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Beschreibung:
We present a new approach to study limit cycles of planar systems of autonomous differential equations with a cylindrical phase space $Z$. It is based on an extension of the Dulac function which we call Dulac-Cherkas function $\Psi$. The level set $W:=\{\vf,y) \in Z: \Psi(\vf,y)=0\}$ plays a key role in this approach, its topological structure influences existence, location and number of limit cycles. We present two procedures to construct Dulac-Cherkas functions. For the general case we describe a numerical approach based on the reduction to a linear programming problem and which is implemented by means of the computer algebra system Mathematica. For the class of generalized Li\'enard systems we present an analytical approach associated with solving linear differential equations and algebraic equations.