“…Therefore, when studying the group w(F 2 ) and groups associated with it, we can use lemmas proved in [5]. In particular, by Lemma 17 of [5], all the nontrivial values of w(x, y) in G = F 2 /[w(F 2 ), F 2 ] form a basis of the free abelian group w(G). Now let V be the subgroup of G generated by all the elements of the form w(X, Y ) and [5] and Lemma 15 of [5], the fact that w(S 1 , T 1 ) = w(S 2 , T 2 ) in G yields that S 1 is conjugate to S 2 and T 1 is conjugate to T 2 in the group F 2 /w(F 2 ).…”