Difference and $(\Delta)$ properties for some new classes

Abstract

\begin{abstract} {In this paper we study difference and $(\Delta)$ properties for the classes of the form $C_0(J,X)$, $\frak {g} \U$, $\U+\frak {g} \V$, where $\U, \V\in \{BUC(J,X), UC(J,X)\}$ and $\frak{g} (t)=e^{it^2}$, $t\in \mathbb{R}$. For functions whose differences belong to $ \F \in\{C_0(J,X)$, $\frak {g} \U\}$, we prove a new stronger $(\Delta)$ property (S$\Delta$P): If $f: J\to X$ and $\Delta_h f \in \F$, $h > 0$, then $f\in C(J,X)$ and $(f-M_hf)\in \F$, $h > 0$. \noindent See (Lemma 2.5, Theorems 3.1, 3.2, Lemma 3.4, Theorem 3.7). These results enabled us to prove $(\Delta)$ for $\U+\frak {g} \V$ even when $\U, \V\in UC(J,X) $ (Theorem 4.2). We give a new proof of a theorem of De Bruijn [10] stating: if $J\in \{\Rdb_+, \Rdb\}$, $\phi\in \Cdb^J$ and $\Delta_s \phi \in C(J,\Cdb)$ for each $s >0$, then $\phi= G+H$, where $G \in C(J,\Cdb)$ and $H(t+s)=H(t)+H(s)$, $t,s\in J$, for functions $\phi: X^J $.} \end{abstract}