HRUBINOVÁ A.: Asymptotic properties of solutions of nth order nonlinear differential equations with advanced argument. (in Slovak). Dissertation, Department of mathematics VŠT Košice, 1984.
 SINGH B., KUSANO T.: Asymptotic behavior of oscillatory solutions of a difìerential equation with deviating arguments
. J. Math. Anal. Appl., 83, 1981, 395-407. MR 0641341