Some nonexistence results for positive solutions of elliptic equations in unbounded domains

We prove some Liouville type theorems for positive solutions of semilinear elliptic equations in the whole space R-N, N greater than or equal to 3, and in the half space R-+(N) with different boundary conditions, using the technique based on the Kelvin transform and the Alexandrov-Serrin method of moving hyperplanes. In particular we get new nonexistence results for elliptic problems in half spaces satisfying mixed (Dirichlet-Neumann) boundary conditions.