A025720 Index of 7^n within the sequence of the numbers of the form 2^i*7^j.

1, 4, 10, 19, 31, 46, 63, 83, 106, 132, 161, 192, 226, 263, 303, 346, 391, 439, 490, 544, 601, 660, 722, 787, 855, 926, 999, 1075, 1154, 1236, 1321, 1409, 1499, 1592, 1688, 1787, 1889, 1993, 2100, 2210, 2323, 2439, 2557, 2678, 2802, 2929, 3059, 3191, 3326

Offset

0,2

Comments

Positions of zeros in A025637. - R. J. Mathar, Jul 06 2025

Links

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

Formula

From Amiram Eldar, Nov 13 2025: (Start)

A003591(a(n)) = 7^n.

a(n) = 1 + Sum_{k=0..n} ceiling(k * c), where c = log_2(7) (A020860).

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

Mathematica

Accumulate[Table[Ceiling[n * Log2[7]], {n, 0, 60}]] + 1 (* Amiram Eldar, Nov 13 2025 *)

Crossrefs

Cf. A000420, A003591, A020860, A025637.

Sequence in context: A160425 A152946 A162505 * A022793 A005448 A381647

Adjacent sequences: A025717 A025718 A025719 * A025721 A025722 A025723

Keyword

nonn,easy

Author

David W. Wilson

Extensions

Offset corrected by Amiram Eldar, Nov 13 2025

Status

approved