Large Deviation Asymptotics for Occupancy Problems

In the standard formulation of the occupancy problem one considers the distribution of r balls in n cells, with each ball assigned independently to a given cell with probability 1/n. Although closed form expressions can be given for the distribution of various interesting quantities (such as the fraction of cells that contain a given number of balls), these expressions are often of limited practical use. Approximations provide an attractive alternative, and in the present paper we consider a large deviation approximation as r and n tend to infinity. In order to analyze the problem we first consider a dynamical model, where the balls are placed in the cells sequentially and "time" corresponds to the number of balls that have already been thrown. A complete large deviation analysis of this "process level" problem is carried out, and the rate function for the original problem is then obtained via the contraction principle.