\dots$, $x_N$ $\in X$. We denote $A_{p,N}(X)$, the supremum of all such constants $\lambda$ satisfying \eqref{e-0.1}. We obtain the lower and upper bounds of $\widetilde{R}_{p,N}(X)$ in terms of $A_{p,N}(X)$. In this paper, for $p\in [2,\infty)$ and each $N \in \mathbb{N}$, we prove that a complex Banach space $X$ is $p$-uniformly $\mathbb{C}$-convex of order $N$ if, and only if, the $p$-Bohr radius of order $N$ $\widetilde{R}_{p,N}(X)>0$. We also study the $p$-Bohr radius of order $N$ for the Lebesgue spaces $L^q (\mu)$ and the sequence spaces $l^q$ for $1\leq p<q<\infty$ or $1\leq q \leq p <2$.