Lower Tail Probability Estimates for Subordinators and Nondecreasing Random Walks

Abstract

Let $X_1, X_2,\ldots$ be nonnegative i.i.d. random variables and $S_n = X_1 + \cdots + X_n; EX_1 = \mu \leq \infty$ and $a$ is the infimum of the support of the distribution of $X_1$. For $a < x_n < \mu$ we obtain the asymptotic behavior of $\log P\{S_n \leq nx_n\}$ as $n \rightarrow \infty$. Under the additional assumption of stochastic compactness a stronger result is obtained which gives the asymptotic behavior of $P\{S_n \leq nx_n\}$ itself. Analogues of these results are given for subordinators when $t \rightarrow \infty$ or $t \rightarrow 0$.