1. Nonlinear Oscillations;Minorsky,1962
2. Nonlinear Oscillations;Nayfeh,1979
3. Multiple-Input Describing Functions and Nonlinear Systems Design;Gelb,1968
4. Functional analysis and periodic solutions of nonlinear differential equations;Cesari;Contribut. Differen. Eqs.,1963
5. Galerkin’s procedure for nonlinear periodic systems;Urabe;Arch. Rational Mech. Anal.,1965