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On the LIL for Self-Normalized Sums of IID Random Variables
Authors:Evarist Giné  David M Mason
Abstract:Let 
$$X,X_i ,i \in \mathbb{N},$$
be i.i.d. random variables and let, for each 
$$n \in \mathbb{N},S_n  = \sum\nolimits_{i = 1}^n {X_i }$$
and 
$$V_n^2  = \sum\nolimits_{i = 1}^n {X_i^2 }$$
. It is shown that 
$$\lim \sup _{n \to \infty } {{|S_n |} \mathord{\left/ {\vphantom {{|S_n |} {(V_n \sqrt {\log \log n} ) < \infty }}} \right. \kern-\nulldelimiterspace} {(V_n \sqrt {\log \log n} ) < \infty }}$$
a.s. whenever the sequence of self-normalized sums S n /V n is stochastically bounded, and that this limsup is a.s. positive if, in addition, X is in the Feller class. It is also shown that, for X in the Feller class, the sequence of self-normalized sums is stochastically bounded if and only if 
$$\lim \sup _{t \to \infty } {{t|\mathbb{E}XI(|X| \leqslant t)|} \mathord{\left/ {\vphantom {{t|\mathbb{E}XI(|X| \leqslant t)|} {\mathbb{E}X^2 I(|X| \leqslant t)] < \infty }}} \right. \kern-\nulldelimiterspace} {\mathbb{E}X^2 I(|X| \leqslant t)] < \infty }}$$
Keywords:Self-normalized sums  law of the iterated logarithm  Feller class  Student t-statistic
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