Electronic Journal of Qualitative Theory of Differential Equations (Electron. J. Qual. Theory Differ. Equ.) (20110101), No 57, 13~pp. eISSN: 1417-3875.
Electronic Journal of Qualitative Theory of Differential Equations (Electron. J. Qual. Theory Differ. Equ.) (20080101), No 22, 11~pp. eISSN: 1417-3875.