1. Oscillation theory of ordinary linear differential equations
2. : Leçons de Méthodes de Sturm, Gauthier-Villars, Paris, 1917
3. : Disconjugacy, Lecture Notes in Mathematics 220, Springer-Verlag, Berlin-Heidelberg-New York, 1971
4. and : Linear Operators, Part II: Spectral Theory, Wiley, Interscience, New York, 1988