# Problem 551: Sum of digits sequence

![graphic](img551.gif)

Let a0, a1, a2, ... be an integer sequence defined by: a0 = 1; for n ≥
1, an is the sum of the digits of all preceding terms. The sequence
starts with 1, 1, 2, 4, 8, 16, 23, 28, 38, 49, ... You are given
a106 = 31054319. Find a1015.