bifurcation diagram blue sky catastrophe bogdanov-takens bifurcation catastrophe theory delay differential equation differential equation equilibrium point heteroclinic bifurcation homoclinic bifurcation hopf bifurcation infinite-period bifurcation integral curve jacobian mathematics ordinary differential equation partial differential equation period-doubling bifurcation phase portrait pitchfork bifurcation saddle-node bifurcation saddle point transcritical bifurcation vector field