Multiplicity and ellipticity of closed characteristics on compact star-shaped hypersurfaces in ${\bf R}^{2n}$

Abstract: In this paper, we firstly generalize some theories developed by I. Ekeland and H. Hofer in [EkH] for closed characteristics on compact convex hypersurfaces in ${\bf R}^{2n}$ to star-shaped hypersurfaces. As applications, we use Ekeland-Hofer theory and index iteration theory to prove that if a compact star-shaped hypersuface in ${\bf R}^4$ satisfying some suitable pinching condition carries exactly two geometrically distinct closed characteristics, then both of them must be elliptic. We also conclude that the theory developed by Y. Long and C. Zhu in [LoZ] still holds for dynamically convex star-shaped hypersurfaces, and combining it with the results in [WHL], [LLW], [Wan3], we obtain that there exist at least $n$ closed characteristics on every dynamically convex star-shaped hypersurface in ${\bf R}^{2n}$ for $n=3, 4$.