Topological degree in analysis of chaotic behavior in singularlyperturbed systems

A scheme of applying topological degree theory to the analysis of chaotic behavior in singularly perturbed systems is suggested. The scheme combines one introduced by Zgliczyński [ Topol. Methods Nonlinear Anal. 8 , 169 ( 1996 ) ] with the method of topological shadowing, but does not rely on computer based proofs. It is illustrated by a three-dimensional system with piecewise linear slow surface. This approach, when applicable, guarantees abundance of periodic orbits with arbitrarily large periods, each of which is a canard-type trajectory: at first it passes along, and close to, an attractive part of the slow surface of the singularly perturbed system and then continues for a while along the repulsive part of the slow surface. These periodic trajectories are robust in a topological sense with respect to small disturbances in the right-hand sides of the system under consideration, but typically not stable in the Lyapunov sense. Methods of localization of such periodic trajectories are briefly discussed, and numerical examples of localizations are given. The periodic trajectories that are useful from the applications point of view can be stabilized via an appropriate feedback control, for instance, the Pyragas control.