1. On the mean-value theorem corresponding to a given linear homogeneous differential equation;Polya;Trans. Amer. Math. Soc.,1922
2. Some problems bearing on the oscillation of linear differential equations;Levin;Soviet Math. Dokl.,1963
3. Distribution of the zeros of solutions of a linear differential equation;Levin;Soviet Math. Dokl.,1964
4. Theory of Ordinary Differential Equations;Coddington,1955
5. Disconjugacy of generalized linear differential equations of third and fourth order;Barrett;Amer. Math. Soc. Notices,1962