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4. Nonoscillation theorems for a class of nonlinear differential equations;Moore, Richard A.;Trans. Amer. Math. Soc.,1959
5. On the oscillatory aspect of solutions of the differential equation 𝑢”=𝑓(𝑢,𝑢’,𝑡);Petropavlovskaya, R. V.;Dokl. Akad. Nauk SSSR (N.S.),1956