Fonda, AlessandroKlun, GiulianoSfecci, Andrea2021-01-082021-01-082021-01-08https://openscience.sissa.it/handle/1963/35410The aim of this paper is to extend the theory of lower and upper solutions to the periodic problem associated with planar systems of differential equations. We generalize previously given definitions and we are able to treat both the well-ordered case and the non-well-ordered case. The proofs involve topological degree arguments, together with a detailed analysis of the solutions in the phase plane.enPreprint