[1] Cesari L.: Asimptotičeskoje povedenije i ustojčivosť rešenij obyknovennych differenciaľnych uravnenij. Moskva 1964.

[2] Ráb M.: On the Boundedness of Solutions of Second-order Differential Equations. (Lеcturе һеld on Wintеr-School of Diffеrеntial Equations in 1974).

[3] Wiswanatham B.: A generalization of Bellman's lemma. Proc. Аmеr. Math. Soc. 14 (1963), 15-18.