title: Statistical properties of passive tracers in a positive four-point vortex model
abstract: Stochastic properties of systems formed by many passive particles conducted by four point vortices, each one with positive circulation, are investigated. A statistical chi(2) test is developed in order to study the spatial distribution of particles in the chaotic background (lambda(L)>0). The fact that the uniform distribution is an invariant measure of the spatial distribution of particles is used to debug the chi(2) test. This procedure is applied in the same conditions as described in Babiano et al. in order to study the uniformity of passive particles. It is observed that uniformity is not attained up to times of order 10(5) natural time unity, when either a Gaussian or a uniform initial distribution is considered in a small region away from the vortices.
authors.affiliation: Departamento de Matem&tacute;ica Aplicada-Faculdade de Ciencias da Universidade do Porto and Centro de Matematica Aplicada da Universidade do Porto, Rua das Taipas 135, 4050-600 Porto, Portugal and Observatoire de la Coinsertion markte d'Az.
authors.initials: S
authors.lastname: Gama
authors.initials: P
authors.lastname: Milheiro-Oliveira