Published online by Cambridge University Press: 20 November 2018
We prove that a Hamiltonian
$p\in {{C}^{\infty }}({{T}^{*}}{{\mathbf{R}}^{n}})$
is locally integrable near a non-degenerate critical point
${{\rho }_{0}}$
of the energy, provided that the fundamental matrix at
${{\rho }_{0}}$
has rationally independent eigenvalues, none purely imaginary. This is done by using Birkhoff normal forms, which turn out to be convergent in the
${{C}^{\infty }}$
sense. We also give versions of the Lewis-Sternberg normal form near a hyperbolic fixed point of a canonical transformation. Then we investigate the complex case, showing that when
$p$ is holomorphic near
${{\rho }_{0}}\in {{T}^{*}}{{\mathbf{C}}^{n}},$ then Re
$p$ becomes integrable in the complex domain for real times, while the Birkhoff series and the Birkhoff transforms may not converge, i.e.,
$p$ may not be integrable. These normal forms also hold in the semi-classical frame.