1. On the equation x′' + f(x)h(x′)x′ + g(x) = e(t);Burton;Annali Mat. pura appl.,1970
2. On the generalized Liénard equation with negative damping;Graef;J. diff. Eqns,1972
3. Stability by Liapunov's Direct Method with Applications;Lasalle,1961
4. Qualitative Theorie Nichtlinearer Differentialgleichungen;Reissig,1963
5. Non-Linear Differential Equations;Sansone,1964