[1] Bartušek M.: On existence of oscillatory solution of the system of differential equations. Arch. Math., XVII, 7-10, 1981. MR 0672483

[2] Kiguradze T. I.: Some singular boundary value problems for ordinary differential equations. (in Russian), Tbilisi Univ. Press, Tbilisi, 1975. MR 0499402

[3] Chanturia T. A.: On a comparison theorem for linear differential equations. (in Russian). Izvestija AN SSSR, 40, No 5, 1129-1142, 1976.

[4] Chanturia T. A.: Asymptotic properties of solutions of some sets of nonautonomous ordinary differential equations. Mat. Zametki, 32, No 4, 577-588, 1982 (in Russian). MR 0679249

[5] Koplatadze R. T., Chanturia T. A.: On oscillatory properties of differential equations with deviating argument. Tbilisi Univ. Press, Tbilisi, 1977 (in Russian).