Hi; In chapter 9 of Solid state physics of Ashcroft&Mermin(Electrons in a weak periodic potential), there is a General Approch to the Schrodinger Equation when the Potential is Weak. i can't understand what is meant by the term DEGENERACY? or what does "nearly degenerate free electron levels mean"? how should we visuallize degenerate and non-degenerate energy levels? what are the differences and how can we interpret them? i'm totally confused. Thank u All.