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 A131728 #11 Jun 13 2015 00:52:24
%S A131728 0,1,1,0,1,3,2,0,2,5,3,0,3,7,4,0,4,9,5,0,5,11,6,0,6,13,7,0,7,15,8,0,8,
%T A131728 17,9,0,9,19,10,0,10,21,11,0,11,23,12,0,12,25,13,0,13,27,14,0,14,29,
%U A131728 15,0,15,31,16,0,16,33,17,0,17,35,18,0,18,37,19,0,19,39,20,0,20,41,21,0,21
%N A131728 a(4n) = n, a(4n+1) = 2n+1, a(4n+2) = n+1, a(4n+3) = 0.
%H A131728 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (2,-3,4,-3,2,-1).
%F A131728 a(n)=A077814(n+1). - _R. J. Mathar_, Jun 13 2008
%t A131728 LinearRecurrence[{2,-3,4,-3,2,-1},{0,1,1,0,1,3},90] (* _Harvey P. Dale_, Aug 15 2013 *)
%K A131728 nonn,easy
%O A131728 0,6
%A A131728 _Paul Curtz_, Sep 17 2007