1. Theory of Oscillations;Andronov,1949
2. Asymptotic Methods in the Theory of Non-Linear Oscillations;Bogoliubov,1961
3. Qualitative integration of a system of n differential equations in a region containing a singular point and a limit cycle;Brushinskaya;Dokl. Akad. Nauk SSSR,1961
4. Qualitative integration of a system of n differential equations in a region containing a singular point and a limit cycle;Brushinskaya;Dokl. Akad. Nauk MR 26 # 5212,1963
5. Theory of Ordinary Differential Equations;Coddington,1955