An interplay between attraction and repulsion in infinite populations

Abstract

We propose and study a model describing an infinite population of point entities arriving in and departing from X=mathbb {R}^d, dge 1. The already existing entities force each other to leave the population (repulsion) and attract the newcomers. The evolution of the population states is obtained by solving the corresponding Fokker-Planck equation. Without interactions, the evolution preserves states in which the probability p(n,Lambda ) of finding n points in a compact vessel Lambda subset X obeys the Poisson law. As we show, for pure attraction the decay of p(n,Lambda ) with nrightarrow +infty may be essentially slower. The main result is the statement that in the presence of repulsion—even of an arbitrary short range—the evolution preserves states in which the decay of p(n,Lambda ) is at most Poissonian. We also derive the corresponding kinetic equation, the numerical solutions of which can provide more detailed information on the interplay between attraction and repulsion. Further possibilities in studying the proposed model are also discussed.