A025745 Index of 10^n within the sequence of the numbers of the form 7^i*10^j.

1, 3, 6, 10, 15, 21, 29, 38, 48, 59, 71, 85, 100, 116, 133, 151, 170, 191, 213, 236, 260, 285, 312, 340, 369, 399, 430, 462, 496, 531, 567, 604, 642, 682, 723, 765, 808, 852, 897, 944, 992, 1041, 1091, 1142, 1195, 1249, 1304, 1360, 1417, 1475, 1535, 1596, 1658

Offset

0,2

Comments

Positions of zeros in A025671. - R. J. Mathar, Jul 06 2025

Links

Amiram Eldar, Table of n, a(n) for n = 0..10000

Formula

From Amiram Eldar, Nov 14 2025: (Start)

A025632(a(n)) = 10^n.

a(n) = 1 + Sum_{k=0..n} ceiling(k * c), where c = log_7(10) (A154158).

a(n) ~ c * n^2 / 2, where c is defined above. (End)

Mathematica

Accumulate[Table[Ceiling[n / Log10[7]], {n, 0, 60}]] + 1 (* Amiram Eldar, Nov 14 2025 *)

Crossrefs

Cf. A011557, A025632, A025671, A154158.

Sequence in context: A147846 A062099 A115650 * A175724 A335184 A101551

Adjacent sequences: A025742 A025743 A025744 * A025746 A025747 A025748

Keyword

nonn,easy

Author

David W. Wilson

Extensions

Offset corrected by Amiram Eldar, Nov 14 2025

Status

approved