Hamiltonian systems with linearly dependent constraints (irregular systems),
are classified according to their behavior in the vicinity of the constraint
surface. For these systems, the standard Dirac procedure is not directly
applicable. However, Dirac's treatment can be slightly modified to obtain, in
some cases, a Hamiltonian description completely equivalent to the Lagrangian
one. A recipe to deal with the different cases is provided, along with a few
pedagogical examples.Comment: To appear in Proceedings of the XIII Chilean Symposium of Physics,
Concepcion, Chile, November 13-15 2002. LaTeX; 5 pages; no figure