Fig. 10

Phase plane portrait for trajectories in the Fitzhugh–Nagumo model for \(D = 0.01\) (solid line), \(D = 0.015\) (dashed), and \(D=0.02\) (dotted). For \(D = (0.015, 0.02)\), one can see the large excursions away from the fixed point which appear as the bifurcation becomes closer. These excursions contribute to the ensemble averages and thus the divergence in the diagrammatic series