Vertex reinforced random walks with exponential interaction on complete graphs

We describe a model for m vertex reinforced interacting random walks on complete graphs with d≥2 vertices. The transition probability of a random walk to a given vertex depends exponentially on the proportion of visits made by all walks to that vertex. The individual proportion of visits is modulated by a strength parameter that can be set equal to any real number. This model covers a large variety of interactions including different vertex repulsion and attraction strengths between any two random walks as well as self-reinforced interactions. We show that the process of empirical vertex occupation measures defined by the interacting random walks converges (a.s.) to the limit set of the flow induced by a smooth vector field. Further, if the set of equilibria of the field is formed by isolated points, then the vertex occupation measures converge (a.s.) to an equilibrium of the field. These facts are shown by means of the construction of a strict Lyapunov function. We show that if the absolute value of the interaction strength parameters are smaller than a certain upper bound, then, for any number of random walks (m≥2) on any graph (d≥2), the vertex occupation measures converge towards a unique equilibrium. We provide two additional examples of repelling random walks for the cases m=d=2 and m∈{3,4,5} with d=2. The latter is used to study some properties of three, four and five exponentially repelling random walks on Z.