Labutin, Denis2015-12-132015-12-130002-9939http://hdl.handle.net/1885/90857For an arbitrary open set Ω ⊂ ℝn we characterize all functions G on the real line such that G o u ∈ W1,p(Ω) for all u ∈ W1,p. New element in the proof is based on Maz'ya's capacitary criterion for the imbedding W1,p(Ω) → L∞(Ω).Keywords: Sobolev spaces; Superposition operator20002015-12-12