The initially palindromic numbers 1, 121, 12321, 1234321, 123454321, ... (OEIS A002477). For the first through ninth terms, the sequence is given by the generating function -(10x + 1)/((x - 1)(10x - 1)(100x - 1)) = 1 + 121x + 12321x^2 + 1234321x^3 + ... (Plouffe 1992, Sloane and Plouffe 1995). The definition of this sequence is slightly ambiguous from the tenth term on, but the most common convention follows from the following observation.