On the asymptotic behavior of the spectral function and expansion in eigenfunctions of the equation [delta]u+{[lambda]-q(x¹, x², x³)}u=0 / B.M. Levitan -- Spectral analysis of non-self-adjoint operators / M.A. Naĭmark -- The method of finite differences in the theory of partial differential equations / O.A. Ladyženskaya -- A priori estimate in L[sub p] and generalized solutions of elliptic equations and systems / A.I. Košelev -- Some questions in the qualitative theory of elliptic and parabolic equations / E.M. Landis -- Regular degeneration and boundary layer for linear differential equations with small parameter / M.I. Višik and L.A. Lyusternik.