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2. On the mean-value theorem corresponding to a given linear homogeneous differential equation;Polya;Trans. Amer. Math. Soc.,1922
3. Trench's canonical form for a disconjugate nth-order linear difference equation;Harris;PanAmerican Journals,1998
4. Canonical forms and principal systems for general disconjugate equations;Trench;Trans. Amer. Math. Soc.,1974
5. Disconjugacy criteria for second order linear difference equations;Ahlbrandt,1987