Some explorations on two conjectures about Rademacher sequences

Abstract

In this paper, we explore two conjectures about Rademacher sequences. Let $(\epsilon_i)$ be a Rademacher sequence, i.e., a sequence of independent $\{-1,1\}$-valued symmetric random variables. Set $S_n=a_1\epsilon_1+\cdots+a_n\epsilon_n$ for $a=(a_1,\dots,a_n)\in \mathbb{R}^n$. The first conjecture says that $P\ (\ |S_n\ |\leq \|a\|\ )\geq\frac{1}{2}$ for all $a\in \mathbb{R}^n$ and $n\in \mathbb{N}$. The second conjecture says that $P\ (\ |S_n\ |\geq\|a\|\ )\geq \frac{7}{32}$ for all $a\in \mathbb{R}^n$ and $n\in \mathbb{N}$. Regarding the first conjecture, we present several new equivalent formulations. These include a topological view, a combinatorial version and a strengthened version of the conjecture. Regarding the second conjecture, we prove that it holds true when $n\leq 7$.