# Lacunary sequence

2020 Mathematics Subject Classification: *Primary:* 11B05 *Secondary:* 42A55 [MSN][ZBL]

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The Lacunary sequence is a sequence of numbers $\seq{n_k}$ such that $n_{k+1} / n_k \geq \lambda > 1$; this class of sequences is denoted by $\Lambda$ and is used, in particular, in the theory of lacunary series and in the theory of lacunary trigonometric series. There are generalizations of the class $\Lambda$. For example, the class $B_2$: $\seq{n_k} \in B_2$ if there is an $A$ such that the number of solutions of the equations $[n_{k_1} \pm n_{k_2}] = m$ (where $n_{k_1} > n_{k_2}$ and $[a]$ is the integer part of the number $a$) does not exceed $A$ for any integer $m$; the class $R$: $\seq{n_k} \in R$ if there is an $A$ such that the number of solutions of the equations $[n_{k_1} \pm \cdots \pm n_{k_p}] = m$ (where $n_{k_1} > \cdots > n_{k_p}$) does not exceed $A^p$ for any $p=2,3,\ldots$ and any integer $m$; and the classes $\Lambda_\sigma$, $B_{2\sigma}$, $R_\sigma$, consisting of sequences that split into finitely-many sequences of the classes $\Lambda$, $B_2$, $R$, respectively.

#### References

[Ba] | N.K. [N.K. Bari] Bary, "A treatise on trigonometric series", Pergamon (1964) (Translated from Russian) MR0171116 Zbl 0129.28002 |

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Lacunary sequence.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Lacunary_sequence&oldid=25922