Contactifications: a Lagrangian description of compact Hamiltonian systems *

If η is a contact form on a manifold M such that the orbits of the Reeb vector field form a simple foliation on M, then the presymplectic 2-form on M induces a symplectic structure ω on the quotient manifold . We call a contactification of the symplectic manifold . First, we present an explicit geometric construction of contactifications of some coadjoint orbits of connected Lie groups. Our construction is a far going generalization of the well-known contactification of the complex projective space , being the unit sphere in , and equipped with the restriction of the Liouville 1-form on . Second, we describe a constructive procedure for obtaining contactification in the process of the Marsden–Weinstein–Meyer symplectic reduction and indicate geometric obstructions for the existence of compact contactifications. Third, we show that contactifications provide a nice geometrical tool for a Lagrangian description of Hamiltonian systems on compact symplectic manifolds , on which symplectic forms never admit a ‘vector potential’.