This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A025721 #14 Jul 06 2025 13:35:04 %S A025721 1,3,7,13,21,30,41,54,69,85,103,123,145,169,194,221,250,281,313,347, %T A025721 383,421,460,501,544,589,636,684,734,786,840,895,952,1011,1072,1134, %U A025721 1198,1264,1332,1402,1473,1546,1621,1698,1776,1856,1938,2022,2108,2195,2284 %N A025721 Index of 7^n within sequence of numbers of form 3^i*7^j for i >= 0, j >= 1. %C A025721 Positions of zeros in A025642. - _R. J. Mathar_, Jul 06 2025 %H A025721 Robert Israel, <a href="/A025721/b025721.txt">Table of n, a(n) for n = 1..10000</a> %e A025721 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 A025721 dA:= map(t -> 1+ilog[3](7^t), [$0..100]): %p A025721 ListTools:-PartialSums(dA); # _Robert Israel_, Aug 20 2024 %Y A025721 Cf. A003594. %K A025721 nonn %O A025721 1,2 %A A025721 _David W. Wilson_ %E A025721 Name clarified by _Robert Israel_, Aug 20 2024