# Besicovitch almost-periodic functions

A class of almost-periodic functions in which the analogue of the Riesz–Fischer theorem is valid: Any trigonometric series

$$\sum _ { n } a _ {n} e ^ {i \lambda _ {n} x } ,$$

where

$$\sum _ { n } | a _ {n} | ^ {2} < \infty ,$$

is the Fourier series of some $B ^ {2}$- almost-periodic function. The definition of these functions [1], [2] is based on a generalization of the concept of an almost-period, and certain additional ideas must be introduced in it. A set $E$ of real numbers is called sufficiently homogeneous if there exists an $L > 0$ such that the ratio between the largest number of members of $E$ in an interval of length $L$ and the smallest number of members in an interval of the same length $L$ is less than 2. A sufficiently homogeneous set is also relatively dense. A complex-valued function $f(x)$, $- \infty < x < \infty$, summable to degree $p$ on any finite interval of the real axis, is called a Besicovitch almost-periodic function if to each $\epsilon > 0$ there corresponds a sufficiently homogeneous set of numbers (the so-called $(B ^ {p} , \epsilon )$- almost-periods of $f(x)$):

$${} \dots < \tau _ {-2} < \tau _ {-1} < \tau _ {0} < \tau _ {1} < \dots ,$$

such that for each $i$

$$\overline{M}\; _ {x} \{ | f (x + \tau _ {i} ) - f (x) | ^ {p} \} < \epsilon ^ {p} ,$$

and for each $c > 0$

$$\overline{M}\; _ {x} \overline{M}\; _ {i} { \frac{1}{c} } \int\limits _ { x } ^ { x+c } | f ( \xi + \tau _ {i} ) - f (x) | ^ {p} d \xi < \epsilon ^ {p} ,$$

where

$$\overline{M}\; _ {x} \{ F (x) \} = \ \overline{\lim\limits _ {\tau \rightarrow \infty } } \ { \frac{1}{2 \tau } } \int\limits _ {- \tau } ^ \tau F (x) dx,$$

$$\overline{M}\; _ {i} \{ F (i) \} = \overline{\lim\limits _ {n \rightarrow \infty } } { \frac{1}{2n+1} } \sum _ { i=-n } ^ { n } F (i).$$

Here $F(x)$ is a real-valued function, defined, respectively, for a real variable and an integer argument.

#### References

 [1] A.S. Besicovitch, "On mean values of functions of a complex and of a real variable" Proc. London Math. Soc. (2) , 27 (1927) pp. 373–388 [2] A.S. Besicovitch, "On Parseval's theorem for Dirichlet series" Proc. London Math. Soc. (2) , 26 (1927) pp. 25–34 [3] B.M. Levitan, "Almost-periodic functions" , Moscow (1953) (In Russian)

As is implicit in the article, for each $p \geq 1$ there is a class of almost-periodic functions, denoted by $B ^ {p}$. The first part of the article deals with $B ^ {2}$, the rest is more general. General references may be found under Almost-periodic function.