“…Delay differential equations exhibit very richer bifurcation phenomena, including Hopf bifurcation (Hale and Verduyn Lunel [62], Hassard et al [63], Faria and Magalhães [49], Guo and Wu [60]), Bautin bifurcation (Bi and Ruan [13], Ion [66]), Bogdanov-Takens bifurcation (Faria and Magalhães [50], Xiao and Ruan [125]), Fold-Hopf (zero-Hopf) bifurcation (Choi and LeBlanc [27], Guo et al [59], Jiang et al [67], Jiang and Wang [68], Wu and Wang [122]), Hopf-Hopf bifurcation (Campbell and Belair [25], Bruno and Bélair [21], Wu and Wang [124]), and triple zero singularities (Campbell and Yuan [26], LeBlanc [77]). In applications it is usually interesting but difficult to show that a specific biological or physical model undergoes any of these bifurcations in particular the degenerate ones.…”