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 A023721 #20 Apr 14 2025 05:33:35
%S A023721 1,2,3,4,6,7,8,9,11,12,13,14,16,17,18,19,21,22,23,24,31,32,33,34,36,
%T A023721 37,38,39,41,42,43,44,46,47,48,49,56,57,58,59,61,62,63,64,66,67,68,69,
%U A023721 71,72,73,74,81,82,83,84,86,87,88,89,91,92,93
%N A023721 Numbers with no 0's in their base-5 expansion.
%H A023721 Nathaniel Johnston, <a href="/A023721/b023721.txt">Table of n, a(n) for n = 1..10000</a>
%H A023721 Robert Baillie and Thomas Schmelzer, <a href="https://library.wolfram.com/infocenter/MathSource/7166/">Summing Kempner's Curious (Slowly-Convergent) Series</a>, Mathematica Notebook kempnerSums.nb, Wolfram Library Archive, 2008.
%H A023721 <a href="/index/Ar#5-automatic">Index entries for 5-automatic sequences</a>.
%F A023721 Sum_{n>=1} 1/a(n) = 8.1899922882413061715479525413921657841497267151276815624858907606158756278085270372763455153366655369098... (calculated using Baillie and Schmelzer's kempnerSums.nb, see Links). - _Amiram Eldar_, Apr 14 2025
%p A023721 seq(`if`(numboccur(0,convert(n,base,5))=0,n,NULL),n=1..127); # _Nathaniel Johnston_, Jun 27 2011
%t A023721 Select[ Range[ 120 ], (Count[ IntegerDigits[ #, 5 ], 0 ]==0)& ]
%t A023721 Select[Range[120], DigitCount[#, 5, 0] == 0 &] (* _Amiram Eldar_, Apr 14 2025 *)
%o A023721 (PARI) is(n)=while(n, if(n%5==0, return(0)); n\=5); 1 \\ _Charles R Greathouse IV_, Feb 12 2017
%Y A023721 Cf. A020654, A023725, A023729, A023733.
%K A023721 nonn,base,easy
%O A023721 1,2
%A A023721 _Olivier Gérard_