Sandwich groups and (strong) left 3-Engel elements in groups

In this paper we prove a group theoretic analogue of the well known local nilpotence theorem for sandwich Lie algebras due to Kostrikin and Zel’manov [Trudy Mat. Inst. Steklov. 183 (1990), pp. 106–111, 225]. We introduce the notion of a strong left 3-Engel element of a group G and show that these are always in the locally nilpotent radical of G. This generalises a previous result of Jabara and Traustason [Proc. Amer. Math. Soc. 147 (2019), pp. 1921–1927] that showed that a left 3-Engel element a of a group G is in the locally nilpotent radical of G whenever a is of odd order.