1. Probleme général de la stabilité du mouvement, (french translation of a russian paper dated 1893);Liapunov;Ann. Fac. Sci. Univ. Toulouse,1907
2. On the nonexistence of conjugate points;Wintner;Am. J. Math.,1951
3. Selected Works;Azbelev,2012
4. Ordinary Differential Equations;Hartman,1982
5. Lyapunov and Wirtinger inequalities;Lee;Appl. Math. Lett.,2004