Action-angle coordinates at singularities for analytic integrable systems. Hidekazu Ito — 1991 Mathematische Zeitschrift
Integrability of Hamiltonian systems and Birkhoff normal forms in the simple resonance case. Hidekazu Ito — 1992 Mathematische Annalen
Convergence of Birkhoff normal forms for integrable systems. Hidekazu Ito — 1989 Commentarii mathematici Helvetici