“…This is well understood today for U 1 q [9,20], but the theory is far for complete if j ≥ 2. However, a number of important theorems have been obtained by Erdős et [12,33,6,4,5,27,36,37,19]. In order to mention some of these results concerning the two-digit case M = 1, let us denote by B j the set of bases q for which U j q is non-empty.…”