Hereditary Hsu-Robbins-Erdös Law of Large Numbers

Abstract

We show that every sequence $f_1,f_2,\cdots$ of real-valued random variables with $\sup_{n \in \N} \E (f_n^2) < \infty$ contains a subsequence $f_{k_1},f_{k_2},\cdots$ converging in \textsc{Cesàro} mean to some $f_{\infty }\in {\mathbb{L}}^2$ {\it completely,} to wit, $ \sum_{N \in \N} \, ¶\left( \bigg| \frac{1}{N} \sum_{n=1}^N f_{k_n} - f_\infty \bigg| > \eps \right)< \infty\,, \quad \forall ~ \eps > 0\,; $ and {\it hereditarily,} i.e., along all further subsequences as well. We also identify a condition, slightly weaker than boundedness in ${\mathbb{L}}^2,$ which turns out to be not only sufficient for the above hereditary complete convergence in \textsc{Cesàro} mean, but necessary as well.