Bifurcation of Periodic Delay Differential Equations at Points of 1:4 Resonance

BIFURCATION OF PERIODIC DELAY DIFFERENTIAL EQUATIONS AT POINTS OF 1:4 RESONANCE ∗ G. RÖST † Abstract. The time-periodic scalar delay differential equation ẋ(t) = γf(t, x(t − 1)) is considered, which leads to a resonant bifurcation of the equilibrium at critical values of the parameter. Using Floquet theory, spectral projection and center manifold reduction, we give conditions for the stability properties of the bifurcating invariant curves and fourperiodic orbits. The coefficients of the third order normal form are derived explicitly. We show that the 1:4 resonance has no effect on equations of the form ż(t) = −γr(t)g(x(t−1)).