On Moments and Symmetrical Sequences

Abstract

In this article we consider questions related to the behavior of the moments $M_{m}\left( \left\{ z_{j}\right\} \right) $ when the indices are restricted to specific subsequences of integers, such as the even or odd moments. If $n\geq2$ we introduce the notion of symmetrical series of order $n,$ showing that if $\left\{ z_{j}\right\} \ $is symmetrical then $M_{m}\left( \left\{ z_{j}\right\} \right) =0$ whenever $n\nmid m;$ in particular, the odd moments of a symmetrical series of order $2$ vanish. We prove that when $\left\{ z_{j}\right\} \in l^{p}$ for some $p$ then several results characterizing the sequence from its moments hold. We show, in particular, that if $M_{m}\left( \left\{ z_{j}\right\} \right) =0$ whenever $n\nmid m$ then $\left\{ z_{j}\right\} $ is a rearrangement of a symmetrical series of order $n.$ We then construct examples of sequences whose moments vanish with required density. Lastly, we construct counterexamples of several of the results valid in the $l^{p}$ case if we allow the moment series to be all conditionally convergent. We show that for each arbitrary sequence of real numbers $\left\{ μ_{m}\right\} _{m=0}^{\infty}$ there are real sequences $\left\{ u_{j}\right\} _{j=0}^{\infty}$ such that \[ \sum_{j=0}^{\infty}u_{j}^{2m+1}=μ_{m}\,,\ \ \ m\geq0\,. \]