A note on a piecewise-linear Duffing-type system
  In a recent work it was suggested that the number of limit cycles in a piecewise-linear system could be closely related to the number of zones, that is the number of parts of the phase plane where the system is linear. In this note we construct an example of a class of perturbed piecewise systems with n zones such that the first variation of the displacement function is identically zero. Then we conjecture that the system has no limit cycles using the second variation of the displacement function expressed for continuous functions. This system can be seen as a feedback system in control theory. 
G. Tigan, A. Astolfi