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2. Comparison and Oscillation Theory of Linear Differential Equations;Swanson,1968
3. On a class of nonlinear second-order differential equations;Nehari;Amer. Math. Soc. Trans.,1960
4. Theory of Ordinary Differential Equations;Coddington,1955
5. Comparison and monotonity theorems for second order nonlinear differential equations;Das;Acta Math. Hung.,1964