A note on focus-focus singularities
arXiv:math/0110147
Abstract
We give a topological and geometrical description of focus-focus singularities of integrable Hamiltonian systems. In particular, we explain why the monodromy around these singularities is non-trivial, a result obtained before by J.J. Duistermaat and others for some concrete systems.
old paper put here for archival purposes