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On Zeros of Solutions of the Differential Equation $(p(t)y')' +f(t,y,y') = 0$. Arch. Math. XI, No 4, 187-192. 
MR 0409975 | 
Zbl 0342.34018[3] T. Dlotko: 
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MR 0133499[4] K. M. Das: 
Comparison and Monotonity Theorems for Second Order Non-linear Differential Equations. Acta Math. Acad. Sci. Hung., 15, No 3-4, 1964, 449-456. 
MR 0176155 | 
Zbl 0135.14102[5] I. Bihari: 
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