“…In this paper we study the on-line shortest path problem, a representative example of structured expert classes that has received attention in the literature for its many applications, including, among others, routing in communication networks; see, e.g., Takimoto and Warmuth [28], Awerbuch et al [3], or György and Ottucsák [17], and adaptive quantizer design in zero-delay lossy source coding; see, György et al [13,14,16]. In this problem, a weighted directed (acyclic) graph is given whose edge weights can change in an arbitrary manner, and the decision maker has to pick in each round a path between two given vertices, such that the weight of this path (the sum of the weights of its composing edges) be as small as possible.…”