1. On self-adjoint differential equations of second order;Leighton;J. Lond. Math. Soc.,1952
2. Oscillation criteria for forced second-order linear differential equation;Wong;J. Math. Anal. Appl.,1999
3. Ordinary Differential Equations;Hartman,2002
4. The Theory of Differential Equations: Classical and Qualitative;Kelley,2004
5. Principal and nonprincipal solutions of impulsive differential equations with applications;Özbekler;Appl. Math. Comput.,2010