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 A082853 #10 Jul 24 2021 11:04:17
%S A082853 0,0,0,1,0,1,2,3,4,0,1,2,3,4,5,6,7,8,9,10,11,12,13,0,1,2,3,4,5,6,7,8,
%T A082853 9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,
%U A082853 32,33,34,35,36,37,38,39,40,41,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17
%N A082853 Integers 0 to Catalan(n)-1 followed by integers 0 to Catalan(n+1)-1 etc.
%H A082853 Harvey P. Dale, <a href="/A082853/b082853.txt">Table of n, a(n) for n = 0..1000</a>
%F A082853 a(n) = n - A082852(n)
%t A082853 Table[Range[0,CatalanNumber[n]-1],{n,0,6}]//Flatten (* _Harvey P. Dale_, Jul 24 2021 *)
%Y A082853 a(n) = A082854(n)-1. Cf. A002262, A000108.
%K A082853 nonn
%O A082853 0,7
%A A082853 _Antti Karttunen_, Apr 17 2003