1. Hale, J. K., Asymptotic behavior of the solutions of differential-difference equations. Proc. Symp. Nonlinear Oscillations, IUTAM, Kiev, USSR, September 1961.
2. Hale, J. K., Functional-differential equations with parameters. Contributions to Differential Equations 1, 401–410 (1963).
3. Hale, J. K., Periodic and almost periodic solutions of functional-differential equations. Arch. Rational Mech. Anal. 15, 289–304 (1964).
4. Krasovskii, N. N., Some problems in the theory of stability of motion. Moscow 1959, English translation, Stanford: Stanford Univ. Press 1963.
5. Lasalle, J. P., & S. Lefschetz, Stability by Liapunov's direct method with applications. New York: Academic Press 1961. See also J. P. LaSalle, A study of synchronous asymptotic stability, Annals of Math. 65 (1957), 571–581.