  In this paper we obtain asymptotic formulas of arbitrary order for the Bloch
eigenvalue and the Bloch function of the periodic Schrodinger operator of
arbitrary dimension, when corresponding quasimomentum lies near a diffraction
hyperplane. Besides, writing the asymptotic formulas for the Bloch eigenvalue
and the Bloch function, when corresponding quasimomentum lies far from the
diffraction hyperplanes, obtained in my previous papers in improved and
enlarged form, we obtain the complete perturbation theory for the
multidimensional Schrodinger operator with a periodic potential. Moreover, we
estimate the measure of the isoenergetic surfaces in the high energy region
which implies the validity of the Bethe-Sommerfeld conjecture for arbitrary
dimension and arbitrary lattice.
