Symmetry Reduction and Periodic Solutions in Hamiltonian Vlasov Systems

In this paper, we discuss a general approach to finding periodic solutions bifurcating from equilibrium points of classical Vlasov systems. The main access to the problem is chosen through the Hamiltonian representation of any Vlasov system, first put forward in [J. Fröhlich, A. Knowles, and S. Schwarz, Comm. Math. Phys., 288 (2009), pp. 1023--1059] and generalized in [R. A. Neiss and P. Pickl, J. Stat. Phys., 178 (2020), pp. 472--498; R. A. Neiss, Arch. Ration. Mech. Anal., 231 (2019), pp. 115--151]. The method transforms the problem into a setup of complex valued $\mathcal{L}^2$ functions with phase equivariant Hamiltonian. Through Marsden--Weinstein symmetry reduction [J. Marsden and A. Weinstein, Rep. Math. Phys., 5 (1974), pp. 121--130], the problem is mapped on a Hamiltonian system on the quotient manifold $\mathbb{S}^{\mathcal{L}^2}/\mathbb{S}^1$, which actually proves to be necessary to close many trajectories of the dynamics. As a toy model to apply the method we use the harmonic Vlasov system, a nonrelativistic Vlasov equation with attractive harmonic two-body interaction potential. The simple structure of this model allows us to compute all of its solutions directly and therefore test the benefits of the Hamiltonian formalism and symmetry reduction in Vlasov systems.

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