Ct. Fulton et Sa. Pruess, EIGENVALUE AND EIGENFUNCTION ASYMPTOTICS FOR REGULAR STURM-LIOUVILLE PROBLEMS, Journal of mathematical analysis and applications, 188(1), 1994, pp. 297-340
In this paper, we obtain asymptotic formulas for eigenvalues, eigenfun
ctions, and the reciprocals of the eigenfunction norms for eigenvalue
problems associated with the general Sturm-Liouville equation (pu')' (lambda r - q) u = 0 having regular endpoints. The method is based on
a conversion to Liouville Normal Form and an iterative procedure of s
olving the associated Volterra integral equation, producing an asympto
tic expansion of the solution in higher powers of 1/lambda(1/2) as lam
bda --> infinity. (C) 1994 Academic Press, Inc.