Return maps of folded nodes and folded saddle-nodes.

Folded nodes occur in generic slow-fast dynamical systems with two slow variables. Open regions of initial conditions flow into a folded node in an open set of such systems, so folded nodes are an important feature of generic slow-fast systems. Twisting and linking of trajectories in the vicinity of a folded node have been studied previously, but their consequences for global dynamical behavior have hardly been investigated. One manifestation of the twisting is as "mixed mode oscillations" observed in chemical and neural systems. This paper presents the first systematic numerical study of return maps for trajectories that flow through a region with a folded node. These return maps are approximated by rank-1 maps, and the local twisting of trajectories near a folded node gives rise to multiple turning points in the approximating one dimensional maps. A variant of the forced van der Pol system is used here to illustrate that folded nodes can be a "chaos-generating" mechanism. Folded saddle-nodes occur in generic one-parameter families of slow-fast dynamical systems with two slow variables. These bifurcations give birth to folded nodes. Numerical simulations demonstrate that return maps of systems that are close to a folded saddle-node can be even more complex than those of folded nodes that are far from folded saddles.