On the number of limit cycles in discontinuous piecewise linear   differential systems with two pieces separated by a straight line
  In this paper we study the maximum number $N$ of limit cycles that can exhibit a planar piecewise linear differential system formed by two pieces separated by a straight line. More precisely, we prove that this maximum number satisfies $2\leq N \leq 3$ if one of the two linear differential systems has its equilibrium point on the straight line of discontinuity. 
Rodrigo D. Euz\'ebio and Jaume Llibre
math.DS