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 A019520 #36 Feb 16 2025 08:32:33
%S A019520 2,24,246,2468,246810,24681012,2468101214,246810121416,24681012141618,
%T A019520 2468101214161820,246810121416182022,24681012141618202224,
%U A019520 2468101214161820222426,246810121416182022242628,24681012141618202224262830,2468101214161820222426283032
%N A019520 a(n) is the concatenation of the first n positive even numbers.
%D A019520 H. Ibstedt, A Few Smarandache Sequences, Smarandache Notions Journal, Vol. 8, No. 1-2-3, 1997, 170-183.
%D A019520 F. Smarandache, "Collected Papers", Vol. II, Tempus Publ. Hse., Bucharest, Romania, 1996.
%D A019520 S. Smarandoiu, Convergence of Smarandache continued fractions, Abstract 96T-11-195, Abstracts Amer. Math. Soc., 17 (No. 4, 1996), 680.
%H A019520 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ConsecutiveNumberSequences.html">Consecutive Number Sequences</a>
%F A019520 Lim_{n->oo} A019519(n)/a(n) = 0 (see A067095). - _Bernard Schott_, Dec 18 2021
%t A019520 Table[FromDigits[Flatten[IntegerDigits/@(2Range[n])]],{n,20}] (* _Harvey P. Dale_, Mar 24 2013 *)
%o A019520 (Python)
%o A019520 def a(n): return int("".join(str(2*i) for i in range(1, n+1)))
%o A019520 print([a(n) for n in range(1, 17)]) # _Michael S. Branicky_, Dec 18 2021
%Y A019520 Cf. A019519 (similar, with odd numbers), A067095, A108728.
%K A019520 base,nonn,easy,less
%O A019520 1,1
%A A019520 R. Muller
%E A019520 More terms from _Erich Friedman_
%E A019520 More terms from _Harvey P. Dale_, Mar 24 2013