The two-sided exit problem for a random walk on $\mathbb{Z}$ and having infinite variance II

Abstract

Let $F$ be a distribution function on the integer lattice $\mathbb{Z}$ and $S=(S_n)$ the random walk with step distribution $F$. Suppose $S$ is oscillatory and denote by $U_{\rm a}(x)$ and $u_{\rm a}(x)$ the renewal function and sequence, respectively, of the strictly ascending ladder height process associated with $S$. Putting $A(x) =\int_0^x [1-F(t)-F(-t)] dt$, $H(x)=1-F(x)+F(-x)$ we suppose $$A(x)/\big(xH(x)\big) \to -\infty \quad (x\to\infty).$$ Under some additional regularity condition on the positive tail of $F$, we show that $$u_{\rm a}(x) \sim U_{\rm a}(x)[1-F(x)]/|A(x)|$$ as $x\to\infty$ and uniformly for $0\leq x\leq R\in \mathbb{Z}$, as $R\to\infty$ $$P [ S\; \mbox{leaves $[0,R]$ on its upper side}\, |\, S_0=x] \, \sim\, c^{-1}A(x)u_{\rm a}(x), $$ where $c= \sum_{n=1}^\infty P[S_{n}>S_0;\, S_k < S_0$ for $0<k<n]$ and the regularity condition is satisfied at least if $S$ is recurrent, $\limsup [1-F(x)]/F(-x)<1$, and $x[1-F(x)]/L(x) $ ($x\geq 1$) is bounded away from zero and infinity for some slowly varying function $L$. We also give some asymptotic estimates of the probability that $S$ visits $R$ before entering the negative half-line for asymptotically stable walks and obtain asymptotic behaviour of the probability that $R$ is ever hit by $S$ conditioned to avoid the negative half-line forever.