2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/62390We 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 purposesDynamical SystemsDifferential Geometry58F07,70H05text