Key points are not available for this paper at this time.
The Schroedinger equation for a particle moving in a perturbed periodic potential is transformed into a representation called the ``Crystal momentum representation.'' It is shown that by use of this representation previous approximate ``effective mass'' equations can be readily derived and extended to arbitrarily high approximation. A theorem concerning weak space dependent perturbations is discussed.
E. N. Adams (1953) studied this question.