Polynomial Solutions of Linear Differential Equations: Spectral Problems, Asymptotic and WKB-Analysis

Linear ordinary differential equations with polynomial coefficients and their solutions in the complex plane have been an active area of mathematical research since the mid-19th century, with contributions from such prominent figures as G. Lamé, M. Bôcher, E. Heine, F. Klein, T. Stieltjes, G. D. Birkhoff, and many others. During the 19th century, interest focused mainly on second-order equations because of their importance in physics. The existence of a polynomial solution to a linear differential equation is a very special property, implying in particular the existence of a one-dimensional invariant subspace under the action of the monodromy group on the space of solutions. Polynomial solutions rarely occur for individual linear differential equations, but they typically arise within families of such equations. In this book, we discuss in detail two major topics: exactly solvable linear differential operators, closely related to the Bochner–Krall problem in orthogonal polynomials, and the (generalized) Heine–Stieltjes theory, both of which originated more than a century ago. Our main emphasis is on the study of the root asymptotics of various polynomial sequences appearing in both problems. We also establish connections between our asymptotic analysis and special classes of quadratic and higher-order differentials, as well as other objects of geometric origin. In addition, we present a wealth of numerical results and suggest many open problems for the interested reader.