Unchained polygons and the N-body problem
  The simplest solutions of the N-body problem --symmetric relative equilibria-- are shown to be organizing centers from which stem some recently studied classes of periodic solutions. We focus on the relative equilibrium of the equal-mass regular N-gon, assumed horizontal, and study the families of Lyapunov quasi-periodic solutions bifurcating from them in the vertical direction. The proof of the local existence of such solutions relies on the fact that the restriction to the corresponding directions of the quadratic part of the energy is positive definite. We then discuss the possibility of continuing the families globally as action minimizers under symmetry constraints by using the fact that, in rotating frames where they become periodic, these solutions are highly symmetric. The paradigmatic examples are the "Eight" families for an odd number of bodies and the "Hip-Hop" families for an even number. We argue that it is precisely for these two families that global minimization may be used. We also study the relation with the regular N-gon, of the so-called "chain" choreographies (see C. Sim\'o, New families of Solutions in N-Body Problems, Progr. Math. 201, 2001): here, only a local minimization property is true (except for N=3) and moreover the parity plays a deciding role, in particular through the value of the angular momentum. 
Alain Chenciner (IMCCE), Jacques F\'ejoz (IMCCE, IMJ)