Nonexistence of positive supersolutions of elliptic equations via the maximum principle

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We introduce a new method for proving the nonexistence of positive supersolutions of elliptic inequalities in unbounded domains of Rⁿ. The simplicity and robustness of our maximum principle-based argument provides for its applicability to many elliptic inequalities and systems, including quasilinear operators such as the p-Laplacian, and nondivergence form fully nonlinear operators such as Bellman-Isaacs operators. Our method gives new and optimal results in terms of the nonlinear functions appearing in the inequalities, and applies to inequalities holding in the whole space as well as exterior domains and cone-like domains.