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On Asymptotic Properties and Distribution of Zeros of Solutions of $y'' = q(t) y$. Acta F.R.N. Univ. Comenian. To appear. 
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Distribution of Zeros of Solutions of $y'' = q(t) y$ in Relation to Their Behaviour in Large. Acta Math. Acad. Scien. Hungaricae. 8 (1973) 177-185. 
MR 0333344 | 
Zbl 0286.34050