# Large deviations for maximizer of random walk with drift

Is it easy to write down the large deviations rate for the maximizer of a random walk with negative drift?

Let $X_i$ be the (iid, mean $-\mu$, variance $\sigma$, arbitrarily nice tails) jumps of a random walk $S_i$. I am interested in the location of the maximum of $S_i$, i.e. in $\arg\max_k S_k$. (If $X_i$ are continuous, the maximizer is almost surely unique.)

There's a trivial exponential upper bound $$\mathbb{P}(\arg\max_k(S_k) > n) \leq \mathbb{P}(S_{n+1} > 0) = \mathbb{P}(\sum_{i=1}^n X_i > 0)$$ and the trivial exponential lower bound $$\mathbb{P}(\arg\max_k(S_k) > n) \geq \mathbb{P}(X_i > 0\ \forall i \leq n)$$ and they (of course) don't match. Is there a limit $$\lim_{n \to \infty}n^{-1}\log\mathbb{P}(\arg\max_k(S_k) > n),$$ and is it possible to write it down in terms of the distribution of the jumps $X_i$?

• The answer of course depends on the tail assumptions for the $X_i$s. Do they have exponential tails? In that case, you have a $\log$ missing in your question, and the answer can be read off large deviations for the maximum (which are covered by e.g. Iglehart's 1972 paper). – ofer zeitouni Dec 7 '14 at 22:58
• Thank you, Ofer! Yes, of course, it should be $\log\mathbb{P}(...)$. – Elena Yudovina Dec 10 '14 at 16:29
• The system won't let me write a comment cause I'm a new user, so I'll write an answer instead. I have a question in the OP, in the upper bound part, why is it that $$\mathbb{P}(\arg \max_k (S_k)>n)\leq\mathbb{P}(S_{n+1}>0)$$ I don't quite get that part... Any help would be appreciated :-) – Oliver May 15 '17 at 15:42
• I don't remember if I had a better reason three years ago, but it would definitely true as $\mathbb{P}(\text{arg max}_k(S_k) = n) \leq \mathbb{P}(S_{n+1} > 0)$, which should be close enough from a large deviations standpoint. – Elena Yudovina May 16 '17 at 21:57

The upper bound gives the correct large deviations; that is, the answer is $$n^{-1}\log P(\mbox{ arg max}_k S_k>n)\to -I(0)\,,$$ where $I(0)=-\inf_\lambda \log E(e^{\lambda X_1})$.
The upper bound appears in the OP. For the lower bound, just consider the event $${\cal A}_n:=\{S_n\geq \epsilon n \}$$ and note that $P(\mbox{ argmax}_k S_k <n(1-\delta)|{\cal A}_n)$ goes to $0$ as $n\to\infty$ if $\delta>>\epsilon$ (but both small). But $n^{-1}\log P({\cal A}_n)\to-I(\epsilon)$. Now use the continuity of $I$ in $\epsilon$ (here you need to assume something on the tails; finite logarithmic mgf is enough).