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 A213917 #10 Mar 05 2013 13:58:11
%S A213917 -1,1,31,5,24,37,180,13,213,34,165,49,81,194,430,29,142,231,329,54,
%T A213917 104,187,399,73,418,107,60916,222,290,460,60716,61,535,176,352,267,
%U A213917 353,367,1444,94,60861,146,842,231,335,445,60653,121,526,468,722,159,281
%N A213917 Difference between sum of all even and the sum of all odd numbers in Collatz (3x+1) trajectory of n.
%H A213917 T. D. Noe, <a href="/A213917/b213917.txt">Table of n, a(n) for n = 1..10000</a>
%F A213917 a(n) = A213909(n) - A213916(n).
%t A213917 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Table[c = Collatz[n]; Total[Select[c, EvenQ]] - Total[Select[c, OddQ]], {n, 100}] (* _T. D. Noe_, Mar 05 2013 *)
%Y A213917 Cf. A213909, A213916.
%K A213917 sign
%O A213917 1,3
%A A213917 _Jayanta Basu_, Mar 05 2013