“…This follows from well-known commutator formulae (and for any p-group); see, for example,[20, Lemma 4.1].In particular, for any z ∈ N by using (6.2) we obtain[z, c y 2 m(c−1) ] = [az, c y 2 m(c−1) ] = 1,(6.3)since az, y 2 m(c−1) is a subgroup of az, y , which is nilpotent of class c by (6.1).Our aim is to show that there is a uniform bound, in terms of |G : K|, c, and m, for the nonsoluble length of all finite quotients of G by open normal subgroups. Let M be an open normal subgroup of G and let the bar denote the images in Ḡ = G/M.…”