Nonlinear boundary value problems relative to harmonic functions

We study the problem of finding a function u verifying --$Δ$u = 0 in $Ω$ under the boundary condition $\partial$u $\partial$n + g(u) = $μ$ on $\partial$$Ω$ where $Ω$ $\subset$ R N is a smooth domain, n the normal unit outward vector to $Ω$, $μ$ is a measure on $\partial$$Ω$ and g a continuous nondecreasing function. We give sufficient condition on g for this problem to be solvable for any measure. When g(r) = |r| p--1 r, p > 1, we give conditions in order an isolated singularity on $\partial$$Ω$ be removable. We also give capacitary conditions on a measure $μ$ in order the problem with g(r) = |r| p--1 r to be solvable for some $μ$. We also study the isolated singularities of functions satisfying --$Δ$u = 0 in $Ω$ and $\partial$u $\partial$n + g(u) = 0 on $\partial$$Ω$ \ {0}.

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