Fonda, AlessandroKlun, GiulianoSfecci, Andrea2020-05-042020-05-042020-05https://openscience.sissa.it/handle/1963/35350We prove existence results for systems of boundary value problems involving elliptic second order differential operators. The as-sumptions involve lower and upper solutions, which may be either well-ordered, or not at all. The results are stated in an abstract framework, and can be translated also for systems of parabolic type.enPreprintelliptic operator; boundary value problems; Dirichlet and Neumann problem; lower and upper solutions; degree theory