When time delays and phase lags are not the same: higher-order phase reduction unravels delay-induced synchronization in oscillator networks

Coupled oscillators with time-delayed network interactions are critical to understand synchronization phenomena in many physical systems. Phase reductions to finite-dimensional phase oscillator networks allow for their explicit analysis. However, first-order phase reductions - where delays correspond to phase lags - fail to capture the delay-dependence of synchronization. We develop a systematic approach to derive phase reductions for delay-coupled oscillators to arbitrary order. Already the second-order reduction can predict delay-dependent (bi-)stability of synchronized states as demonstrated for Stuart-Landau oscillators.