Exactly solvable model of electron in the Lame potential and   singularities of the electron thermodynamic potential
  One-gap and two-gap separable Lame potentials are studied in detail. For the one-dimensional case, we construct the dispersion relation graph E(k) and for the three-dimensional case we construct the Fermi surfaces in the first and second bands. The pictures illustrate a passage from the limit case of free electrons to the limit case of tight binding electrons. These results are used to describe the Lifshits electron phase transition of 2.5 kind and derive some exact expressions. We also examine the singularities of the second derivative of magnetic momentum in an external magnetic field. The parameter of the singularities depends on corresponding effective mass. 
Victor G. Baryakhtar, Eugene D. Belokolos, Oleksandr V. Dmytriiev