1. Baumslag B. and Wiegold J., A Sufficient Condition for Nilpotency in a Finite Group. arXiv:1411.2877v1[math.GR].
2. Kassabov M. and Nikolov N., “Words with few values in finite simple groups,” Q. J. Math., 64, 1161–1166 (2013).
3. Gorenstein D., Finite Groups, Chelsea Publ. Co., New York (1980).