Let m, n be positive integers, v a multilinear commutator word and w = v m . We prove that if G is a residually finite group in which all wvalues are n-Engel, then the verbal subgroup w(G) is locally nilpotent. We also examine the question whether this is true in the case where G is locally graded rather than residually finite. We answer the question affirmatively in the case where m = 1. Moreover, we show that if u is a non-commutator word and G is a locally graded group in which all u-values are n-Engel, then the verbal subgroup u(G) is locally nilpotent. then x is a (left) n-Engel element. A group G is called n-Engel if all elements of G are n-Engel. It is a long-standing problem whether any n-Engel group is locally nilpotent. Following Zelmanov's solution of the restricted Burnside problem [16,17], Wilson proved that this is true if G is residually finite [14]. Later the second author showed that if in a residually finite group G all commutators [x 1 , . . . , x k ] are n-Engel, then the subgroup [x 1 , . . . , x k ] | x i ∈ G is locally nilpotent [10,11]. This suggests the following conjecture.Conjecture. Let w be a group-word and n a positive integer. Assume that G is a residually finite group in which all w-values are n-Engel. Then the corresponding verbal subgroup w(G) is locally nilpotent.
Preliminary resultsGiven subgroups X and Y of a group G, we denote by X Y the smallest subgroup of G containing X and normalized by Y .Lemma 2.1. Let x and y be elements of a group G satisfying [x, n y m ] = 1, for some n, m ≥ 1. Then x y is finitely generated.Proof. Set X = x y m . Then X is finitely generated by [9, Exercise 12.3.6]. Since x y = X y i | i = 0, . . . , m − 1 , the lemma follows.Corollary 2.2. Let y be an element of a group G and H a finitely generated subgroup. If y m is Engel for some m ≥ 1, then H y is finitely generated.The following lemma is well-known. We supply the proof for the reader's convenience.