Atkinson, F.V. (1964). Discrete and Continuous Boundary Problems, Academic Press.
Gesztesy, F.; Simon, B.; Teschl, G. (1996). Zeros of the Wronskian and renormalized oscillation theory, Am. J. Math. 118, 571–594.
Kreith, K. (1973). Oscillation Theory, Lecture Notes in Mathematics 324, Springer.
Krüger, H; Teschl G. (2009). Relative oscillation theory, weighted zeros of the Wronskian, and the spectral shift function, Commun. Math. Phys. 287, 613–640.
Sturm, J.C.F. (1836). Memoire sur les equations diferentielles lineaires du second ordre, J. Math. Pures Appl. 1, 106–186.
Swanson, C.A. (1968). Comparison and Oscillation Theory of Linear Differential Equations, Academic Press.