Periodic orbits for real planar polynomial vector fields of degree<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>n</mml:mi></mml:math>having<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>n</mml:mi></mml:math>invariant straight lines taking into account their multiplicities
Electronic journal on the qualitative theory of differential equations