2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/141540The purpose of this paper is to provide tools for analyzing the compactness of sequences in Sobolev spaces, in particular if the sequence gets mapped onto a compact set by some nonlinear operator. Here, our focus lies on a very general class of nonlinear operators arising in quasilinear systems of partial differential equations of second order, in divergence form. Our approach, based on a suitable decomposition lemma, admits the discussion of problems with some inherent loss of compactness, for example due to a domain with infinite measure or a lower order term with critical growth. As an application, we obtain a characterization of properness which is considerably easier to verify than the definition. The methods presented can also be used to check Palais--Smale conditions for variational problems.63 pagesAnalysis of PDEs35G30, 35J55, 35A35, 49J45text