On the planar integrable differential systems

Under very general assumptions we prove that the planar differential systems having a first integral are essentially the linear differential systems u˙ = u, ˙v = v. Additionally such systems always have a Lie symmetry. We improve these results for polynomial differential systems defined in R2 or C2.The first author is partially supported by a MCYT/FEDER grant number MTM2008-00694 and by a CIRIT grant number 2005SGR 00550. The second author is partially supported by a MCYT/FEDER grant number MTM2008-03437 and by a CIRIT grant number 2005SGR 00550