2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/161076The periodic Lorentz gas describes an ensemble of non-interacting point particles in a periodic array of spherical scatterers. We have recently shown that, in the limit of small scatterer density (Boltzmann-Grad limit), the macroscopic dynamics converges to a stochastic process, whose kinetic transport equation is not the linear Boltzmann equation--in contrast to the Lorentz gas with a disordered scatterer configuration. The present paper focuses on the two-dimensional set-up, and reports an explicit, elementary formula for the collision kernel of the transport equation.10 pages, 5 figuresMathematical PhysicsDynamical Systems37A60; 37D50; 82C05; 82C40text