A Classical Limit Theorem Without Invariance or Reflection

Abstract

A sequence of stopping or first passage times is utilized to derive the limiting distribution of the maximum of partial sums of independent, identically distributed random variables with mean zero and finite variance and concomitantly the limit distribution of the stopping times themselves. The result, due to Erdos and Kac, first appeared in the paper which launched the extremely fruitful invariance principle; reflection enters in the calculations relating to the choice of a specific distribution for the $\{X_n\}$. Moreover, it is noted when the $\{X_n\}$ are $\operatorname{i.i.d.}$ with mean $\mu > 0$ and variance $\sigma^2 < \infty$ that $\max_{1\leqq j\leqq n} S_j/j^\alpha$ has a limiting standard normal distribution for any $\alpha$ in [0, 1).