A025642 Exponent of 3 (value of i) in n-th number of form 3^i*7^j.

0, 1, 0, 2, 1, 3, 0, 2, 4, 1, 3, 5, 0, 2, 4, 6, 1, 3, 5, 7, 0, 2, 4, 6, 8, 1, 3, 5, 7, 0, 9, 2, 4, 6, 8, 1, 10, 3, 5, 7, 0, 9, 2, 11, 4, 6, 8, 1, 10, 3, 12, 5, 7, 0, 9, 2, 11, 4, 13, 6, 8, 1, 10, 3, 12, 5, 14, 7, 0, 9, 2, 11, 4, 13, 6, 15, 8, 1, 10, 3, 12, 5, 14, 7, 0, 16, 9, 2, 11, 4, 13, 6, 15, 8, 1, 17, 10

Offset

1,4

Links

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

Formula

a(n) = A007949(A003594(n)). - Amiram Eldar, Nov 08 2025

Maple

N:= 10^9: # for 3^i * 7^j <= N
R:= sort([seq(seq(3^i*7^j, j=0 .. ilog[7](N/3^i)), i=0..ilog[3](N))]):
map(padic:-ordp, R, 3); # Robert Israel, Jun 04 2025

Mathematica

seq[lim_] := IntegerExponent[Flatten[Table[3^i*7^j, {i, 0, Log[3, lim]}, {j, 0, Log[7, lim/3^i]}]] // Sort, 3]; seq[10^9] (* Amiram Eldar, Nov 08 2025 *)

Crossrefs

Cf. A003594, A007949, A025665.

Sequence in context: A215344 A025641 A025649 * A384688 A025643 A127478

Adjacent sequences: A025639 A025640 A025641 * A025643 A025644 A025645

Keyword

nonn,easy

Author

David W. Wilson

Status

approved