1. R. Bellman, “The stability of solutions of a differential equation,” Duke Mathematical Journal, vol. 10 (1943), pp. 643–648.
2. L. Cesari, “Sulla stabilitá delle soluzioni delle equazioni differenziali lineari,” Ann. Scuola norm, super. Pisa, vol. 8 (1939), pp. 131–148.
3. N. Levinson, “ The growth of solutions of a differential equation,” Duke Mathematical Journal, vol. 8 (1941), pp. 1–10.
4. A. Wintner, “Small perturbations,” American Journal of Mathematics, vol. 67 (1945), pp. 417–430.