A025721 Index of 7^n within sequence of numbers of form 3^i*7^j for i >= 0, j >= 1.

1, 3, 7, 13, 21, 30, 41, 54, 69, 85, 103, 123, 145, 169, 194, 221, 250, 281, 313, 347, 383, 421, 460, 501, 544, 589, 636, 684, 734, 786, 840, 895, 952, 1011, 1072, 1134, 1198, 1264, 1332, 1402, 1473, 1546, 1621, 1698, 1776, 1856, 1938, 2022, 2108, 2195, 2284

Offset: 1

Author

David W. Wilson

Keywords

• nonn

Comments

Positions of zeros in A025642. - R. J. Mathar, Jul 06 2025

Examples

			a(1) = 1, a(2) = 3 and a(3) = 7 because the first 7 numbers of the form 3^i * 7^j with i >= 0 and j >= 1 are 7, 21, 49, 63, 147, 189 and 343. - _Robert Israel_, Aug 20 2024
		

Links

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

Crossrefs

Cf. A003594.

Programs

Maple

dA:= map(t -> 1+ilog[3](7^t), [$0..100]):
ListTools:-PartialSums(dA); # Robert Israel, Aug 20 2024

Extensions

Name clarified by Robert Israel, Aug 20 2024