A note on the combinatorial derivation of nonsmall sets

摘要

Given an infinite group G and a subset A of G we let Δ(A) = {g ∈ G : |gA ∩ A|=∞} (this is sometimes called the combinatorial derivation of A). A subset A of G is called: large if there exists a finite subset F of G such that FA=G; Δ-large if Δ(A) is large and small if for every large subset L of G, (G setminus A) ∩ L is large. In this note we show that every nonsmall set is Δ-large, answering a question of Protasov.