  We present details and expand on the framework leading to the recently
introduced degenerate adiabatic perturbation theory [Phys. Rev. Lett. 104,
170406 (2010)], and on the formulation of the degenerate adiabatic theorem,
along with its necessary and sufficient conditions given in [Phys. Rev. A 85,
062111 (2012)]. We start with the adiabatic approximation for degenerate
Hamiltonians that paves the way to a clear and rigorous statement of the
associated degenerate adiabatic theorem, where the non-abelian geometric phase
(Wilczek-Zee phase) plays a central role to its quantitative formulation. We
then describe the degenerate adiabatic perturbation theory, whose zeroth order
term is the degenerate adiabatic approximation, in its full generality. The
parameter in the perturbative power series expansion of the time-dependent wave
function is directly associated to the inverse of the time it takes to drive
the system from its initial to its final state. With the aid of the degenerate
adiabatic perturbation theory we obtain rigorous necessary and sufficient
conditions for the validity of the adiabatic theorem of quantum mechanics.
Finally, to illustrate the power and wide scope of the methodology, we apply
the framework to a degenerate Hamiltonian, whose closed form time-dependent
wave function is derived exactly, and also to other non-exactly-solvable
Hamiltonians whose solutions are numerically computed.
