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 A078998 #10 Aug 28 2020 03:06:38
%S A078998 1,11,111,1111,11111,111111,1111111,11111111,111111111,12222222121,
%T A078998 1222222212101,122222221210101,12222222121010101,1222222212101010101,
%U A078998 122222221210101010101,12222222121010101010101
%N A078998 Choose a(n) so that a(1)+a(2)+...+a(n) = concatenation of n first natural numbers.
%F A078998 a(1)=1; for n>1, a(n) = A007908(n)-A007908(n-1)
%t A078998 b = {}; a = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[k]]], {k, 1, Length[w]}]; p = FromDigits[a]; AppendTo[b, p], {n, 1, 30}]; c = {}; Do[AppendTo[c, b[[n + 1]] - b[[n]]], {n, 1, Length[b] - 1}]; c (* _Artur Jasinski_, Mar 30 2008 *)
%Y A078998 Cf. A000422, A116504, A007908, A116505, A104759, A138789, A138790, A138793.
%K A078998 nonn,base
%O A078998 1,2
%A A078998 _Benoit Cloitre_, Jan 12 2003