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 A137318 #15 Jan 02 2024 08:59:19
%S A137318 1,31,313,1313,13131,313131,3131313,13131313,131313131,3131313131,
%T A137318 31313131313,131313131313,1313131313131,31313131313131,
%U A137318 313131313131313,1313131313131313,13131313131313131,313131313131313131
%N A137318 Concatenation of segments of the digit sequence 1 3 1 3 1 3 1 3 1 3 1 3 1 3 1 3 1 3....
%C A137318 A000042 is 1,11,111,1111,11111,... concatenation of 111111111111111....
%C A137318 A002276 is 2,22,222,2222,22222,... concatenation of 222222222222222....
%C A137318 A133013 is 2,35,71113,... concatenation of 2 3 5 7 11 13 17 19 23 29,...
%H A137318 Harvey P. Dale, <a href="/A137318/b137318.txt">Table of n, a(n) for n = 1..1000</a>
%F A137318 O.g.f.: x*(100x^4 + 200x^3 + 83x^2 + 20x + 1)/((10x-1)(100x^2+1)(x-1)(x^2+1)). - _R. J. Mathar_, Apr 09 2008
%t A137318 Module[{nn=20},FromDigits/@TakeList[PadRight[{},(nn(nn+1))/2,{1,3}],Range[ nn]]] (* _Harvey P. Dale_, Aug 04 2021 *)
%Y A137318 Cf. A000040, A000042.
%K A137318 nonn,base,easy
%O A137318 1,2
%A A137318 _Ctibor O. Zizka_, Apr 06 2008
%E A137318 More terms from _R. J. Mathar_, Apr 09 2008