  We consider an invariant quantum Hamiltonian $H=-\Delta_{LB}+V$ in the
$L^{2}$ space based on a Riemannian manifold $\tilde{M}$ with a discrete
symmetry group $\Gamma$. Typically, $\tilde{M}$ is the universal covering space
of a multiply connected manifold $M$ and $\Gamma$ is the fundamental group of
$M$. To any unitary representation $\Lambda$ of $\Gamma$ one can relate another
operator on $M=\tilde{M}/\Gamma$, called $H_\Lambda$, which formally
corresponds to the same differential operator as $H$ but which is determined by
quasi-periodic boundary conditions. We give a brief review of the Bloch
decomposition of $H$ and of a formula relating the propagators associated to
the Hamiltonians $H_\Lambda$ and $H$. Then we concentrate on the example of the
Aharonov-Bohm effect with two vortices. We explain in detail the construction
of the propagator in this case and indicate all essential intermediate steps.
