Leading All The Way

Abstract

Xavier and Yushi run a ``random race'' as follows. A continuous probability distribution $\mu$ on the real line is chosen. The runners begin at zero. At time $i$ Xavier draws $\mathbf{X}_i$ from $\mu$ and advances that distance, while Yushi advances by an independent drawing $\mathbf{Y}_i$. After $n$ such moves, Xavier wins a valuable prize provided he not only wins the race but leads after every step; that is, $\sum_{i=1}^k \mathbf{X}_i > \sum_{i=1}^k \mathbf{Y}_i$ for all $k = 1,2, \dots, n$. What distribution is best for Xavier, and what then is his probability of getting the prize?