“…Let us, for instance, address the problem of estimating the length of the longest arithmetic progression in lA (where A is a subset of [n]), as a function of l, n and |A|. In special cases sharp results have been obtained, thanks to the works of several researchers, including Bourgain, Freiman, Halberstam, Ruzsa and Sárközy [2], [6], [8], [17]. Our method, combined with additional arguments, allows us to derive a sharp bound for this length for a wide range of l and |A|.…”