On A Characterization of Compactness and the Abel-Poisson Summability of Fourier Coefficients In Banach Spaces

Abstract

Let $ T $ be the topological group of the unit circle with Euclidean topology, $ H $ be a complex Banach space, $ \alpha $ be a strongly continuous isometric linear representation of $ T $ in $ H $, $ x\in H $ and $ \lbrace F_{k}^{\alpha}(x) \rbrace_{k \in\mathbb{Z}}$ be the family of Fourier coefficients of $ x $ with respect to $ \alpha $. In this paper, an integral representation for Abel-Poisson mean operator of the family $ \lbrace F_{n}^{\alpha}(x)\rbrace $ of an $ x\in H $ is given, then by means of this representation it is proved that the family $ \lbrace F_{k}^{\alpha}(x) \rbrace_{k \in\mathbb{Z}} $ is Abel-Poisson summable to $x$, and some tests for relatively compactness for a subset of $H$ are given in terms of $ \alpha $.