1. Asymptotic behavior of the solutions of (ry(m))(k) ± qy = 0;Hinton;J. Differential Equations,1968
2. Linear Differential Operators Part II;Naimark,1968
3. Asymptotic methods in the theory of one-dimensional singular differential operators;Fedorjuk;Trans. Moscow Math. Soc.,1966
4. Theory of Ordinary Differential Equations;Coddington,1955
5. The asymptotic nature of the solutions of linear systems of differential equations;Levinson;Duke Math. J.,1948