A025666 Exponent of 7 (value of j) in n-th number of form 4^i*7^j.

0, 0, 1, 0, 1, 2, 0, 1, 2, 0, 3, 1, 2, 0, 3, 1, 4, 2, 0, 3, 1, 4, 2, 0, 5, 3, 1, 4, 2, 0, 5, 3, 1, 6, 4, 2, 0, 5, 3, 1, 6, 4, 2, 7, 0, 5, 3, 1, 6, 4, 2, 7, 0, 5, 3, 8, 1, 6, 4, 2, 7, 0, 5, 3, 8, 1, 6, 4, 9, 2, 7, 0, 5, 3, 8, 1, 6, 4, 9, 2, 7, 0, 5, 10, 3, 8, 1, 6, 4, 9, 2, 7, 0, 5, 10, 3, 8, 1, 6, 11, 4, 9, 2, 7, 0, 5

Offset

1,6

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Formula

4^A025648(n)*7^a(n) = A025619(n). - Robert Israel, Aug 11 2019

a(n) = A214411(A025619(n)). - Amiram Eldar, Nov 08 2025

Maple

N:= 10^10: # to consider all terms with 4^i*7^j<=N
S:= {seq(seq(4^i*7^j, j=0..floor(log[7](N/4^i))), i=0..floor(log[4](N)))}:
map(padic:-ordp, sort(convert(S, list)), 7); # Robert Israel, Aug 11 2019

Mathematica

seq[lim_] := IntegerExponent[Flatten[Table[4^i*7^j, {i, 0, Log[4, lim]}, {j, 0, Log[7, lim/4^i]}]] // Sort, 7]; seq[10^10] (* Amiram Eldar, Nov 08 2025 *)

Crossrefs

Cf. A025619, A025648, A214411.

Sequence in context: A173863 A022337 A025687 * A025679 A071491 A156537

Adjacent sequences: A025663 A025664 A025665 * A025667 A025668 A025669

Keyword

nonn,easy

Author

David W. Wilson

Status

approved