A025722 Index of 7^n within the sequence of the numbers of the form 4^i*7^j.

1, 3, 6, 11, 17, 25, 34, 44, 56, 69, 84, 100, 117, 136, 156, 178, 201, 225, 251, 278, 307, 337, 368, 401, 435, 471, 508, 546, 586, 627, 670, 714, 759, 806, 854, 904, 955, 1007, 1061, 1116, 1173, 1231, 1290, 1351, 1413, 1477, 1542, 1608, 1676, 1745, 1816, 1888

Offset

0,2

Comments

Positions of zeros in A025648. - 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)

A025619(a(n)) = 7^n.

a(n) = 1 + Sum_{k=0..n} ceiling(k * c), where c = log_4(7) (A153615).

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

Mathematica

Accumulate[Table[Ceiling[n * Log[4, 7]], {n, 0, 60}]] + 1 (* Amiram Eldar, Nov 13 2025 *)

Crossrefs

Cf. A000420, A025619, A025648, A153615.

Sequence in context: A109413 A294397 A003022 * A022775 A025743 A022338

Adjacent sequences: A025719 A025720 A025721 * A025723 A025724 A025725

Keyword

nonn,easy

Author

David W. Wilson

Extensions

Offset corrected by Amiram Eldar, Nov 13 2025

Status

approved