A theorem on the structural stability of non-autonomous differential equations И.У. Бронштейн, В.П. Бурдаев (1982) Matematiceskie issledovanija
An algebraic condition for qualitative stability of first order linear autonomous ordinary differential systems Dele Oluwade (2001) Kragujevac Journal of Mathematics
Asymptotic expansions of quasiperiodic solutions L. Chierchia, E. Zehnder (1989) Annali della Scuola Normale Superiore di Pisa - Classe di Scienze