The paper focuses on the connection between the existence of infinitely many
periodic orbits for a Hamiltonian system and the behavior of its action or index
spectrum under iterations. We use the action and index spectra to show that any
Hamiltonian diffeomorphism of a closed, rational manifold with zero first Chern class
has infinitely many periodic orbits and that, for a general rational manifold, the
number of geometrically distinct periodic orbits is bounded from below by
the ratio of the minimal Chern number and half of the dimension. These
generalizations of the Conley conjecture follow from another result proved
here asserting that a Hamiltonian diffeomorphism with a symplectically
degenerate maximum on a closed rational manifold has infinitely many periodic
orbits.
We also show that for a broad class of manifolds and/or Hamiltonian
diffeomorphisms the minimal action-index gap remains bounded for some infinite
sequence of iterations and, as a consequence, whenever a Hamiltonian diffeomorphism
has finitely many periodic orbits, the actions and mean indices of these orbits must
satisfy a certain relation. Furthermore, for Hamiltonian diffeomorphisms of
with
exactly
periodic orbits a stronger result holds. Namely, for such a Hamiltonian
diffeomorphism, the difference of the action and the mean index on a periodic
orbit is independent of the orbit, provided that the symplectic structure on
is
normalized to be in the same cohomology class as the first Chern class.