A Remark on a Paper of Luca and Walsh

Abstract In 2002, F. Luca and P. G. Walsh studied the Diophantine equations of the form (ak – 1)(bk – 1) = x 2, for all (a, b) in the range 2 ≤ b < a ≤ 100 with sixty-nine exceptions. In this paper, we solve two of the exceptions. In fact, we consider the equations of the form (ak – 1)(bk – 1) = x 2, with (a, b) = (13, 4), (28, 13).