Multiple Periodic Solutions for $\Gamma$-symmetric Newtonian Systems
  The existence of periodic solutions in $\Gamma$-symmetric Newtonian systems $\ddot{x}=-\nabla f(x)$ can be effectively studied by means of the $(\Gamma\times O(2))$-equivariant gradient degree with values in the Euler ring $U(\Gamma\times O(2))$. In this paper, we show that in the case of $\Gamma$ being a finite group, the Euler ring $U(\Gamma\times O(2))$ and the related basic degrees are effectively computable using Euler ring homomorphisms, the Burnside ring $A(\Gamma\times O(2))$ and the reduced $(\Gamma\times O(2))$-degree with no free parameters. We present several examples of Newtonian systems with various symmetries, for which we show existence of multiple periodic solutions. We also provide exact value of the equivariant topological invariant for those problems. 
Mieczyslaw Dabkowski and Wieslaw Krawcewicz and Yanli Lv and Hao-pin   Wu
math.DS