Figure 12

Phase portraits near the Hopf–Hopf bifurcation point \((\tau _{1}^{*}, \tau _{\mathrm{s}}^{*}) =(0.0451, 5.215)\) for the delay \((\tau _{1}, \tau _{\mathrm{s}})\) chosen as (a) \((0.02, 4)\), (b) \((0.02, 5)\), (c) \((0.02, 6)\), (d) \((0.16, 5.5)\), (e) \((0.16, 6)\), and (f) \((0.16, 6.9)\) with the fixed parameters \(k _{1} =1\), \(k _{2} =0.6\), \(c _{1} =0.8\)