[1] T. A. Canturija:
Integral criteria of oscillation of solutions of linear differential equations of higher order. (Russian). Differ urav., XVI, 3, 1980, 470-482.
MR 0569776
[4] V. N. Ševelo N. V. Varech:
On certain properties of solutions of differential equations with a delay. (Ukrainian) UMŽ, 24, 6, 1972, 807-813.
MR 0313612