Посилання:Torsion-free groups with every proper homomorphic image an N₁-group / S. Ercan // Algebra and Discrete Mathematics. — 2004. — Vol. 3, № 2. — С. 56–58. — Бібліогр.: 7 назв. — англ.
In this article it is proved that a torsion-free
locally nilpotent groups with non-trivial Fitting subgroup and every
proper homomorphic image an N₁-group is an N₁-group(and so it
is nilpotent).