1. A.D. Myshkis, Linear differential equations with retarded argument. (Russian)Nauka, Moscow, 1972.
2. Delay and Functional Differential Equations and Their Application;G. Ladas,1972
3. R.G. Koplatadze and T.A. Chanturia, On oscillatory and monotone solutions of first-order differential equations with retarded argument. (Russian)Differentsial'nye Uravneniya 18(1982), No. 8, 1463–1465.
4. L. H. Erbe and B.G. Zhang, Oscillation of first-order linear differential equations with deviating arguments.Differential Integral Equations 1(1988), 305–314.
5. J. Chao, Oscillation of linear differential equations with deviating arguments.Theory Practice Math. 1(1991), 32–41.