WKB approach to zero distribution of solutions of linear second order differential equations

Given a second-order linear differential equation y″(z)+S(z)y(z)=0, the distribution of zeros of its solutions is defined by v= Σ_y(z)=0 δ_z, where δ_z stands for the Dirac delta at the point z. Some techniques of approximation of the restriction of v to ℝ directly from S(z) are considered. In particular, for the WKB method error bounds are provided and some related results established. In the second part, formulas for the appropriate scaling in the holonomic case are given. As an illustration, we obtain the asymptotic distribution of the real zeros of some families of polynomials.