@article{120188, author = {J\'an Andres}, title = {Symmetry in a certain simply perturbed Hamiltonian system}, journal = {Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica}, volume = {26}, year = {1987}, pages = {131-138}, zbl = {0699.34046}, mrnumber = {1033334}, language = {en}, url = {http://dml.mathdoc.fr/item/120188} }
Andres, Ján. Symmetry in a certain simply perturbed Hamiltonian system. Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica, Tome 26 (1987) pp. 131-138. http://gdmltest.u-ga.fr/item/120188/
Ordinary Differential Equations, (2nd edit.). Nauka, Moscow, 1984 (in Russian). (1984) | MR 0799024
Stability Theory by Liapunov's Second Method, Math. Soc. Japan, Tokyo, 1966. (1966) | MR 0208086