A025704 Index of 4^n within the sequence of the numbers of the form 4^i*7^j.

1, 2, 4, 7, 10, 14, 19, 24, 30, 37, 45, 53, 62, 72, 82, 93, 105, 118, 131, 145, 160, 175, 191, 208, 226, 244, 263, 283, 303, 324, 346, 369, 392, 416, 441, 466, 492, 519, 547, 575, 604, 634, 664, 695, 727, 760, 793, 827, 862, 897, 933, 970, 1008, 1046, 1085, 1125, 1165

Offset

0,2

Comments

Positions of zeros in A025666. - 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)) = 4^n.

a(n) = 1 + Sum_{k=0..n} ceiling(k * c), where c = log_7(4) (A153103).

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

Mathematica

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

Crossrefs

Cf. A000302, A025619, A025666, A153103.

Sequence in context: A025711 A117634 A022776 * A025710 A023536 A196126

Adjacent sequences: A025701 A025702 A025703 * A025705 A025706 A025707

Keyword

nonn,easy

Author

David W. Wilson

Extensions

Offset corrected by Amiram Eldar, Nov 13 2025

Status

approved