A031026 a(n)=least k such that the base 7 representation of n begins at s(k), where s=A031018.

1, 7, 2, 9, 3, 11, 4, 58, 6, 1, 8, 65, 10, 68, 7, 71, 14, 74, 16, 77, 18, 63, 15, 83, 22, 2, 24, 89, 9, 92, 23, 95, 30, 98, 32, 3, 34, 17, 36, 31, 38, 110, 40, 11, 42, 25, 44, 39, 46, 4, 49, 531, 52, 559, 55, 506, 58, 604, 61, 608, 64, 612, 67, 6