Periodic Solutions for $N$-Body-Type Problems
  We consider non-autonomous $N$-body-type problems with strong force type potentials at the origin and sub-quadratic growth at infinity, and using Ljusternik-Schnirelmann theory, we prove the existence of unbounded sequences of critical values for the Lagrangian action corresponding to non-collision periodic solutions. 
Fengying Li and Shiqing Zhang
math-ph math.MP