Random Networks with Preferential Growth and Vertex Death

Abstract

A dynamic model for a random network evolving in continuous time is defined, where new vertices are born and existing vertices may die. The fitness of a vertex is defined as the accumulated in-degree of the vertex and a new vertex is connected to an existing vertex with probability proportional to a functionbof the fitness of the existing vertex. Furthermore, a vertex dies at a rate given by a functiondof its fitness. Using results from the theory of general branching processes, an expression for the asymptotic empirical fitness distribution {pk} is derived and analyzed for a number of specific choices ofbandd. Whenb(i) =i+ α andd(i) = β, that is, linear preferential attachment for the newborn and random deaths, thenpk∼k-(2+α). Whenb(i) =i+ 1 andd(i) = β(i+ 1), with β < 1, thenpk∼ (1 + β)−k, that is, if the death rate is also proportional to the fitness, then the power-law distribution is lost. Furthermore, whenb(i) =i+ 1 andd(i) = β(i+ 1)γ, with β, γ < 1, then logpk∼ -kγ, a stretched exponential distribution. The momentaneous in-degrees are also studied and simulations suggest that their behaviour is qualitatively similar to that of the fitnesses.