Chemical oscillator — limit cycles and Hopf bifurcation
The Brusselator (Prigogine, 1968) is a minimal model for autocatalytic chemical oscillations:
Fixed point: X* = A, Y* = B/A. Stability: eigenvalues of Jacobian at fixed point. Hopf bifurcation occurs at B = 1 + A².
For B > 1+A², the fixed point destabilizes and a stable limit cycle appears — the system oscillates indefinitely. This models the Belousov–Zhabotinsky reaction.