Theory of the topical coverage of multiple databases

We present a model that describes which fraction of the literature on a certain topic we will find when we use n (n = 1, 2, …) databases. It is a generalization of the theory of discovering usability problems. We prove that, in all practical cases, this fraction is a concave function of n, the number of used databases, thereby explaining some graphs that exist in the literature. We also study limiting features of this fraction for n very high and we characterize the case that we find all literature on a certain topic for n high enough.