The forced response curve is a graph showing the amplitude of the response of a periodically forced system as the forcing frequency is varied. Zhang and Golubitsky (SIADS, 10(4) (2011) 1272–1306) classified the existence and multiplicity of periodic responses to small amplitude periodic forcing of a system near a Hopf bifurcation point, where the forcing frequency, ωF , is close to the Hopf fre...