Finite-by-nilpotent groups and a variation of the BFC-theorem

Abstract: For a group G and an element a in G let |a|k denote the cardinality of the set of commutators [a,x_1,...,x_k], where x_1,...,x_k range over G. The main result of the paper states that a group G is finite-by-nilpotent if and only if there are positive integers k and n such that |x|_k < n for every x in G. More precisely, if |x|_k < n for every x in G then \gamma{k+1}(G) has finite (k,n)-bounded order.