1. Asymptotic Methods and Perturbation Theory;Bender,1999
2. Asymptotic Expansions;Erdélyi,1987
3. Asymptotic Analysis: Linear Ordinary Differential Equations;Fedoryuk,1993
4. On matrix-valued Herglotz functions;Gesztesy;Math. Nachr.,2000
5. D.J. Gilbert, Subordinacy and spectral analysis of Schrödinger operators, Ph.D. Thesis, University of Hull, 1984.