  Given an invariant gauge potential and a periodic scalar potential \tilde{V}
on a Riemannian manifold \tilde{M} with a discrete symmetry group \Gamma,
consider a \Gamma-periodic quantum Hamiltonian
\tilde{H}=-\tilde{\Delta}_{B}+\tilde{V} where \tilde{\Delta}_{B} is the Bochner
Laplacian. Both the gauge group and the symmetry group \Gamma can be
noncommutative, and the gauge field need not vanish. On the other hand, \Gamma
is supposed to be of type I. To any unitary representation \Lambda of \Gamma
one relates a Hamiltonian H^{\Lambda}=-\Delta_{B}^{\Lambda}+V on
M=\tilde{M}/\Gamma where V is the projection of \tilde{V} to M. We describe a
construction of the Bloch decomposition of \tilde{H} into a direct integral
whose components are H^{\Lambda}, with \Lambda running over the dual space
\hat{\Gamma}. The evolution operator and the resolvent decompose
correspondingly. Conversely, given \Lambda\in\hat{\Gamma}, one can express the
propagator \mathcal{K}_{t}^{\Lambda}(y_{1},y_{2}) (the kernel of
\exp(-itH^{\Lambda})) in terms of the propagator
\tilde{\mathcal{K}}_{t}(y_{1},y_{2}) (the kernel of \exp(-it\tilde{H})) as a
weighted sum over \Gamma. Such a formula is known in theoretical physics for
the case when the gauge field vanishes and \tilde{M} is a universal covering of
a multiply connected space M. We show that these constructions are mutually
inverse. Analogous formulas exist for resolvents and their kernels (Green
functions) as well.
