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 A213909 #21 Mar 05 2013 13:59:13
%S A213909 0,2,40,6,30,46,234,14,276,40,212,58,100,248,562,30,178,294,424,60,
%T A213909 126,234,516,82,538,126,81178,276,366,592,80910,62,688,212,446,330,
%U A213909 444,462,1894,100,81096,168,1090,278,416,562,80816,130,666,588,926,178,340
%N A213909 Sum of all even numbers in Collatz (3x+1) trajectory of n.
%H A213909 T. D. Noe, <a href="/A213909/b213909.txt">Table of n, a(n) for n = 1..10000</a>
%e A213909 a(5)=30 since Collatz trajectory of 5 is 5,16,8,4,2,1.
%t A213909 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Table[Total[Select[Collatz[n], EvenQ]], {n, 100}] (* _T. D. Noe_, Mar 05 2013 *)
%Y A213909 Cf. A033493.
%K A213909 nonn
%O A213909 1,2
%A A213909 _Jayanta Basu_, Mar 05 2013