[1] M. Бapтyшeк: O нyляx кoлeблющuxcя peшeнuй ypaвнeнuя  $(p(t) x')' + f(t, x, x') = 0$. Дифф. ypaв. XII, Но. 4, 621-625.
[2] M. Bartušek: 
On Zeros of Solutions of the Differential Equation (p(t)y')' + f(t,y,y') = 0. Arch. Math. XI, No 4, 187-192. 
MR 0409975 | 
Zbl 0342.34018 
[З] M. Bartušek: 
Monotonicity Theorems Concerning Differential Equations y" + f(t,y,y') = 0. Arch. Math. XII, No 4, 169-178. 
MR 0430410 | 
Zbl 0354.34031 
[4] I. Bihari: 
Oscillation and Monotonity Theorems Concerning Non-linear Differential Equations of the Second Order. Acta Math. Acad. Sci. Hung., IX, No 1-2, 1958, 83-104. 
MR 0095321