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 A345672 #5 Jun 24 2021 13:43:06
%S A345672 0,1,2,3,4,5,6,7,8,9,10,11,1,13,14,15,16,17,18,19,20,12,22,2,24,25,26,
%T A345672 27,28,29,30,13,23,33,3,35,36,37,38,39,40,14,24,34,44,4,46,47,48,49,
%U A345672 50,15,25,35,45,55,5,57,58,59,60,16,26,36,46,56,66,6,68
%N A345672 a(n) is the least m such that the decimal expansion of n appears in the concatenation of m, ..., m+k for some k >= 0 and Sum_{i = m..m+k-1} A055642(i) < A055642(n) <= Sum_{i = m..m+k} A055642(i).
%C A345672 This sequence is similar to A055642; here we consider decimal expansions, there binary expansions.
%H A345672 Rémy Sigrist, <a href="/A345672/a345672.gp.txt">PARI program for A345672</a>
%F A345672 a(n) <= n.
%F A345672 a(n) = 1 iff n belongs to A252043.
%e A345672 For n = 21:
%e A345672 - the decimal expansion of 21 first appears in the concatenation of 12 and 13,
%e A345672 - so a(21) = 12.
%o A345672 (PARI) See Links section.
%Y A345672 Cf. A007376, A055642, A083653 (binary analog), A252043.
%K A345672 nonn,base
%O A345672 0,3
%A A345672 _Rémy Sigrist_, Jun 22 2021