View source for Almost-period

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A concept from the theory of almost-periodic functions (cf. [[Almost-periodic function|Almost-periodic function]]); a generalization of the notion of a period. For a uniformly almost-periodic function $f(x)$, $-\infty<x<\infty$, a number $\tau=\tau_f(\epsilon)$ is called an $\epsilon$-almost-period of $f(x)$ if for all $x$,

$$|f(x+\tau)-f(x)|<\epsilon.$$

For generalized almost-periodic functions the concept of an almost-period is more complicated. For example, in the space $S_l^p$ an $\epsilon$-almost-period $\tau$ is defined by the inequality

$$D_{S_l^p}[f(x+\tau),f(x)]<\epsilon,$$

where $D_{S_l^p}[f,\phi]$ is the distance between $f(x)$ and $\phi(x)$ in the metric of $S_l^p$.

A set of almost-periods of a function $f(x)$ is said to be relatively dense if there is a number $L=L(\epsilon,f)>0$ such that every interval $(\alpha,\alpha+L)$ of the real line contains at least one number from this set. The concepts of uniformly almost-periodic functions and that of Stepanov almost-periodic functions may be defined by requiring the existence of relatively-dense sets of $\epsilon$-almost-periods for these functions.

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  B.M. Levitan,   "Almost-periodic functions" , Moscow  (1953)  (In Russian)</TD></TR></table>



====Comments====
For the definition of $S_l^p$ and its metric $D_{S_l^p}$ see [[Almost-periodic function|Almost-periodic function]]. The Weyl, Besicovitch and Levitan almost-periodic functions can also be characterized in terms of $S_l^p$ $\epsilon$-periods. These characterizations are more complicated. A good additional reference is [[#References|[a1]]], especially Chapt. II.

====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  A.S. Besicovitch,   "Almost periodic functions" , Cambridge Univ. Press  (1932)</TD></TR></table>