“…In a class of larger spaces, the Zhidkov spaces X k (R d ), it has been solved in X 1 (R) and in X 2 (R 2 ) by Zhidkov, by Gallo and by Goubet ([23], [24], [8], [13]). Also, considered in the natural energy space {f ∈ H 1 loc , ∇f ∈ L 2 , |f | 2 − 1 ∈ L 2 }, it has been solved for d ∈ {2, 3} and for d = 4 with smallness assumption, by Gérard in [10].…”