[1] Breuer S., Gottlieb D.: 
Separation of roots and oscillation in ordinary linear differential equations of second order. Proc. Amer. Math. Soc. 29 (1971), 487-493. 
MR 0276545 | 
Zbl 0224.34024[2] Caratheodory C.: Vorlesungen über reelle Funktionen. 2. Aufl. Teubner Leipzig 1927.
[3] Etgen G. J., Pawlowski J. F.: 
Oscillation criteria for second order self adjoint differential systems. Pacific J. Math. 66 (1976) 99-110. 
MR 0440147 | 
Zbl 0355.34017[4] Fite W. B.: 
Concerning the zeros of the solutions of certain differential equations. Trans. Amer. Math. Soc. 19 (1917), 341-352. 
MR 1501107[5] Gagliardo E.: 
Sul comportamento asintotico degli integrali dell'equazione differenziale $y" + A(x) y =0$ con $A(x) \geq 0$. Boll. Unione Mat. Ital. (3) 8 (1953), 177-185. 
MR 0056779[6] Guggenheimer H.: 
Hill equations with coexisting periodic solutions. J. Diff. Equations 5 (1969), 159-166. 
MR 0239193 | 
Zbl 0186.41003[7] Hartman P., Wintner A.: 
On non-conservative linear oscillators of low frequency. Amer. J. Math. 70 (1948), 529-539. 
MR 0026736 | 
Zbl 0037.19002[8] Kreith K.: 
Oscillation theory. Lecture Notes in Math. 324, Springer-Verlag Berlin, Heidelberg, New York 1973. 
Zbl 0258.35001[10] Leighton W.: 
Principal quadratic functionals and self adjoint second-order differential equations. Proc. Nat. Acad. Sci. 35 (1949) 192-193. 
MR 0030672 | 
Zbl 0032.34603[11] Leighton W.: 
On self adjoint differential equations of second order. J. London Math. Soc. 27 (1952), 37-47. 
MR 0046506 | 
Zbl 0048.06503[12] Moore R. A.: 
The behavior of solutions of a linear differential equation of second order. Pacific J. Math. 5 (1955), 125-145. 
MR 0068690[13] Neuman F.: 
Centroaffine invariants of plane curves in connection with the theory of the second order linear differential equations. Archivum Math. (Brno) 4 (1968), 201-216. 
MR 0267512 | 
Zbl 0218.34007[14] Opial Z.: 
Sur une critère d'oscillation des integrales de l'equation différentielle $(Q(t) x')' + f(t)x = 0$. Ann. Polon. Math. 6 (1959/1960), 99-104. 
MR 0104876[15] Pfaff R.: 
Zur Theorie der gewöhnlichen linearen Differentialgleichung zweiter Ordnung mit Distributionskoeffizient. Dissertation Darmstadt 1978. 
MR 0481259[16] Potter R. L.: 
On self-adjoint differential equations of second order. Pacific J. Math. 3 (1953),. 467-491. 
MR 0056156 | 
Zbl 0051.06502[17] Ráb M.: 
Kriterien für die Oszillation der Lösungen der Differentialgleichung $[p(x)y']' + q(x)y = 0$. Časopis Pěst. Mat. 84 (1959), 335-370. 
MR 0114964[18] Swanson C. A.: 
Comparison and oscillation theory of linear differential equations. Academic Press New York, London 1968. 
MR 0463570 | 
Zbl 0191.09904[19] Willet D.: 
Classification of second order linear differential equations with respect to oscillation. In: Lectures on ordinary differential equations. Ed. R. McKelvey, Academic Press New York, London 1970. 
MR 0280800[20] Wintner A.: 
A criterion of oscillatory stability. Quarterly Appl. Math. 7 (1949), 115-117. 
MR 0028499 | 
Zbl 0032.34801