Let A be a nonlinear differential operator on an open set X ⊂ Rn and S a closed subset of X . Given a class F of functions in X , the set S is said to be removable for F relative to A if any weak solution of A(u) = 0 in X \S of class F satisfies this equation weakly in all of X. For the most extensively studied classes F, we show conditions on S which guarantee that S is removable for F relative to A.
