A. F. El-Bassiouny Response of a three-degree-of-freedom system with cubic non-linearities to harmonic excitation. 65-84 1999 104 Applied Mathematics and Computation 1 http://dx.doi.org/10.1016/S0096-3003(98)10051-6 db/journals/amc/amc104.html#El-Bassiouny99