Generalized Bohr inequalities for certain classes of functions and their applications

Abstract

Let $ \mathcal{B}:=\{f(z)=\sum_{n=0}^{\infty}a_nz^n\; \mbox{with}\; |f(z)|<1\;\mbox{for all}\; z\in\mathbb{D}\} $. The improved version of the classical Bohr's inequality \cite{Bohr-1914} states that if $ f\in\mathcal{B} $, then the associated majorant series $ M_f(r):=\sum_{n=0}^{\infty}|a_n|r^n\leq 1 $ holds for $ |z|=r\leq 1/3 $ and the constant $ 1/3 $ cannot be improved. Bohr's original theorem and its subsequent generalizations remain active fields of study, driving investigations in a wide range of function spaces. In this paper, first we establish a generalized Bohr inequality for the class $\mathcal{B}$ by allowing a sequence $\{\varphi_n(r)\}_{n=0}^{\infty}$ of non-negative continuous functions on $[0, 1)$ in the place of $\{r^n\}_{n=0}^{\infty}$ of the majorant series $M_f(r)$ introducing a weighted sequence of non-negative continuous functions $\{\Phi_n(r)\}_{n=0}^{\infty}$ on $[0, 1)$. Secondly, as a generalization, we obtain a refined version of the Bohr inequality for a certain class $\tilde{G}^0_{\mathcal{H}}(\beta)$ of harmonic mappings. All the results are proved to be sharp.