“…Several papers deal with the Diophantine equation (1.1) when S contains at least two distinct odd primes. Thus, all solutions of the Diophantine equation (1.1) were given in [2] for S = {5, 13}, in [28] for S = {5, 17}, in [30], [31] for S = {7, 11} -except for the case when ax is odd and b is even-, in [6] for S = {11, 17}, in [17] for S = {2, 5, 13}, in [9] for S = {2, 3, 11}, in [15] for S = {2, 5, 17}. In [27], Pink gave all the non-exceptional solutions of the equation (1.1) (according to the terminology of that paper) for S = {2, 3, 5, 7}.…”