A025721 - OEIS

%I #12 Aug 20 2024 11:27:45

%S 1,3,7,13,21,30,41,54,69,85,103,123,145,169,194,221,250,281,313,347,

%T 383,421,460,501,544,589,636,684,734,786,840,895,952,1011,1072,1134,

%U 1198,1264,1332,1402,1473,1546,1621,1698,1776,1856,1938,2022,2108,2195,2284

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

%H Robert Israel, <a href="/A025721/b025721.txt">Table of n, a(n) for n = 1..10000</a>

%e 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

%p dA:= map(t -> 1+ilog[3](7^t), [$0..100]):

%p ListTools:-PartialSums(dA); # _Robert Israel_, Aug 20 2024

%K nonn

%O 1,2

%A _David W. Wilson_

%E Name clarified by _Robert Israel_, Aug 20 2024