Zeno dynamics and constraints

We investigate an example of quantum Zeno dynamics, when a systemundergoes very frequent (projective) measurements that ascertain whetherit is within a given spatial region. The evolution is found to beunitary and the generator of the Zeno dynamics is the Hamiltonian withhard-wall (Dirichlet) boundary conditions. We then look at the case of aprojection onto a lower-dimensional space: in such a situation the Zenoansatz turns out to be a procedure to impose constraints.