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 A029521 #30 Jan 13 2025 19:21:05
%S A029521 1,3,15,969,1107,136193,538019,2177475,2799315,5948555
%N A029521 Numbers k such that k divides the (left) concatenation of all numbers <= k written in base 4 (most significant digit on right and removing all least significant zeros before concatenation).
%C A029521 This sequence differs from A061957 in that all least significant zeros are removed before concatenation.
%C A029521 No more terms < 10^7. [_Lars Blomberg_, Sep 02 2011]
%H A029521 <a href="/index/N#concat">Index entries for related sequences</a>
%e A029521 See A029519 for example.
%t A029521 b = 4; c = {}; Select[Range[10^4], Divisible[FromDigits[c = Join[IntegerDigits[IntegerReverse[#, b], b], c], b], #] &] (* _Robert Price_, Mar 13 2020 *)
%Y A029521 Cf. A029447-A029470, A029471-A029494, A029495-A029518, A029519-A029542, A061931-A061954, A061955-A061978.
%K A029521 nonn,base,more
%O A029521 1,2
%A A029521 _Olivier Gérard_
%E A029521 Edited and updated by Larry Reeves (larryr(AT)acm.org), Apr 12 2002; Aug 25 2002
%E A029521 a(8)-a(10) from _Lars Blomberg_, Sep 02 2011