1. A. A. Movchan, The direct method of Liapunov in stability problems of elastic systems, Prikl. Mat. i Mekh. 23, 483-493 (1959)
2. English translation, Appl. Math. and Mech. 3, 686-700 (1959)
3. R. H. Plaut, A study of the dynamic stability of continuous elastic systems by Liapunov’s direct method, Rept. No. AM-67-3, College of Engineering, University of California, Berkeley, May 1967
4. Dynamical systems and stability;Hale, Jack K.;J. Math. Anal. Appl.,1969