Steady-state responses of system (1): (a) c1 = 0.1, c3 = 0.0; (b) c1 = 0.1, c3 = −0.1; (c) c1 = 0.1, c3 = 0.1. Solid lines: analytical results; the marks of ◼, *, and •: numerical results; dashed line: stability boundaries; and dash-dotted lines: Backbone curves.