How eigenvalues are affected by a change in the Operator
Examples.
Energy levels.
From an isolated well to a periodic potential [Coh, p.381, l.-11-l.-1].
From the dashed line to the solid line in [Coh, p.379, Fig. 4.b].
The way the tight-bounding periodic potential and the interatomic spacing affect the energy levels [Ashc, p.183, Fig. 10.4].
The Kronig-Penney potential [Mer2, p.168, l.-6-l.-4]
is a case that falls between the tight-binding approximation and the model of a free electron perturbed by a weak potential.