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 A116551 #10 Sep 21 2017 04:09:16
%S A116551 0,3,1,6,4,2,9,7,5,12,10,8,15,13,11,18,16,14,21,19,17,24,22,20,27,25,
%T A116551 23,30,28,26,33,31,29,36,34,32,39,37,35,42,40,38,45,43,41,48,46,44,51,
%U A116551 49,47,54,52,50,57,55,53,60,58,56,63,61,59,66,64,62,69,67,65,72,70,68,75
%N A116551 Permutation of natural numbers generated by 3-rowed array shown below.
%C A116551 0 3 6 9 12 15 18 21 24 ... a(n)= 3n
%C A116551 1 4 7 10 13 16 19 22 25 ... a(n)= 3n+1
%C A116551 2 5 8 11 14 17 20 23 26 ... a(n)= 3n+2
%D A116551 M. Cerasoli, F. Eugeni and M. Protasi, Elementi di Matematica Discreta, Bologna 1988.
%D A116551 Emanuele Munarini and Norma Zagaglia Salvi, Matematica Discreta, UTET, CittaStudiEdizioni, Milano 1997.
%H A116551 G. C. Greubel, <a href="/A116551/b116551.txt">Table of n, a(n) for n = 1..1000</a>
%H A116551 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,1,-1).
%F A116551 Starting at the term a(3), a(n+3k) = a(n) + 3k, with k>=1.
%F A116551 From _Chai Wah Wu_, Jul 10 2016: (Start)
%F A116551 a(n) = a(n-1) + a(n-3) - a(n-4) for n > 7.
%F A116551 G.f.: x^2*(2*x^5 - 5*x^3 + 5*x^2 - 2*x + 3)/(x^4 - x^3 - x + 1). (End)
%t A116551 Rest[CoefficientList[Series[x^2*(2*x^5 - 5*x^3 + 5*x^2 - 2*x + 3)/(x^4 - x^3 - x + 1), {x,0,50}], x]] (* or *) Join[{0, 3, 1, 6, 4, 2, 9}, LinearRecurrence[{1,0,1,-1}, {7, 5, 12, 10}, 50]] (* _G. C. Greubel_, Sep 20 2017 *)
%o A116551 (PARI) x='x+O('x^50); Vec(x^2*(2*x^5 - 5*x^3 + 5*x^2 - 2*x + 3)/(x^4 - x^3 - x + 1)) \\ _G. C. Greubel_, Sep 20 2017
%Y A116551 Cf. A115302.
%K A116551 easy,nonn
%O A116551 1,2
%A A116551 _Giovanni Teofilatto_, Mar 17 2006