“…The first system, (1.1), has been thoroughly studied in the literature [5,6,7,8,10,12,14,23,25,26,28]. The main result -as shown first by Poincaré -is that for every solution r, there is a cone with vertex at the origin on which r is a geodesic (it follows from (1.1) that |9 r| is constant).…”