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 A061974 #18 Dec 05 2020 11:16:46
%S A061974 1,5,25,125,155,160,421,1409,729175,2804576
%N A061974 Numbers n such that n divides the (left) concatenation of all numbers <= n written in base 21 (most significant digit on right).
%C A061974 This sequence differs from A029538 in that all least significant zeros are kept during concatenation.
%C A061974 No more terms < 7*10^6. - _Lars Blomberg_, Jul 27 2011
%H A061974 <a href="/index/N#concat">Index entries for related sequences</a>
%e A061974 See A061955 for example.
%t A061974 b = 21; c = {}; Select[Range[10^4], Divisible[FromDigits[c = Join[Reverse[IntegerDigits[#, b]], c], b], #] &] (* _Robert Price_, Mar 09 2020 *)
%o A061974 (PARI) lista(nn, m=21) = my(c, t); for(k=1, nn, t+=m^c*fromdigits(Vecrev(digits(k, m)), m); c+=logint(m*k, m); if(t%k==0, print1(k, ", "))); \\ _Jinyuan Wang_, Dec 05 2020
%Y A061974 Cf. A029447-A029470, A029471-A029494, A029495-A029518, A029519-A029542, A061931-A061954, A061955-A061978.
%K A061974 nonn,base,more
%O A061974 1,2
%A A061974 Larry Reeves (larryr(AT)acm.org), May 24 2001
%E A061974 Edited and updated by Larry Reeves (larryr(AT)acm.org), Apr 12 2002
%E A061974 a(9)-a(10) from _Lars Blomberg_, Jul 27 2011