We generalize multivariate Hawkes processes mainly by including a dependence with respect to the age of the process, i.e. the delay since the last point.Within this class, we investigate the limit behaviour, when n goes to infinity, of a system of n mean-field interacting age-dependent Hawkes processes. We prove that such a system can be approximated by independent and identically distributed age dependent point processes interacting with their own mean intensity. This result generalizes the study performed by Delattre, Fournier and Hoffmann in [17].In continuity with [11], the second goal of this paper is to give a proper link between these generalized Hawkes processes as microscopic models of individual neurons and the age-structured system of partial differential equations introduced by Pakdaman, Perthame and Salort in [42] as macroscopic model of neurons.Keywords: Hawkes process, mean-field approximation, interacting particle systems, renewal equation, neural network Mathematical Subject Classification: 60G55, 60F05, 60G57, 92B20
I IntroductionIn the recent years, the self-exciting point process known as the Hawkes process [29] has been used in very diverse areas. First introduced to model earthquake replicas [32] or [41] (ETAS model), it has been used in criminology to model burglary [39], in genomic data analysis to model occurrences of genes [27,50], in social networks analysis to model viewing or popularity [3,14], as well as in finance [1,2]. We refer to [35] or [56] for more extensive reviews on applications of Hawkes processes. A univariate (nonlinear) Hawkes process is a point process N admitting a stochastic intensity of the formwhere Φ : R → R + is called the intensity function, h : R + → R is called the selfinteraction function (also called exciting function or kernel function in the literature) and N (dz) denotes the point measure associated with N . We refer to [53,54,55,57,58] for recent papers dealing with nonlinear Hawkes process. Such a form of the intensity is motivated by practical cases where all the previous points of the process may impact the rate of appearance of a new point. The influence of the past points is formulated in terms of the delay between those past occurrences and the present time, through the weight function h. In the natural framework where h is nonnegative and Φ increasing, this choice of interaction models an excitatory phenomenon: * Corresponding author: e-mail: julien.chevallier@unice.fr 1 each time the process has a jump, it excites itself in the sense that it increases its intensity and thus the probability of finding a new point. A classical case is the linear Hawkes process for which h is non-negative and Φ(x) = µ + x where µ is a positive constant called the spontaneous rate. Note however that Hawkes processes can also describe inhibitory phenomena. For example, the function h may take negative values, Φ being the positive part modulo the spontaneous rate µ, i.e. Φ(x) = max(0, µ + x).Multivariate Hawkes processes consist of multivariate point processes...