On inequalities for sums of bounded random variables

Abstract

Let ${\eta }_1,{\eta }_2,...$ be independent (not necessarily identically distributed) zero-mean random variables (r.v.'s) such that $|{\eta }_i|\le 1$ almost surely for all $i$, and let $Z$ stand for a standard normal r.v. Let $a_1,a_2,...$ be any real numbers such that $a_1^2+a_2^2+...=1.$ It is shown that then $$ ¶(a_1η_1+a_2η_2+...\ge x) \le ¶(Z\ge x-\la/x) \forall x>0, $$ where $\la := \ln\frac{2e^3}9=1.495...$. The proof relies on (i) another probability inequality and (ii) a l'Hospital-type rule for monotonicity, both developed elsewhere. A multidimensional analogue of this result is given, based on a dimensionality reduction device, also developed elsewhere. In addition, extensions to (super)martingales are indicated.

6 pages; the result in the previous version is strengthened and extended